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Relative Abundance, Growth, and Mortality of Five Estuarine Age-0 Fishes in Relation to River Discharge at the Suwannee ...

Permanent Link: http://ufdc.ufl.edu/UFE0021795/00001

Material Information

Title: Relative Abundance, Growth, and Mortality of Five Estuarine Age-0 Fishes in Relation to River Discharge at the Suwannee River, Florida
Physical Description: 1 online resource (43 p.)
Language: english
Creator: Purtlebaugh, Caleb H
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: discharge, estuary, fish, flow, growth, mortality, river, suwannee
Fisheries and Aquatic Sciences -- Dissertations, Academic -- UF
Genre: Fisheries and Aquatic Sciences thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Understanding relationships between river discharge and recruitment of estuarine fishes is important due to hydrology alterations from anthropogenic water withdrawals. Variation in river discharge alters salinity, turbidity, nutrient and detrital concentrations which influence all estuarine biota. The Suwannee River system is one of the few remaining large river systems in the United States that has no major impoundments. I assessed the relationship between seasonal river discharge and age-0 fish relative abundance, growth, and mortality for five estuarine-dependent species in the Suwannee River estuary. Analyses included nine years of data (1997-2005) collected as part of a long-term fisheries-independent monitoring program. I found a positive relationship between age-0 fish relative abundance and seasonal mean river discharge for spotted seatrout Cynoscion nebulosus, sand seatrout Cynoscion arenarius, and red drum Sciaenops ocellatus. Pinfish Lagodon rhomboides was the only species for which relative abundance was negatively related to river discharge, and spot Leiostomus xanthurus relative abundance was not significantly related to changes in river discharge. Instantaneous daily growth estimates were positively related to river discharge for all species except spotted seatrout, for which a negative correlation was found. Instantaneous daily mortality estimates were positively correlated with river discharge for sand seatrout, pinfish, and red drum. Changes in fresh water discharge clearly affected the abundance, growth, and survival of these juvenile fish, stressing the importance of water allocation decisions to estuarine fishes and the fisheries they support.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Caleb H Purtlebaugh.
Thesis: Thesis (M.S.)--University of Florida, 2007.
Local: Adviser: Allen, Micheal S.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021795:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021795/00001

Material Information

Title: Relative Abundance, Growth, and Mortality of Five Estuarine Age-0 Fishes in Relation to River Discharge at the Suwannee River, Florida
Physical Description: 1 online resource (43 p.)
Language: english
Creator: Purtlebaugh, Caleb H
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: discharge, estuary, fish, flow, growth, mortality, river, suwannee
Fisheries and Aquatic Sciences -- Dissertations, Academic -- UF
Genre: Fisheries and Aquatic Sciences thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Understanding relationships between river discharge and recruitment of estuarine fishes is important due to hydrology alterations from anthropogenic water withdrawals. Variation in river discharge alters salinity, turbidity, nutrient and detrital concentrations which influence all estuarine biota. The Suwannee River system is one of the few remaining large river systems in the United States that has no major impoundments. I assessed the relationship between seasonal river discharge and age-0 fish relative abundance, growth, and mortality for five estuarine-dependent species in the Suwannee River estuary. Analyses included nine years of data (1997-2005) collected as part of a long-term fisheries-independent monitoring program. I found a positive relationship between age-0 fish relative abundance and seasonal mean river discharge for spotted seatrout Cynoscion nebulosus, sand seatrout Cynoscion arenarius, and red drum Sciaenops ocellatus. Pinfish Lagodon rhomboides was the only species for which relative abundance was negatively related to river discharge, and spot Leiostomus xanthurus relative abundance was not significantly related to changes in river discharge. Instantaneous daily growth estimates were positively related to river discharge for all species except spotted seatrout, for which a negative correlation was found. Instantaneous daily mortality estimates were positively correlated with river discharge for sand seatrout, pinfish, and red drum. Changes in fresh water discharge clearly affected the abundance, growth, and survival of these juvenile fish, stressing the importance of water allocation decisions to estuarine fishes and the fisheries they support.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Caleb H Purtlebaugh.
Thesis: Thesis (M.S.)--University of Florida, 2007.
Local: Adviser: Allen, Micheal S.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021795:00001


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26405d878e1a4f0f49210e0eb0503756ace40e23







RELATIVE ABUNDANCE, GROWTH, AND MORTALITY OF FIVE ESTUARINE AGE-0
FISH IN RELATION TO DISCHARGE OF THE SUWANNEE RIVER, FLORIDA





















By

CALEB HUNTER PURTLEBAUGH


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA



2007

































2007 Caleb Hunter Purtlebaugh




























To my family and son "Hunter"









ACKNOWLEDGMENTS

Gratitude is expressed to the many people who helped me carry out this project. Special

thanks are given to Dr. Mike Allen who took me on as a student and helped initiate my project. I

also thank Jered Jackson, Bill Pine, and Tom Frazer for their reviews and advice. Data for this

study was collected over nine years by FWCC Fish and Wildlife Research Institute personnel at

the Senator George G. Kirkpatrick Marine Laboratory in Cedar Key, Florida. I thank all those

who participated in field work, data collection, data entry, and data proofing during that time. I

also thank the USGS and SRWMD for flow and precipitation data that they have made available

in public domain. Support for this study was provided in part by funds from Florida

Recreational Saltwater Fishing License sales and the Department of Interior, U.S. Fish and

Wildlife Service, Federal Aid for Sport Fish Restoration, Project number F-43.









TABLE OF CONTENTS


A C K N O W L E D G M E N T S ..............................................................................................................4

LIST OF TABLES ................. .............. ........... .................. 6.

LIST O F FIG U RE S ................................................................. 7

A B S T R A C T ......... ....................... .................. .......................... ................ .. 8

CHAPTER

1 INTRODUCTION ............... .......................................................... 10

2 M E T H O D S ................... ...................1...................4..........

Study Location .............................................14
D ata C o lle ctio n ................................................................................................................. 14
A nalyses.................................................... ................... ...................16
Seasonal River Discharge ...................................................... ...... ........... 16
Age-0 Fish Relative Abundance and Seasonal River Discharge ................ ...............16
G ro w th ................... ...................1...................7..........
M mortality ........................................................ ........... ................ .. 18
Growth and M ortality Validation .............................................................. ............. 19

3 R E SU L T S .............. ... ................................................................20

Age-0 Fish Relative Abundance and Seasonal River Discharge............... ......... ....20
G row th and M mortality .......................................................2 1
G row th and M mortality V alidation ...............................................................................22

4 DISCUSSION ..................... ............. .. ..................... 31

L IST O F R E F E R E N C E S ..................................................................................... ....................38

B IO G R A PH IC A L SK E T C H .............................................................................. .....................43









LIST OF TABLES


Table page

3-1 Summary of species, period fish were recruiting into the estuary and vulnerable to
the gear (Recruitment window), and SL (standard length) cut-off for fish used in
calculation of relative abundance of fish considered to be within age-0 classification.....23

3-2 Significant multiple regression equations predicting age-0 fish relative abundance
within zones (1997 2005) from seasonal mean Suwannee River discharge rates
betw een years .................. ........................... ....... ...................... 24

3-3 Summary table of species and their mean relative growth rates reported as
millimeters per day (mm-d-1), standard error (SE), and minimum and maximum
relative growth rates for fish captured from 1997-2005.......... .............. .............. 25









LIST OF FIGURES


Figure pe

1-1 Map of the sampling area around the Suwannee River estuary, Florida. ..........................13

3-2 Relationship between log transformed age-0 fish relative abundance (fish-100 m-2)
and seasonal mean Suwannee River discharge between years......................................26

3-3 Relationship between yearly instantaneous daily growth estimates (G) and mean
Suw annee R iver discharge........................................................................ ..................27

3-4 Relationship between yearly instantaneous mortality rate estimates (Z) and mean
Suw annee R iver discharge........................................................................ ...................28

3-5 Observed (bars) length frequency of fast (A) and slow (B) growing pinfish with the
predicted length frequency (solid line) overlaid, after calculated growth and mortality
w ere applied to observed catches............................................. .............................. 29

3-6 Observed (bars) length frequency of fast growing red drum with the predicted length
frequency (solid line) overlaid, after calculated growth and mortality were applied to
ob served catches. ........................................................ ................. 30









Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

RELATIVE ABUNDANCE, GROWTH, AND MORTALITY OF FIVE ESTUARINE AGE-0
FISH IN RELATION TO DISCHARGE OF THE SUWANNEE RIVER, FLORIDA
By

Caleb Hunter Purtlebaugh

December 2007

Chair: Micheal Allen
Major: Fisheries and Aquatic Sciences

Understanding relationships between river discharge and recruitment of estuarine fishes is

important due to hydrology alterations from anthropogenic water withdrawals. Variation in river

discharge alters salinity, turbidity, nutrient and detrital concentrations which influence all

estuarine biota. The Suwannee River system is one of the few remaining large river systems in

the United States that has no major impoundments. I assessed the relationship between seasonal

river discharge and age-0 fish relative abundance, growth, and mortality for five estuarine-

dependent species in the Suwannee River estuary. Analyses included nine years of data (1997-

2005) collected as part of a long-term fisheries-independent monitoring program. I found a

positive relationship between age-0 fish relative abundance and seasonal mean river discharge

for spotted seatrout Cynoscion nebulosus, sand seatrout Cynoscion arenarius, and red drum

Sciaenops ocellatus. Pinfish Lagodon rhomboides was the only species for which relative

abundance was negatively related to river discharge, and spot Leiostomus xanthurus relative

abundance was not significantly related to changes in river discharge. Instantaneous daily

growth estimates were positively related to river discharge for all species except spotted seatrout,

for which a negative correlation was found. Instantaneous daily mortality estimates were

positively correlated with river discharge for sand seatrout, pinfish, and red drum. Changes in









fresh water discharge clearly affected the abundance, growth, and survival of these juvenile fish,

stressing the importance of water allocation decisions to estuarine fishes and the fisheries they

support.









CHAPTER 1
INTRODUCTION

River discharge affects many abiotic and biotic characteristics of estuaries. River

discharge influences salinity and turbidity as well as nutrient and detrital concentrations, and

these changes can strongly influence estuarine biota (Wilber 1994; Garcia et al. 2003; North and

Houde 2003; Binett et al. 1995; Crivelli et al. 1995; Livingston 1991; Livingston 1997;

Winemiller and Leslie 1992). Freshwater input provides nutrients for primary production in

estuaries (Strydom et al. 2002; Wooldridge and Bailey 1982; Baird and Heymans 1996) and

quality habitat for many estuary-dependent larval and juvenile fishes recruiting into estuarine

nursery areas (Whitfield 1994). Global-scale atmospheric circulation anomalies and patterns

such as El Nifio and La Nifia events have caused unusual precipitation and drought, leading to

variation in river discharge in many areas of the world (Molles and Dahm 1990; Gillanders and

Kingsford 2002). Modified reductions in freshwater discharge into estuarine ecosystems are of

particular concern as potable water withdrawals are increased to meet the demands of growing

human populations (Browder 1991). Conversely, land use changes can actually increase

freshwater discharge and disrupt the natural timing of water delivery to an estuary (Drinkwater

and Frank 1994; Gillanders and Kingsford 2002). Changes in the timing and magnitude of

freshwater discharge have the potential to impact recruitment, growth, and mortality of age-0

fishes that use estuarine nursery habitat during their first year of life. Annual changes in age-0

fish abundance, growth, and mortality may subsequently impact year-class strength of fish that

support important fisheries.

Several hypotheses have been published concerning how river discharge influences fish

recruitment in estuaries. The short-food hypothesis states that recruitment would be enhanced in

the vicinity of river plumes because fish larvae experience superior feeding conditions, resulting









in faster growth and lower mortality (Govoni et al. 1989; Finucane et al. 1990; Grimes and

Finucane 1991). The total-larval-production hypothesis postulates that nutrients associated with

river discharge support high total production of fish larvae and that specific dynamics of growth

and mortality are not relevant (Grimes and Finucane 1991). The third hypothesis contends that

plumes facilitate retention of more fish larvae within a limited area, and it is the physical

retention rather than production that explains the effects of discharge on fish recruitment

(Sinclair 1988; Grimes and Kingsford 1996). In all of these cases, variation in river discharge

may lead to changes in relative abundance and potentially growth and mortality of age-0 fishes.

Interactions among river discharge, estuarine productivity, and fisheries has been

reported in many regions of the world (Caddy and Bakun 1995; Deegan et al. 1986; Martins et

al. 2001). Major fisheries have been negatively impacted as a result of altering river discharge.

For example, totoaba, Totoaba macdonaldi, once supported important commercial and

recreational fisheries in the northern Gulf of California. Totoaba was placed on the endangered

species list in 1976 after diversion of the Colorado River altered spawning and nursery areas

(Barrera-Guevara 1990). Similarly, the Aswan Dam, Egypt, decreased river discharge by 40 km3

year, with a concomitant decline in primary fisheries production in estuarine waters and

adjacent Mediterranean Sea (Bebars and Lasserre 1983; Bishara 1984). Anthropogenic alteration

of river discharge regimes has been detrimental to fisheries (Baird and Heymans 1996; Grange et

al. 2000; Strydom and Whitfield 2000). However, fisheries have also been shown to flourish in

estuaries that received increased freshwater discharge. In the Kariega estuary, South Africa,

Strydom et al. (2002) found a positive correlation between catches of juvenile fish and river

discharge. In Australia a positive correlation was reported to exist between barramundi Lates

calcarifer year class strength and river discharge (Staunton-Smith et al. 2004).









I examined fish responses to river discharge in the Suwannee River estuary, Florida. This

watershed is one of the few remaining large river systems in the United States with no major

impoundments constructed within its drainage system (Mattson and Rowan 1989). The

headwaters originate in the Okefenokee Swamp of Georgia and the river flows 426 km to the

Gulf of Mexico in Florida (Figure 1). The bulk of human water consumption and river baseflow

is supplied by groundwater, which is intricately connected to the surface water via abundant

springs. The biological communities within the Suwannee River estuary could be impacted by

impending increases in water withdrawal from the Suwannee River system (Tsou and Matheson

2002).

I utilized an existing long term fishery-independent monitoring database to evaluate

whether relative abundance, growth, and mortality of age-0 fish was related to river discharge at

the Suwannee River estuary. My objectives were to 1) determine if relative abundance of age-0

fish varied with seasonal river discharge among years, and 2) assess potential mechanisms that

might underline any relations with river discharge by evaluating growth and mortality of each

species. An evaluation of these relationships may have implications for setting water withdrawal

policy for rivers and estuaries.




















j, *~-d--.


North


Gulf
of
Mexico


South


0 1,252.5 5 7.5 10
... .............. -,K iom eters


Figure 1-1. Map of the sampling area around the Suwannee River estuary, Florida.









CHAPTER 2
METHODS

Study Location

The Suwannee River estuary is a relatively pristine estuary with mostly undeveloped

shorelines and is located in the Big Bend region of Florida's west coast. Unlike most estuaries,

the Suwannee River estuary is an open system, lacking a barrier island (Figure 1). The

shorelines are dominated by salt marshes and the bottom substrate is primarily mud, sand, and

oyster reef. The Suwannee River has the second largest discharge in Florida with an average

discharge rate near the mouth of the river of 125 m3 s-1 (USGS 2006). In general, there are two

peaks of freshwater discharge to the estuary: a relatively large peak between February and April

and a somewhat smaller peak between August and October (Mattson and Rowan 1989; Tsou and

Matheson 2002). However, seasonal discharge of the Suwannee River is highly variable across

years.

Data Collection

Spotted seatrout Cynoscion nebulosus, sand seatrout Cynoscion arenarius, red drum

Sciaenops ocellatus, spot Leiostomus xanthurus (family: Sciaenidae), and pinfish Lagodon

rhomboides, (family: Sparidae) were collected in the Suwannee River estuary during monthly

stratified-random sampling efforts from January 1997 through December 2005. These fish

species were selected due to their recreational or commercial importance and also because of

their dependence upon estuary habitats during the juvenile life stage. The estuary was divided

into two zones (North and South) (Figure 1). Water chemistry in the north zone was directly

influenced by discharge from the Suwannee River and surrounding tidal creeks. Water

chemistry in the south zone was minimally affected by changes in river discharge with the

exception of extremely high flow events. Fish were collected during daylight hours and during









all tidal stages using a center bag seine that measured 21.3-m x 1.8-m with a 3.2-mm #35

knotless nylon Delta mesh, deployed in water depths ranging from 0.3 1.8 m. Three

deployment techniques were used to set the bag seine to ensure that predominate habitat types

were effectively sampled. Shoreline deployments sampled shorelines with emergent vegetation,

mangrove fringes, seawalls, and beaches. Offshore deployments sampled shallow waters at least

5 m away from a shoreline and sampled vegetated and unvegetated flats. River deployments

sampled the shorelines of tidal creeks and the lower Suwannee River. All collections were

standardized with regard to amount of area covered in each haul. The area sampled with

shoreline and offshore deployments was 140 m2 and for river deployments was 68 m2. Effort

among the three deployment techniques was roughly proportional to the available habitat within

the sampling universe.

All fish collected were counted and up to 40 individuals per species and sample were

measured to the nearest millimeter standard length (mm SL). Length measurements were then

extrapolated to the unmeasured portion of the sample by species. Collections containing more

than 1,000 fish by species were subsampled with a modified Motoda box splitter and the total

number of individuals was estimated by fractional expansion of the subsampled portion (Winner

and McMichael 1997). Length-frequency histograms were developed by month and year for

each species to identify the timing of recruitment into the estuary and to identify cohort length

modes. For quality control, the first three individuals of each species identified in the field, and

up to ten individuals of species that were unidentifiable in the field were retained and later

identified at the lab using dichotomous keys. At each sample site, salinity (psu), water

temperature (C), and dissolved oxygen (mg-L1) were measured 0.2-m and every 1.0-m bellow









the water surface, down to 0.2-m from the bottom. Water depth (m), and location (degrees,

minutes, seconds) were measured and recorded at all sample sites.

Suwannee River discharge (m3's-1) was provided by the U.S. Geological Survey (USGS)

and was based upon measurements at Wilcox, Florida, approximately 51 kilometers from the

river mouth. Monthly precipitation data (cm) was provided by the Suwannee River Water

Management District (SRWMD) and was based upon measurements at Manatee Springs, Florida

Analyses

Seasonal River Discharge

Seasons were determined by evaluating environmental variables using Principal

Component Analysis (PCA). Within each month of each year (1997-2005), environmental

parameters included average monthly water temperature, salinity, dissolved oxygen, river

discharge, and precipitation. To reduce variability during PCA analyses, monthly river discharge

and precipitation values were calculated as monthly proportions of the entire annual values for

each variable. To determine if principal component scores differed for each month, general

linear models (GLM; a=0.05) were used. Months were then grouped into each of four seasons

based primarily upon Student Newman Keuls (SNK) tests (SAS Institute Inc. 1989). Within

each year and season, monthly discharge data were then averaged, establishing seasonal mean

river discharge.

Age-0 Fish Relative Abundance and Seasonal River Discharge

Age-0 fish relative abundance (i.e., fish-100 m-2) was estimated for each species within

species-specific recruitment windows in both north and south zones, across years. Recruitment

window was defined as the months when newly-recruited fish settled out into the estuary and

remained vulnerable to the sampling gear. Length-frequency histograms were developed by

month for each species to identify timing of recruitment into the estuary and to determine size









ranges that were vulnerable to capture for each species. Fish that were considered vulnerable to

the gear were confirmed to be age-0 based upon size ranges found in literature reviews for each

species. Relative abundance of age-0 fish was calculated separately for north and south zones in

to detect response differences based on distance from the river mouth (i.e. river plume).

Relationships between age-0 fish relative abundance and seasonal river discharge across years

were assessed using multiple linear regression. To determine lagged effects of river discharge on

relative abundance, seasonal river discharge used in regression models included seasons that

occurred up to one year prior to and during recruitment windows for individual species. In

instances when a season occurred during and beyond a recruitment window, a partial season

which only included the months occurring during the recruitment window was used in the model.

Relative abundance and seasonal river discharge data were loglo-transformed prior to

analyses to normalize the data. A Shapiro-Wilk test was used to test for normality (Zar 1996).

Stepwise elimination and Akaike's information criterions (AIC) were used for model selection.

Multicollinearity was assessed by evaluating the variance inflation factor (VIF) (Meyers 1990).

Residuals were inspected to assure the appropriateness of the linear models. All statistical

analyses were conducted using Statistical Analysis System (SAS Institute Inc. 1989). Statistical

tests were considered significant when P < 0.10. A p-value of 0.10 was chosen to reduce

Type-II error (Peterman 1990).

Growth

Instantaneous daily growth rates were estimated for age-0 fish of each species. For

increased sample size, length-frequency data from north and south zones were combined to track

cohort modes and by using the following model:

G ln(L2 Li) (2-1)
T2- T1









where G = the instantaneous daily growth rate;

L = cohort mode length (mm SL); and

T= time in days.

When cohort modes were absent or not evident, growth rate estimates were not calculated

for that particular year and species. Due to bi-modal spawning of spotted seatrout and sand

seatrout during single recruitment windows, separate calculations for growth were estimated for

"early" and "late" recruiting fish. Early-recruitment fish were those that settled into the estuary

at the beginning of the recruitment window while late-recruitment fish were those that were

spawned in the middle of the recruitment window and later settled into the estuary. Growth

calculations began with the first month in which recruiting fish length frequencies were not

truncated and a clear shift in cohort modes could be detected. First and last months included in

growth calculations never varied by more than one month from year to year. Length-frequency

histograms were developed by month for each species to determine at which minimum and

maximum SL each species were vulnerable to the gear. To minimize bias associated with gear

escapement, only cohort modes that fell within the gear vulnerability range were used for growth

estimates. Instantaneous daily growth rate estimates for year classes of each species were related

to mean river discharge using linear regression. Only river discharge that occurred during

months from which growth was calculated was included in the model.

Mortality

Instantaneous daily mortality rates were estimated for age-0 fish of each species. For

increased sample size, length-frequency and abundance data from both north and south zones

were combined to track cohort modes and using the following equation:

Nt = Noez (2-2)









where N = abundance (fish-100 m-2) offish at time t;

No = initial abundance; and

Z = daily instantaneous mortality

t = time interval between No and Nt.

Initial abundance of fish (No) applied to the earliest month at which specific species were

considered to be fully recruited to the sampling gear. To reduce bias associated with smaller fish

still recruiting into the gear and larger fish avoiding the gear, I only included fish that were

captured within + 5-mm SL around the cohort mode when calculating relative abundance. When

cohort modes were absent or not clear, mortality estimates were not calculated for that particular

year and species. Instantaneous daily mortality estimates for each recruitment-class of each

species were correlated with mean river discharge using linear regression. Only river discharge

that occurred during months from which mortality was calculated was included in the model.

Growth and Mortality Validation

To further investigate the possibility of bias associated with gear vulnerability on growth

and mortality estimates, I applied growth and mortality estimates to observed length-frequencies

of pinfish and red drum. Predicted length frequencies were overlaid on top of observed length

frequencies to evaluate the potential for fish to grow into or out of the gear at a different rate than

was predicted from equations 2-1 and 2-2. Fast growth and slow growth pinfish and fast growth

red drum were used as examples.









CHAPTER 3
RESULTS

Age-0 Fish Relative Abundance and Seasonal River Discharge

Principal Component Analysis indicated that water temperature and dissolved oxygen

combined accounted for the highest monthly environmental variation followed by salinity and

river discharge combined. General linear models revealed a significant difference between PCA

scores for each month. Subsequent SNK tests performed on monthly PCA scores resulted in the

assignment of four seasons. Winter was defined as December February; Spring as March -

May; Summer as June September; and Autumn as October and November.

Multiple regression models revealed significant relationships between age-0 fish (summary

of age-0 fish defined in Table 1) relative abundance and changes in seasonal mean Suwannee

River discharge between years (P < 0.10) for all species except spot during the period 1997

through 2005 (Table 2). In general, relative abundance of age-0 spotted seatrout, sand seatrout,

and red drum were positively related to seasonal river discharge occurring prior to juvenile fish

recruiting into the estuary and during recruitment (Table 2 and Figure 2). Relative abundance of

sand seatrout and red drum captured in the north zone, directly influenced by the Suwannee

River discharge, was positively related to seasonal river discharge. No significant relationship

could be found for these species in the south zone. Spotted seatrout was the only species that

exhibited a positive relationship between river discharge and age-0 fish relative abundance in the

south zone. Red drum in the north zone had the best-fitting multiple regression equation,

indicating that age-0 fish relative abundance was positively related to pre-spawn summer and

winter river discharge (Table 2). Relative abundance of age-0 pinfish was negatively related to

seasonal river discharge in both north and south zones (Table 2 and Figure 2). Spring river

discharge was negatively related to relative abundance of pinfish captured in the north zone,









whereas winter discharge was negatively related to abundance in the south zone. No significant

relationship between relative abundance and seasonal mean river discharge was found for spot in

either the north or south zones. None of the species analyzed displayed an opposite relationship

to river discharge between north and south zones. Variation explained by significant regression

models (r2) ranged from 0.37 to 0.88 (Table 2).

Growth and Mortality

Linear regression models for red drum, pinfish, spot, and early-recruitment sand seatrout

indicated a significant increase in instantaneous daily growth rates with higher river discharge (P

< 0.05; Figure 3). Early-recruitment spotted seatrout was the only species that demonstrated a

decrease in growth with higher river discharge (Figure 3). I detected no relationship between

growth and river discharge for late-recruitment spotted seatrout or sand seatrout. Variation

explained by significant regression models (r2) ranged from 0.48 to 0.96 (Figure 3). Relative

daily growth for all five species ranged from 0.083 to 0.833 mm-d-1 (Table 3). Red drum,

pinfish, spot, and early recruiting sand seatrout all had similar average growth rates which ranged

from 0.359 to 0.387 mm-d-1. Late recruitment spotted seatrout (0.181 mm-d-1) and early

recruitment spotted seatrout (0.233 mm-d-1) exhibited slower growth (Table 3).

Linear regression models for pinfish, red drum, and early-recruitment sand seatrout

indicated a significant increase in instantaneous daily mortality rates with river discharge rate (P

< 0.05; Figure 4). River discharge was not logo transformed for the early-recruitment sand

seatrout model. No significant relationship was detected between mortality and river discharge

for spot and late-recruitment sand seatrout. Early-recruitment sand seatrout had the lowest and

highest instantaneous daily mortality across all species, ranging from 0.002 to 0.124. Mortality

rates were not determined for spotted seatrout due to limited data. Variation explained by

significant regression models (r2) ranged from 0.46 to 0.81 (Figure 4).









Growth and Mortality Validation

Comparisons of predicted length frequencies to the observed catches suggested no gear

selectivity issues for the size of fish that I used in this analysis (Figures 3-5 and 3-6). Fish did

not appear to be avoiding the gear or moving out of the sample able areas at larger sizes,

suggesting that the growth and mortality rates I estimated were not biased by gear selectivity. In

general predicted length frequency modes closely resembled the observed modes (Figures 3-5

and 3-6).









Table 3-1. Summary of species, period fish were recruiting into the estuary and vulnerable to the gear (Recruitment window), and SL
(standard length) cut-off for fish used in calculation of relative abundance of fish considered to be within age-0
classification.


Species
spotted seatrout
sand seatrout
red drum
pinfish
spot


Age-0 Fish Relative Abundance
Recruitment Window Age-0 length (SL)
June October < 100 mm
May October < 100 mm
September January < 100 mm
January June < 80 mm
January April < 75 mm


Reference
McMichael and Peters 1989
Nemeth et. al. 2006
Peters and McMichael 1987
Hanson 1969
Livingston 1984









Table 3-2. Significant multiple regression equations predicting age-0 fish relative abundance within zones (1997 2005) from
seasonal mean Suwannee River discharge rates between years.
P-
Model R2 value
Spotted seatrout fish 100m2 (south zone) = -0.4999 + 0.1644 x Log spring 0.37 0.08
Sand seatrout fish 100m2 (north zone) = -1.4082 + 0.5683 x Log summer 0.77 < 0.01
Red drum fish 100m2 (north zone) = -3.8543 + 0.5109 x Log winter + 0.6135 x Log pre-spawn
summer 0.88 0.02
Pinfish fish 100m2 (north zone) = 5.4088 1.0951 x Log spring 0.61 0.01
Pinfish fish 100m2 (south zone) = 4.3659 0.9065 x Log winter 0.56 0.02









Table 3-3. Summary table of species and their mean relative growth rates reported as millimeters per day (mm-d-1), standard error
(SE), and minimum and maximum relative growth rates for fish captured from 1997-2005. Months from which growth
rates were calculated from are listed. Maximum length was the maximum size the cohort mode reached during growth
calculations within any year. All fish captured within the maximum length were considered to be within selectivity of the
gear. Fish captured in north and south zones were combined for growth calculations. Spotted seatrout and sand seatrout
were separated into early and late recruitment due to bi-modal spawning.

Mean Min Max
Species Months Max Length (SL) Growth mm-d' SE Growth mmn-d- Growth mnr-d
spotted seatrout
early recruitment June July < 35 mm 0.233 0.0717 0.083 0.500
late recruitment August September < 35 mm 0.181 0.033 0.083 0.333
sand seatrout
early recruitment May July < 55 mm 0.359 0.078 0.167 0.833
late recruitment August October < 40 mm 0.255 0.070 0.083 0.750
red drum October January < 55 mm 0.368 0.079 0.125 0.667
pinfish March July < 60 mm 0.387 0.017 0.321 0.444
spot February April < 60 mm 0.368 0.079 0.125 0.667










Spotted Seatrout
South zone
R2 = 0.37


C 1.6

. 1.2

0.8

S0.4

0.0
z 1



u1 2.0
0
* 1.6

S 1.2

0 0.8
-J


1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Log Spring River Discharge


2.0 Sand Seatrout
North zone
1.6 -R2 = 0.77

1.2

0.8 -*

0.4 -

0.0
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Log Summer River Discharge

2.0 1 Pinfish
South zone
1.6 -R2 = 0.56

1.2 ,

0.8

0.4

0.0
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0


Log Winter River Discharge


Figure 3-2. Relationship between logio-transformed age-0 fish relative abundance (fish-100 m-
2) and seasonal mean Suwannee River discharge between years. River discharge
is in logo transformed cubic meters per second (m3 s-1).


1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Log Spring River Discharge












0.024


0.018


0.012


Spotted Seatrout
Early recruitment
R2 = 0.82
P<0.05


0.006 -


0.000 ,
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0


Sand Seatrout
Early recruitment
R2 = 0.56
P< 0.05








.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0


Pinfish
R2 = 0.96
P< 0.0001


0.024 1 Red Drum
R2 = 0.70
P < 0.05


0.018


0.012


0.006


0.000


0.024


0.018


S


0.012


0.006


1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0


*








1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Spot
R2 = 0.48
P<0.05


'S


0.024


0.018


0.012


0.006


0.000
1

0.024


0.018


0.012


0.006


0.000


Loglo River Discharge (m3s1)


Figure 3-3. Relationship between yearly instantaneous daily growth estimates (G) and mean
Suwannee River discharge during months in which growth was calculated by year.


0.000 -
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0


I












0.14
0.12 Sand Seatrout
Early Recruitment
N 0.10 R2 = 0.81
S0.08 P< 0.05

- 0.06
0.04
0 0.02
0.00
0 0 100 200 300 400 500 600

S0.14 0.14
U) Pinfish Red Drum
S0.12 R2 = 0.46 0.12 R2 = 0.66
O 0.10 P<0.05 0.10 P=0.05
G)
0.08 0.08
0.06 0.06

CU
- 0.04 0.04
C 0.02 0.02
0.00 .. 0.00
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0


River Discharge (m3 s-l)

Figure3-4. Relationship between yearly instantaneous mortality rate estimates (Z) and mean
Suwannee River discharge during months in which mortality was calculated by year.
River discharge is in loglo-transformed cubic meters per second (m3 -1) for pinfish
and red drum only.









Pinfish Fast Growth
(A) March 2001
N N=1,213


III H
April
SN = 1,050


May
N = 324





June
N = 391





July
N =25


n Ini


lH Il


0 20 40 60 80


0 20 40 60 80 100


Standard Length (mm)


Figure 3-5. Observed (bars) length-frequency of fast (A) and slow (B) growing pinfish with the
predicted length-frequency (solid line) overlaid, after calculated growth and mortality
were applied to observed catches. Dark bars represent + 5-mm increments around the
cohort mode, used in calculating mortality estimates.


200

























0
3

-c

r-
CO
LL


E
3
z


0 20 40


60 80 100


Standard Length (mm)
Figure 3-6. Observed (bars) length-frequency of fast growing red drum with the predicted
length-frequency (solid line) overlaid, after calculated growth and mortality were
applied to observed catches. Dark bars represent + 5-mm increments around the
cohort mode, used in calculating mortality estimates.









CHAPTER 4
DISCUSSION

Previous work has evaluated the effects of river discharge on fish larvae retention,

growth, and survival (Grimes and Kingsford 1996; Grimes and Finucane 1991; Govoni et al.

1989), but few have evaluated the effects that changes in river plumes may have on fish once

they reach a juvenile stage within an estuary and compare these effects to hypotheses associated

with larvae fish. The short-food hypothesis stated higher recruitment in the vicinity of river

plumes due to enhanced feeding conditions, promoting fast growth and lower mortality of fish

larvae (Govoni et al. 1989; Finucane et al. 1990; Grimes and Finucane 1991). The size and

shape of river plumes and their associated fronts is variable, and depends primarily upon the

nature of ocean currents, topography, dimensions and shape of the estuary and adjacent shelf

area, and the rate of freshwater discharge. Rivers with high discharge rates typically produce

large plumes (Gilllanders and Kingsford 2002). I found that relative abundance and growth of

age-0 sand seatrout and red drum increased with river discharge across years. However, these

species also suffered higher mortality rates during increased river discharge which lends some

credence to the total-larval-production hypothesis which states that superior trophic conditions

found during high river discharge support such high total production of fish larvae that specific

dynamics of growth and mortality are not relevant (Grimes and Finucane 1991).

Although both the short-food and total-larval-production hypotheses apply to larval fish,

these concepts may also be applicable to juvenile fish recruiting into an estuary. The physical

mechanisms associated with intensity of river plumes may affect juvenile fish relative abundance

and the relationship between growth and mortality. Changes in mortality may have a much larger

effect on survival (ultimately recruitment success) than incremental changes in growth (Grimes

2001). Effects of these mechanisms on juvenile fishes may vary depending on species.









Therefore, analysis for juvenile fish should be conducted on an individual species basis. In my

study, pinfish suffered lower relative abundance and higher mortality rates with increased river

discharge, despite higher growth with high discharge. Whereas previously mentioned, red drum

and sand seatrout exhibited higher relative abundance, higher growth and higher mortality with

increased river discharge. The third hypothesis stated that it is the physical retention of river

plumes rather than production that explains the variation in recruitment (Sinclair 1988; Grimes

and Kingsford 1996). Loneragan and Bunn (1999) suggested that changes in turbidity and

salinity as a result of changing river discharge may restrict distribution of fish or stimulate their

movements into areas more likely to be caught thus leading to higher densities rather than an

actual increase in fish production. I was not able to evaluate this hypothesis due to the inability

to isolate the relative contribution of nutrients and intensity of the retention mechanism because

both vary with river discharge and mixing with ocean currents (Grimes and Kingsford 1996).

My study found relative abundance of age-0 spotted seatrout from the south zone, and

sand seatrout and red drum from the north zone to be positively related to increases in seasonal

river discharge. None of the five species abundances displayed an opposite relationship to river

discharge between north and south zones which would have indicated a river-induced change in

spawning location or settling location of new recruits transported from offshore waters. Both

spotted seatrout and sand seatrout are nearshore or estuarine spawners and both spawn from

spring through summer months (Walters 2005). The positive relationship found between relative

abundance of spotted seatrout and sand seatrout to spring and summer river discharge

respectively, may be attributed to the higher input of nutrients into the estuary leading to

enhanced primary and secondary production during summer months as opposed to other seasons

(Whitfield 1994; Loneragan and Bunn 1999). Spotted seatrout was the only species which









displayed an increase in relative abundance in the south zone during increased river discharge.

This may have been a result of spotted seatrout avoiding lower salinity areas in order to reduce

osmoregulatory stress, commonly found in marine species (Whitfield and Harrison 2003).

Red drum and spot are two of three estuarine-dependent species I analyzed that spawn

offshore (Rooker et al. 1998; Weinstein 1983) and depend upon ocean currents to transport

larvae back into the estuary. Red drum begin spawning during late summer when Suwannee

River discharge rates are normally relatively low. Many species of marine fishes use changes in

freshwater discharge as physical and chemical stimuli to initiate migration offshore for spawning

and for passive transport of larvae towards estuaries (Champalbert et al. 1989; Champalbert and

Koutsikopoulos 1995). As a result, high recruitment levels should be expected for high river

discharge occurring during optimal periods (Costa et al. 2007). North and Houde (2003) found

similar results in the Chesapeake Bay estuary where juvenile white perch and striped bass

abundances were positively correlated to Susquehanna River discharge during spring. Juvenile

Gilchristella aestuaria abundances were positively correlated with river discharge in the Kariega

Estuary along the coast of South Africa (Strydom et. al. 2002). This may explain the strong

positive association I found between relative abundance of age-0 red drum and river discharge

during pre-spawn summer seasons and winter seasons. No significant relationship was found

between discharge and relative abundance of age-0 spot. Spot spawn during winter and spring

months when Suwannee River discharge is generally at its highest. Although increased river

discharge can be important for nutrient input, extreme high freshwater discharge may actually

flush out nutrients and even create a physical barrier to recruitment of fish (Gillanders and

Kingsford 2002; Costa et al. 2007).









In contrast, other studies of juvenile fishes have shown negative relationships between

abundance and river discharge. Juvenile gulf menhaden Brevoortiapatronus abundance has

been found to be negatively associated with Mississippi River discharge (Govoni 1997), and age-

0 Japanese seaperch Lateolabraxjaponicus abundance exhibited an inverse correlation to the

Chikugo River discharge (Shoji and Tanaka 2006). In this study, pinfish was the only species in

which age-0 fish abundance was negatively related to river discharge. Pinfish are the third of

three species I analyzed that spawn offshore (Hansen 1969) and depend upon ocean currents to

transport larvae back into the estuary. Pinfish spawn during late winter and early spring when

the Suwannee River discharge rates generally are highest. High discharge, may restrict the

shoreward transport of pinfish larvae.

Food supply is perhaps the most important biological factor influenced by changes in

river discharge (Grimes and Finucane 1991). My results showed sand seatrout, red drum,

pinfish, and spot experienced higher growth during years with elevated river discharge,

supporting the argument of enhanced feeding opportunities during increased river discharge.

Spotted seatrout was the only species that displayed slower growth during years with increased

river discharge. Deegan (1990) found a similar relationship with growth of juvenile gulf

menhaden declining with increased discharge from the Mississippi River. Spotted seatrout in my

study may have suffered from prolonged freshwater conditions (lower salinity) during increased

river discharge which can lead to osmoregulatory stress in some marine species (Whitfield and

Harrison 2003), thus leading to slower growth.

My results, however, do not support the contention that higher growth rates of juvenile

fish leads to better survival as is often found with larvae fish (North and Houde 2001; Grimes

and Kingsford 1996). In my study, red drum, sand seatrout, and pinfish mortality increased with









river discharge, despite these species exhibiting faster growth under the same physical

conditions. There is some evidence in the literature supporting a relationship of high mortality

rates with high growth rates in juvenile estuarine-dependent species. Walters and Martell's

(2004) foraging arena theory suggests juvenile fish respond to changes in food concentrations to

maintain constant growth. This theory assumes most predation occurs while fish are foraging

and this attempt to maintain constant growth likely results in linear increase in mortality with

increasing juvenile density. Prey such as juvenile finfish can hide from predators unless the prey

density is so high that it forces that prey to spend more time foraging outside of their preferred

habitat, resulting in higher mortality. Munch and Conover (2003) also found fast growing

Atlantic silversides Menidia menidia suffered higher mortality from predation despite being 40%

larger than the slow-growing population (Munch and Conover 2003). Consumption of larger

meals by fast growth fish may be detrimental to swimming ability and predator evasion

(Billerbeck et al. 2001). Furthermore, the same physical and biological dynamics that lead to

accumulated larval fish and prey may also accumulate their predators (Femandez-Delgado et al.

2007; Govoni et al. 1989) although the number of prey eaten may not depend on the number of

predators (Walters and Martell 2004; Munch and Conover 2003). Cannibalism associated with

high abundance is another major source of mortality common in early life stages that should be

considered (Grimes and Finucane 1991). Thus, my results indicate that faster growth associated

with high river discharge might result in higher mortality for juvenile fishes.

Benefits of increased production associated with river plumes (Gillanders and Kingsford

2002; Grimes and Finucane 1991) may or may not out weigh the disadvantage of increased

mortality. For example, Grimes and Kingsford (1996) showed higher instantaneous natural

mortality rates in the vicinity of the Mississippi river plume than away from the plume for larval









Spanish mackerel Scomberomorus maculatus and king mackerel Scomberomorus cavalla,

despite enhanced growth. Similar results for striped anchovy Anchoa hepsetus suggest that

natural mortality in the plume front and within the river plume was higher than that experienced

in shelf waters (Day 1993). Connel (1998) reported a positive relationship between juvenile reef

fish abundance and mortality rates due to predation. If conditions in the vicinity of river plumes

support higher growth or fish abundances, but also aggregate predators and increase mortality

rate, enhanced recruitment into the fishery may not occur. In my study, red drum, sand seatrout,

and pinfish all suffered from higher mortality despite either rapid growth or higher relative

abundance during high-discharge years.

Differences in mortality rates based on changes in relative abundance with fish size

should be interpreted with caution because the method assumes that fish have equal vulnerability

to capture by the sampling gear. Faster growth rates may lead to biased mortality estimates,

because large fish become less vulnerable to capture and would be under-represented, thus

leading to inflated mortality rates. However, any inherent biases in the mortality estimates

would be present during all years. Moreover, the data used in calculating instantaneous mortality

rates was within the bounds of gear selectivity of each species analyzed. Fish captured during

initial recruitment months in which they appeared to be partially vulnerable the gear, as well as

later months when larger fish would be expected to either leave the area or avoid the gear were

not included in mortality estimates. By only including fish captured + 5-mm around the cohort

mode when calculating cohort abundance, I further reduced the chance of gear selectivity biasing

mortality estimates. This approach appeared to be valid based on the observed and predicted

changes in the length frequency distributions through time.









Results of this study could assist managers in the development and implementation of

strategies to compensate for poor year-class production as a result of changes in river discharge.

In areas where water control devices (i.e., dams and levies) currently exist along major

tributaries, mangers could work with water controlling authorities to manage water levels for

better fish production. This study has broad implications for assessing how river discharge

influences recruitment of ecologically and recreationally important estuarine-dependent species

in an estuary.









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BIOGRAPHICAL SKETCH

Caleb Hunter Purtlebaugh was born February 20, 1975 in Winchester, Virginia, the son of

Michael and Sun Purtlebaugh. He graduated from James Wood High School, Virginia in 1993.

He received his Bachelor of Science degree in Wildlife and Fisheries Resources from West

Virginia University. He became interested in the outdoors while hunting and fishing with his

father. He would often canoe and camp along the Shenandoah River located in the western part

of Virginia. After graduating from college with his Bachelor of Science degree, he briefly

attended graduate school at Frostburg State University, however eventually moved and started a

job working for the Delaware Division of Fish and Wildlife for a couple of years. It was there

that he became interested in marine science. During his time in Delaware, he continued graduate

school at Delaware State University while also working for the Delaware Division of Fish and

Wildlife. It soon became apparent to him that he wanted to broaden his experience in marine

science. In 2000, he accepted ajob as a marine fisheries research scientist with the Florida Fish

and Wildlife Research Institute, located in Cedar Key, Florida. After gaining extensive

experience in the marine science field and after the birth of his son "Hunter", he decided to

finally finish what he had started. He contacted the University of Florida and Dr. Mike Allen

took him on as a graduate student in the fall 2005. After his enrollment into graduate school he

continued to work full-time. After his graduation in December 2007, he plans to continue his

successful career in marine fisheries research.





PAGE 1

1 RELATIVE ABUNDANCE, GROW TH, AND MORTALITY OF FIVE ESTUARINE AGE-0 FISH IN RELATION TO DISCHARGE OF THE SUWANNEE RIVER, FLORIDA By CALEB HUNTER PURTLEBAUGH A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2007

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2 2007 Caleb Hunter Purtlebaugh

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3 To my family and son Hunter

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4 ACKNOWLEDGMENTS Gratitude is expressed to the many people who helped me carry out this project. Special thanks are given to Dr. Mike Alle n who took me on as a student and he lped initiate my project. I also thank Jered Jackson, Bill Pine, and Tom Frazer for their reviews and advice. Data for this study was collected over nine years by FWCC Fish and Wildlife Research Institute personnel at the Senator George G. Kirkpatrick Marine Labora tory in Cedar Key, Florid a. I thank all those who participated in field work, data collection, data entry, and da ta proofing during that time. I also thank the USGS and SRWMD for flow and preci pitation data that they have made available in public domain. Support for this study wa s provided in part by funds from Florida Recreational Saltwater Fishing License sales and the Department of Interior, U.S. Fish and Wildlife Service, Federal Aid for Spor t Fish Restoration, Project number F-43.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........6 LIST OF FIGURES................................................................................................................ .........7 ABSTRACT....................................................................................................................... ..............8 CHAPTER 1 INTRODUCTION..................................................................................................................10 2 METHODS........................................................................................................................ .....14 Study Location................................................................................................................. .......14 Data Collection................................................................................................................ .......14 Analyses....................................................................................................................... ...........16 Seasonal River Discharge................................................................................................16 Age-0 Fish Relative Abundance and Seasonal River Discharge.....................................16 Growth......................................................................................................................... ....17 Mortality...................................................................................................................... ....18 Growth and Mortality Validation....................................................................................19 3 RESULTS........................................................................................................................ .......20 Age-0 Fish Relative Abundance and Seasonal River Discharge............................................20 Growth and Mortality........................................................................................................... ..21 Growth and Mortality Validation...........................................................................................22 4 DISCUSSION..................................................................................................................... ....31 LIST OF REFERENCES............................................................................................................. ..38 BIOGRAPHICAL SKETCH.........................................................................................................43

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6 LIST OF TABLES Table page 3-1 Summary of species, period fish were re cruiting into the estuary and vulnerable to the gear (Recruitment window), and SL (sta ndard length) cut-off for fish used in calculation of relative abundance of fish considered to be within age-0 classification.....23 3-2 Significant multiple regression equations predicting age-0 fish relative abundance within zones (1997 2005) from seasonal mean Suwannee River discharge rates between years.................................................................................................................. ...24 3-3 Summary table of species and their mean relative growth rates reported as millimeters per day (mmd-1), standard error (SE), and minimum and maximum relative growth rates for fish captured from 1997-2005................................................... 25

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7 LIST OF FIGURES Figure page 1-1 Map of the sampling area around the Suwannee River estuary, Florida...........................13 3-2 Relationship between log transformed age0 fish relative abundance (fish m-2) and seasonal mean Suwannee River discharge between years..........................................26 3-3 Relationship between yearly instantaneous daily growth estimates (G) and mean Suwannee River discharge................................................................................................27 3-4 Relationship between yearly instantaneous mortality rate estimates (Z) and mean Suwannee River discharge.................................................................................................28 3-5 Observed (bars) length fre quency of fast (A) and slow (B) growing pinfish with the predicted length frequency (solid line) overl aid, after calculated growth and mortality were applied to observed catches.......................................................................................29 3-6 Observed (bars) length fre quency of fast growing red dr um with the predicted length frequency (solid line) overlaid, after calculated growth and mortality were applied to observed catches............................................................................................................... .30

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8 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science RELATIVE ABUNDANCE, GROW TH, AND MORTALITY OF FIVE ESTUARINE AGE-0 FISH IN RELATION TO DISCHARGE OF THE SUWANNEE RIVER, FLORIDA By Caleb Hunter Purtlebaugh December 2007 Chair: Micheal Allen Major: Fisheries a nd Aquatic Sciences Understanding relationships between river discha rge and recruitment of estuarine fishes is important due to hydrology alterations from anthr opogenic water withdrawals. Variation in river discharge alters salinity, turbid ity, nutrient and detrital con centrations which influence all estuarine biota. The Suwannee River system is one of the few remaining large river systems in the United States that has no major impoundments. I assessed the relationship between seasonal river discharge and age-0 fish relative abundan ce, growth, and mortalit y for five estuarinedependent species in the Suwannee River estuary. Analyses included nine years of data (19972005) collected as part of a long-term fisher ies-independent monitoring program. I found a positive relationship between age0 fish relative abundance and s easonal mean river discharge for spotted seatrout Cynoscion nebulosus sand seatrout Cynoscion arenarius and red drum Sciaenops ocellatus Pinfish Lagodon rhomboides was the only species for which relative abundance was negatively related to river discharge, and spot Leiostomus xanthurus relative abundance was not significantly re lated to changes in river discharge. Instantaneous daily growth estimates were positively related to river discharge for all species except spotted seatrout, for which a negative correlation was found. In stantaneous daily mortality estimates were positively correlated with river discharge for sand seatrout, pinfish, and red drum. Changes in

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9 fresh water discharge clearly aff ected the abundance, growth, and survival of these juvenile fish, stressing the importance of water allocation decisions to estuarine fishes and the fisheries they support.

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10 CHAPTER 1 INTRODUCTION River discharge affects many abiotic and biot ic characteristics of estuaries. River discharge influences salinity and turbidity as well as nutrient and detr ital concentrations, and these changes can strongly influence estuarine biota (Wilber 1994; Garcia et al. 2003; North and Houde 2003; Binett et al. 1995; Crivelli et al. 1995; Livingston 1991; Livingston 1997; Winemiller and Leslie 1992). Freshwater inpu t provides nutrients for primary production in estuaries (Strydom et al. 2002; Wooldridge and Bailey 1982; Baird and Heymans 1996) and quality habitat for many estuary-dependent larval and juvenile fishes recruiting into estuarine nursery areas (Whitfield 1994). Global-scale at mospheric circulation anomalies and patterns such as El Nio and La Nia events have caused unusual precipitation and drought, leading to variation in river discharge in many areas of the world (Molles and Dahm 1990; Gillanders and Kingsford 2002). Modified reductions in freshwat er discharge into estuarine ecosystems are of particular concern as potable wa ter withdrawals are increased to meet the demands of growing human populations (Browder 1991). Conversely, land use changes can actually increase freshwater discharge and disrupt the natural timi ng of water delivery to an estuary (Drinkwater and Frank 1994; Gillanders and Kingsford 2002) Changes in the timing and magnitude of freshwater discharge have the potential to imp act recruitment, growth, and mortality of age-0 fishes that use estuarine nursery habitat during th eir first year of life. Annual changes in age-0 fish abundance, growth, and mortality may subsequen tly impact year-class st rength of fish that support important fisheries. Several hypotheses have been published con cerning how river discharge influences fish recruitment in estuaries. The short-food hypothesis states that re cruitment would be enhanced in the vicinity of river plumes because fish larvae experience superior feed ing conditions, resulting

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11 in faster growth and lower mortality (Govoni et al. 1989; Finucane et al. 1990; Grimes and Finucane 1991). The total-larval-p roduction hypothesis postulates th at nutrients associated with river discharge support high total production of fish larvae and that specific dynamics of growth and mortality are not relevant (Grimes and Finucane 1991). The third hy pothesis contends that plumes facilitate retention of more fish larvae within a limited area, and it is the physical retention rather than producti on that explains the effects of discharge on fish recruitment (Sinclair 1988; Grimes and Kingsford 1996). In a ll of these cases, variat ion in river discharge may lead to changes in relative abundance and pote ntially growth and mortality of age-0 fishes. Interactions among river discharge, es tuarine productivity, and fisheries has been reported in many regions of the world (Caddy and Bakun 1995; Deegan et al. 1986; Martins et al. 2001). Major fisheries have been negatively impacted as a result of altering rive r discharge. For example, totoaba, Totoaba macdonaldi, once supported important commercial and recreational fisheries in the north ern Gulf of California. Toto aba was placed on the endangered species list in 1976 after divers ion of the Colorado River altered spawning and nursery areas (Barrera-Guevara 1990). Similarly, the Aswan Dam, Egypt, decreased river discharge by 40 km3 year-1, with a concomitant decline in primary fi sheries production in estuarine waters and adjacent Mediterranean Sea (Bebars and Lasserre 1983; Bish ara 1984). Anthropogenic alteration of river discharge regimes has been detrimental to fisheries (Baird and He ymans 1996; Grange et al. 2000; Strydom and Whitfield 2000). However, fi sheries have also been shown to flourish in estuaries that received increased freshwater di scharge. In the Kariega estuary, South Africa, Strydom et al. (2002) found a positive correlation between catches of juvenile fish and river discharge. In Australia a positive correla tion was reported to exist between barramundi Lates calcarifer year class strength and river discha rge (Staunton-Smith et al. 2004).

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12 I examined fish responses to river discharge in the Suwannee River estuary, Florida. This watershed is one of the few remaining large ri ver systems in the United States with no major impoundments constructed within its draina ge system (Mattson and Rowan 1989). The headwaters originate in the Okefenokee Swamp of Georgia and the river flows 426 km to the Gulf of Mexico in Florida (Figure 1). The bul k of human water consumption and river baseflow is supplied by groundwater, which is intricatel y connected to the su rface water via abundant springs. The biological communities within th e Suwannee River estuary could be impacted by impending increases in water withdrawal from the Suwannee River system (Tsou and Matheson 2002). I utilized an existing long term fishery-i ndependent monitoring database to evaluate whether relative abundance, growth, and mortality of age-0 fish was related to river discharge at the Suwannee River estuary. My objectives were to 1) determine if relative abundance of age-0 fish varied with seasonal river discharge among years, and 2) assess potential mechanisms that might underline any relations with river discha rge by evaluating growth and mortality of each species. An evaluation of these relationships ma y have implications for setting water withdrawal policy for rivers and estuaries.

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13 Figure 1-1. Map of the sampling area ar ound the Suwannee River estuary, Florida.

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14 CHAPTER 2 METHODS Study Location The Suwannee River estuary is a relatively pristine estuary with mostly undeveloped shorelines and is located in the Big Bend region of Floridas west coast. Unlike most estuaries, the Suwannee River estuary is an open system lacking a barrier isla nd (Figure 1). The shorelines are dominated by salt marshes and the bottom substrate is primarily mud, sand, and oyster reef. The Suwannee River has the second la rgest discharge in Florida with an average discharge rate near the m outh of the river of 125 m3 s-1 (USGS 2006). In general, there are two peaks of freshwater discharge to the estuary: a relatively large peak between February and April and a somewhat smaller peak between August and October (Mattson an d Rowan 1989; Tsou and Matheson 2002). However, seasonal discharge of the Suwannee River is highly variable across years. Data Collection Spotted seatrout Cynoscion nebulosus sand seatrout Cynoscion arenarius red drum Sciaenops ocellatus spot Leiostomus xanthurus (family: Sciaenidae), and pinfish Lagodon rhomboides (family: Sparidae) were collected in the Suwannee River estuary during monthly stratified-random sampling efforts from Janua ry 1997 through December 2005. These fish species were selected due to their recreationa l or commercial importance and also because of their dependence upon estuary habitats during the j uvenile life stage. The estuary was divided into two zones (North and South) (Figure 1). Water chemistry in the no rth zone was directly influenced by discharge from the Suwannee River and surrounding tidal creeks. Water chemistry in the south zone was minimally aff ected by changes in river discharge with the exception of extremely high flow events. Fish were collected during da ylight hours and during

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15 all tidal stages using a center bag seine that measured 21.3-m x 1.8-m with a 3.2-mm #35 knotless nylon Delta mesh, deployed in water depths ranging from 0.3 1.8 m. Three deployment techniques were used to set the bag seine to ensure that predominate habitat types were effectively sampled. Shoreline deployments sampled shorelines with emergent vegetation, mangrove fringes, seawalls, and beaches. Offsho re deployments sampled shallow waters at least 5 m away from a shoreline and sampled vegeta ted and unvegetated flats. River deployments sampled the shorelines of tidal creeks and the lower Suwannee River. All collections were standardized with regard to amount of area covered in each haul. The area sampled with shoreline and offshore deployments was 140 m2 and for river deployments was 68 m2. Effort among the three deployment techniques was roughly proportional to the avai lable habitat within the sampling universe. All fish collected were counted and up to 40 individuals per species and sample were measured to the nearest millimeter standard length (mm SL). Length measurements were then extrapolated to the unmeasured portion of the sa mple by species. Collections containing more than 1,000 fish by species were subsampled with a modified Motoda box splitter and the total number of individuals was estim ated by fractional expansion of the subsampled portion (Winner and McMichael 1997). Length-freq uency histograms were deve loped by month and year for each species to identify the timing of recruitment into the estuary and to identify cohort length modes. For quality control, the first three individual s of each species identif ied in the field, and up to ten individuals of species that were unidentifiable in the field were retained and later identified at the lab using dichotomous keys. At each sample site, salinity (psu), water temperature (oC), and dissolved oxygen (mgL-1) were measured 0.2-m and every 1.0-m bellow

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16 the water surface, down to 0.2-m from the bottom. Water depth (m), and location (degrees, minutes, seconds) were measured and recorded at all sample sites. Suwannee River discharge (m3s-1) was provided by the U.S. Geological Survey (USGS) and was based upon measurements at Wilcox, Florida, approximately 51 kilometers from the river mouth. Monthly precipitation data (cm) was provided by the Suwannee River Water Management District (SRWMD) and was based upon measurements at Manatee Springs, Florida Analyses Seasonal River Discharge Seasons were determined by evaluating environmental variables using Principal Component Analysis (PCA). Within each m onth of each year (1997-2005), environmental parameters included average monthly water te mperature, salinity, dissolved oxygen, river discharge, and precipitation. To reduce variabil ity during PCA analyses, monthly river discharge and precipitation values were cal culated as monthly proportions of the entire annual values for each variable. To determine if principal co mponent scores differed for each month, general linear models (GLM; =0.05) were used. Months were then grouped into each of four seasons based primarily upon Student Newman Keuls (SNK) tests (SAS Institute Inc. 1989). Within each year and season, monthly discharge data were then averaged, establishing seasonal mean river discharge. Age-0 Fish Relative Abundance and Seasonal River Discharge Age-0 fish relative a bundance (i.e., fish m-2) was estimated for each species within species-specific recruitment windows in both north and south zones, across years. Recruitment window was defined as the months when newly-r ecruited fish settled out into the estuary and remained vulnerable to the sampling gear. Length-frequency histogr ams were developed by month for each species to identify timing of recr uitment into the estuary and to determine size

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17 ranges that were vulnerable to capture for each speci es. Fish that were considered vulnerable to the gear were confirmed to be age-0 based upon size ranges found in literature reviews for each species. Relative abundance of age-0 fish was calcu lated separately for north and south zones in to detect response differences based on distan ce from the river mouth (i.e. river plume). Relationships between age-0 fish relative abunda nce and seasonal river discharge across years were assessed using multiple linear regression. To determine lagged effects of river discharge on relative abundance, seasonal rive r discharge used in regression models included seasons that occurred up to one year prior to and during recruitment windows for i ndividual species. In instances when a season occurred during and beyond a recruitment window, a partial season which only included the months o ccurring during the recrui tment window was used in the model. Relative abundance and seasonal ri ver discharge data were log10-transformed prior to analyses to normalize the data. A Shapiro-Wilk test was used to test for normality (Zar 1996). Stepwise elimination and Akaikes information cr iterions (AIC) were used for model selection. Multicollinearity was assessed by evaluating the variance inflation factor (VIF) (Meyers 1990). Residuals were inspected to assure the appropr iateness of the linear models. All statistical analyses were conducted using Statistical Analysis System (SAS Institute Inc. 1989). Statistical tests were considered signi ficant when P < 0.10. A p-value of 0.10 was chosen to reduce Type-II error (Peterman 1990). Growth Instantaneous daily growth rates were estimated for age-0 fish of each species. For increased sample size, length-fre quency data from north and south zones were combined to track cohort modes and by using the following model: 1 2 1 2) ln( T T L L G (2-1)

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18 where G = the instantaneous daily growth rate; L = cohort mode length (mm SL); and T = time in days. When cohort modes were absent or not evident, growth rate estimates were not calculated for that particular year and species. Due to bi-modal spawning of spotted seatrout and sand seatrout during single recruitment windows, separa te calculations for growth were estimated for early and late recruiting fish. Early-recruitment fish were thos e that settled into the estuary at the beginning of the recruitment window while late-recruitment fish were those that were spawned in the middle of the recruitment window and later settled into the estuary. Growth calculations began with the first month in whic h recruiting fish length frequencies were not truncated and a clear shift in c ohort modes could be detected. Fi rst and last months included in growth calculations never varied by more than on e month from year to year. Length-frequency histograms were developed by month for each species to determine at which minimum and maximum SL each species were vulnerable to the gear. To minimize bias associated with gear escapement, only cohort modes that fell within the gear vulnerability range were used for growth estimates. Instantaneous daily growth rate estimat es for year classes of each species were related to mean river discharge using linear regression. Only river discharge that occurred during months from which growth was calculate d was included in the model. Mortality Instantaneous daily mortality rates were estimated for age-0 fish of each species. For increased sample size, length-frequency and ab undance data from both north and south zones were combined to track cohort mode s and using the following equation: Zt te N N0 (2-2)

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19 where Nt = abundance (fish100 m-2) of fish at time t; N0 = initial abundance; and Z = daily instantaneous mortality t = time interval between N0 and Nt Initial abundance of fish (N0) applied to the earliest month at which specific species were considered to be fully recruited to the sampling g ear. To reduce bias associated with smaller fish still recruiting into the gear and larger fish a voiding the gear, I only included fish that were captured within + 5-mm SL around the cohort mode when calculating relative abundance. When cohort modes were absent or not cl ear, mortality estimates were not calculated for that particular year and species. Instantaneous daily mortalit y estimates for each recruitment-class of each species were correlated with mean river discharge using linear re gression. Only river discharge that occurred during months from which mortality was calculated wa s included in the model. Growth and Mortality Validation To further investigate the possi bility of bias associated with gear vulnerability on growth and mortality estimates, I applied growth and mo rtality estimates to observed length-frequencies of pinfish and red drum. Predicted length fre quencies were overlaid on top of observed length frequencies to evaluate the potential for fish to grow into or out of the gear at a different rate than was predicted from equations 2-1 and 2-2. Fast growth and slow growth pinfish and fast growth red drum were used as examples.

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20 CHAPTER 3 RESULTS Age-0 Fish Relative Abundance and Seasonal River Discharge Principal Component Analysis indicated th at water temperature and dissolved oxygen combined accounted for the highest monthly en vironmental variation followed by salinity and river discharge combined. General linear mode ls revealed a significant difference between PCA scores for each month. Subsequent SNK tests performed on monthly PCA scores resulted in the assignment of four seasons. Winter was defi ned as December February; Spring as March May; Summer as June September; and Autumn as October and November. Multiple regression models revealed significant relationships between age-0 fish (summary of age-0 fish defined in Tabl e 1) relative abundance and cha nges in seasonal mean Suwannee River discharge between years (P < 0.10) for all species excep t spot during the period 1997 through 2005 (Table 2). In general, relative a bundance of age-0 spotted seatrout, sand seatrout, and red drum were positively related to seasonal ri ver discharge occurring prior to juvenile fish recruiting into the estuary and during recruitment (Table 2 and Figure 2). Relative abundance of sand seatrout and red drum captured in the nor th zone, directly influenced by the Suwannee River discharge, was positively related to seasonal river discharge. No significant relationship could be found for these species in the south zone Spotted seatrout was the only species that exhibited a positive relationship between river discha rge and age-0 fish relative abundance in the south zone. Red drum in the north zone had the best-fitting multiple regression equation, indicating that age-0 fish relative abundance was positively related to pre-spawn summer and winter river discharge (Table 2) Relative abundance of age-0 pi nfish was negatively related to seasonal river discharge in both north and south zones (Table 2 and Figure 2). Spring river discharge was negatively related to relative a bundance of pinfish captu red in the north zone,

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21 whereas winter discharge was ne gatively related to abundance in the south zone. No significant relationship between relative abund ance and seasonal mean river discharge was found for spot in either the north or south zones. None of the species analyzed displaye d an opposite relationship to river discharge between north and south zones. Variation explained by significant regression models (r2) ranged from 0.37 to 0.88 (Table 2). Growth and Mortality Linear regression models for red drum, pinfis h, spot, and early-recruitment sand seatrout indicated a significant increase in instantaneous da ily growth rates with higher river discharge (P < 0.05; Figure 3). Early-recruitment spotted s eatrout was the only species that demonstrated a decrease in growth with higher river discharge (Figure 3). I detected no relationship between growth and river discharge for late-recruitment spotted seatrout or sand seatrout. Variation explained by significan t regression models (r2) ranged from 0.48 to 0.96 (Figure 3). Relative daily growth for all five species ranged from 0.083 to 0.833 mmd-1 (Table 3). Red drum, pinfish, spot, and early recruiting sand seatrout a ll had similar average growth rates which ranged from 0.359 to 0.387 mmd-1. Late recruitment spotted seatrout (0.181 mmd-1) and early recruitment spotted seatrout (0.233 mmd-1) exhibited slower growth (Table 3). Linear regression models for pinfish, re d drum, and early-recruitment sand seatrout indicated a significant increase in instantaneous daily mortality rates with river discharge rate (P < 0.05; Figure 4). River discharge was not log10 transformed for the early-recruitment sand seatrout model. No significant relationship wa s detected between mortal ity and river discharge for spot and late-recruitment sand seatrout. Earl y-recruitment sand seatrout had the lowest and highest instantaneous daily morta lity across all species, ranging fr om 0.002 to 0.124. Mortality rates were not determined for spotted seatrout due to limited data. Variation explained by significant regression models (r2) ranged from 0.46 to 0.81 (Figure 4).

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22 Growth and Mortality Validation Comparisons of predicted length frequencies to the observed catches suggested no gear selectivity issues for the size of fish that I used in this analysis (Figures 3-5 and 3-6). Fish did not appear to be avoiding the gear or moving out of the sample able areas at larger sizes, suggesting that the growth and mortality rates I es timated were not biased by gear selectivity. In general predicted length freque ncy modes closely resembled th e observed modes (Figures 3-5 and 3-6).

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23Table 3-1. Summary of species, period fish were recruiting into the estuary and vulnerable to th e gear (Recruitment window), a nd SL (standard length) cut-off for fish used in calculation of relative abundance of fish considered to be within age-0 classification. Age-0 Fish Relative Abundance SpeciesRecruitment WindowAge-0 length (SL)Reference spotted seatroutJune October < 100 mm McMichael and Peters 1989 sand seatroutMay October < 100 mm Nemeth et. al. 2006 red drumSeptember January < 100 mm Peters and McMichael 1987 pinfishJanuary June < 80 mm Hanson 1969 spotJanuary April < 75 mm Livingston 1984

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24Table 3-2. Significant multiple regressi on equations predicting age-0 fish relative abundance within zones (1997 2005) from seasonal mean Suwannee River discharge rates between years. Model R2 Pvalue Spotted seatrout fish 100m2 (south zone) = -0.4999 + 0.1644 x Log spring 0.37 0.08 Sand seatrout fish 100m2 (north zone) = -1.4082 + 0.5683 x Log summer 0.77 < 0.01 Red drum fish 100m2 (north zone) = -3.8543 + 0.5109 x L og winter + 0.6135 x Log pre-spawn summer 0.88 0.02 Pinfish fish 100m2 (north zone) = 5.4088 1.0951 x Log spring 0.61 0.01 Pinfish fish 100m2 (south zone) = 4.3659 0.9065 x Log winter 0.56 0.02

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25Table 3-3. Summary table of species and their mean re lative growth rates reported as millimeters per day (mmd-1), standard error (SE), and minimum and maximum relative growth rates for fish captured from 1997-2005. Months from which growth rates were calculated from are listed. Maximum length was the maximum size the cohort mode reached during growth calculations within any year. All fish captured within the maxi mum length were considered to be within selectivity of the gear. Fish captured in north and south zones were combined for growth calculations Spotted seatrout and sand seatrout were separated into early and late recruitment due to bi-modal spawning. MeanMinMax SpeciesMonthsMax Length (SL) Growth mmd-1SE Growth mmd-1Growth mmd-1spotted seatroutearly recruitmentJune July < 35 mm 0.2330.07170.0830.500 late recruitmentAugust September < 35 mm 0.1810.0330.0830.333sand seatroutearly recruitmentMay July < 55 mm 0.3590.0780.1670.833 late recruitmentAugust October < 40 mm 0.2550.0700.0830.750red drumOctober January < 55 mm 0.3680.0790.1250.667pinfishMarch July < 60 mm 0.3870.0170.3210.444spotFebruary April < 60 mm 0.3680.0790.1250.667

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26 Sand SeatroutNorth zone R 2 = 0.77Log Summer River Discharge 1.61.82.02.22.42.62.83. 0 0.0 0.4 0.8 1.2 1.6 2.0 PinfishNorth zone R 2 = 0.61Log Spring River Discharge 1.61.82.02.22.42.62.83.0 0.0 0.4 0.8 1.2 1.6 2.0 Spotted SeatroutSouth zone R 2 = 0.37Log Spring River Discharge 1.61.82.02.22.42.62.83.0Log Age-0 Fish Relative Abundance 0.0 0.4 0.8 1.2 1.6 2.0 PinfishSouth zone R 2 = 0.56Log Winter River Discharge 1.61.82.02.22.42.62.83.0 0.0 0.4 0.8 1.2 1.6 2.0 Figure 3-2. Relationship between log10-transformed age-0 fish relative abundan ce (fish100 m2) and seasonal mean Suwannee River disc harge between years. River discharge is in log10 transformed cubic mete rs per second (m3 s-1).

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27 Spotted SeatroutEarly recruitment R 2 = 0.82 P < 0.05 1.61.82.02.22.42.62.83.0Instantaneous Daily Growth (G) 0.000 0.006 0.012 0.018 0.024 Sand SeatroutEarly recruitment R 2 = 0.56 P < 0.05 1.61.82.02.22.42.62.83.0 0.000 0.006 0.012 0.018 0.024 Red DrumR 2 = 0.70 P < 0.05 1.61.82.02.22.42.62.83.0 0.000 0.006 0.012 0.018 0.024 PinfishR 2 = 0.96 P < 0.0001Log10 River Discharge (m3s-1) 1.61.82.02.22.42.62.83.0 0.000 0.006 0.012 0.018 0.024 SpotR 2 = 0.48 P < 0.05 1.61.82.02.22.42.62.83.0 0.000 0.006 0.012 0.018 0.024 Figure 3-3. Relationship between yearly instantaneous daily gr owth estimates (G) and mean Suwannee River discharge during months in which growth was calculated by year.

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28 0100200300400500600 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Red DrumR 2 = 0.66 P = 0.05River Discharge (m3 s-1) 1.61.82.02.22.42.62.83.0Instantaneous Daily Mortality (Z) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 PinfishR 2 = 0.46 P < 0.05 1.61.82.02.22.42.62.83.0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Sand SeatroutEarly Recruitment R 2 = 0.81 P < 0.05 Figure3-4. Relationship between yearly instanta neous mortality rate estimates (Z) and mean Suwannee River discharge during months in which mortality was calculated by year. River discharge is in log10-transformed cubic meters per second (m3 s-1) for pinfish and red drum only.

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29 Pinfish Fast Growth March 2001 N = 1,213 0 100 200 300 400 500 April N = 1,050 0 100 200 300 May N = 324 Number Fish Captured 0 20 40 60 80 100 June N = 391 0 20 40 60 80 100 July N = 25Standard Len g th ( mm ) 020406080100 0 2 4 6 8 10 Pinfish Slow Growth March 2002 N = 1,268 0 100 200 300 400 500 April N = 887 0 50 100 150 200 250 May N = 403 0 50 100 150 June N = 305 0 20 40 60 80 100 July N = 229 020406080100 0 20 40 60 (A) (B) Figure 3-5. Observed (bars) le ngth-frequency of fast (A) and sl ow (B) growing pinfish with the predicted length-frequency (solid line) overlai d, after calculated growth and mortality were applied to observed catches. Dark bars represent + 5-mm increments around the cohort mode, used in calculating mortality estimates.

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30 Red Drum Fast Growth October 2005 N = 24 0 2 4 6 8 10 November N = 195 Number Fish Captured 0 10 20 30 40 50 60 December N = 69Standard Length (mm) 020406080100 0 2 4 6 8 10 12 14 16 Figure 3-6. Observed (bars) length-frequency of fast growin g red drum with the predicted length-frequency (solid line) overlaid, afte r calculated growth and mortality were applied to observed catches. Dark bars represent + 5-mm increments around the cohort mode, used in calculating mortality estimates.

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31 CHAPTER 4 DISCUSSION Previous work has evaluated the effects of river discharge on fish larvae retention, growth, and survival (Grimes and Kingsford 1996; Grimes and Finucane 1991; Govoni et al. 1989), but few have evaluated the effects that ch anges in river plumes may have on fish once they reach a juvenile st age within an estuary and compare th ese effects to hypotheses associated with larvae fish. The short-food hypothesis stated higher recruitm ent in the vicinity of river plumes due to enhanced feeding conditions, promo ting fast growth and lower mortality of fish larvae (Govoni et al. 1989; Finucane et al 1990; Grimes and Finucane 1991). The size and shape of river plumes and their associated fron ts is variable, and depends primarily upon the nature of ocean currents, topography, dimensions and shape of the estuary and adjacent shelf area, and the rate of freshwater discharge. Rivers with high discharg e rates typically produce large plumes (Gilllanders and Kingsford 2002). I found that relative abundance and growth of age-0 sand seatrout and red drum increased with river discharge across years. However, these species also suffered higher mortality rates duri ng increased river discharge which lends some credence to the total-larval-production hypothesis which states that superior trophic conditions found during high river discharge support such high to tal production of fish larvae that specific dynamics of growth and mortality are not relevant (Grimes and Finucane 1991). Although both the short-food and total-larval-p roduction hypotheses apply to larval fish, these concepts may also be applicable to juvenile fish recruiting into an estuary. The physical mechanisms associated with intensity of river pl umes may affect juvenile fish relative abundance and the relationship between growth and mortality. Changes in mortality may have a much larger effect on survival (ultimately recruitment success) than incremental changes in growth (Grimes 2001). Effects of these mechanisms on juveni le fishes may vary depending on species.

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32 Therefore, analysis for juvenile fish should be conducted on an indi vidual species basis. In my study, pinfish suffered lower relative abundance a nd higher mortality rates with increased river discharge, despite higher growth with high discha rge. Whereas previously mentioned, red drum and sand seatrout exhibited higher relative abundance, highe r growth and higher mortality with increased river discharge. The third hypothesis stated that it is the physical retention of river plumes rather than production th at explains the variation in re cruitment (Sinclair 1988; Grimes and Kingsford 1996). Loneragan and Bunn (1999) suggested that change s in turbidity and salinity as a result of changing ri ver discharge may restrict distribu tion of fish or stimulate their movements into areas more likely to be caught thus l eading to higher densiti es rather than an actual increase in fish production. I was not able to evaluate th is hypothesis due to the inability to isolate the relative contributi on of nutrients and intensity of the retention mechanism because both vary with river discharge and mixing with ocean currents (Grimes and Kingsford 1996). My study found relative abundance of age-0 spot ted seatrout from the south zone, and sand seatrout and red drum from the north zone to be positively related to increases in seasonal river discharge. None of the five species a bundances displayed an opposite relationship to river discharge between north and sout h zones which would have indicat ed a river-induced change in spawning location or settling loca tion of new recruits transporte d from offshore waters. Both spotted seatrout and sand seatrout are nears hore or estuarine spawners and both spawn from spring through summer months (W alters 2005). The positive rela tionship found between relative abundance of spotted seatrout and sand seat rout to spring and summer river discharge respectively, may be attributed to the higher input of nutrients into the estuary leading to enhanced primary and secondary production duri ng summer months as opposed to other seasons (Whitfield 1994; Loneragan and Bunn 1999). Spo tted seatrout was the only species which

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33 displayed an increase in relative abundance in th e south zone during increased river discharge. This may have been a result of spotted seatrout avoiding lower salinity ar eas in order to reduce osmoregulatory stress, commonly found in marine species (Wh itfield and Harrison 2003). Red drum and spot are two of three estuarin e-dependent species I analyzed that spawn offshore (Rooker et al. 1998; Weinstein 1983) and depend upon ocean currents to transport larvae back into the estuary. Red drum be gin spawning during late summer when Suwannee River discharge rates are normally relatively low. Many species of marine fishes use changes in freshwater discharge as physical and chemical stimuli to initiate migration offshore for spawning and for passive transport of larvae towards estuar ies (Champalbert et al. 1989; Champalbert and Koutsikopoulos 1995). As a result high recruitment levels shoul d be expected for high river discharge occurring during optimal periods (Costa et al. 2007). North and Houde (2003) found similar results in the Chesapeake Bay estuary where juvenile white perch and striped bass abundances were positively correlated to Susque hanna River discharge du ring spring. Juvenile Gilchristella aestuaria abundances were positively correlated with river discharge in the Kariega Estuary along the coast of South Africa (Strydom et. al. 2002). This may explain the strong positive association I found between relative abunda nce of age-0 red drum and river discharge during pre-spawn summer seasons and winter seasons. No si gnificant relationship was found between discharge and relative abundance of ag e-0 spot. Spot spawn during winter and spring months when Suwannee River discharge is genera lly at its highest. Although increased river discharge can be important for nutrient input, extreme high freshwater discharge may actually flush out nutrients and even crea te a physical barrier to recrui tment of fish (Gillanders and Kingsford 2002; Costa et al. 2007).

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34 In contrast, other studies of juvenile fishes have shown ne gative relationships between abundance and river discharge. Juvenile gulf menhaden Brevoortia patronus abundance has been found to be negatively associated with Mississippi River discha rge (Govoni 1997), and age0 Japanese seaperch Lateolabrax japonicus abundance exhibited an inverse correlation to the Chikugo River discharge (Shoji and Tanaka 2006). In this study, pinfish was the only species in which age-0 fish abundance was negatively related to river discharge. Pinfish are the third of three species I analyzed that spawn offshore (Hansen 1969) and depend upon ocean currents to transport larvae back into the estuary. Pinfis h spawn during late winter and early spring when the Suwannee River discharge rates generally ar e highest. High discharge, may restrict the shoreward transport of pinfish larvae. Food supply is perhaps the most important biol ogical factor influenced by changes in river discharge (Grimes and Finucane 1991). My results showed sand seatrout, red drum, pinfish, and spot experienced higher growth during years with elevated river discharge, supporting the argument of enhanced feeding oppor tunities during increase d river discharge. Spotted seatrout was the only species that displa yed slower growth during years with increased river discharge. Deegan (1990) found a similar relationshi p with growth of juvenile gulf menhaden declining with increased discharge from the Mississippi Rive r. Spotted seatrout in my study may have suffered from prolonged freshwater conditions (lower sali nity) during increased river discharge which can lead to osmoregulatory stress in some marine species (Whitfield and Harrison 2003), thus leading to slower growth. My results, however, do not support the contenti on that higher growth rates of juvenile fish leads to better surv ival as is often found with larvae fish (North and Houde 2001; Grimes and Kingsford 1996). In my study, red drum, sand seatrout, and pinfish mo rtality increased with

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35 river discharge, despite these species exhi biting faster growth under the same physical conditions. There is some evidence in the liter ature supporting a relationship of high mortality rates with high growth rates in juvenile estu arine-dependent species. Walters and Martells (2004) foraging arena theory suggest s juvenile fish respond to changes in food concentrations to maintain constant growth. This theory assume s most predation occurs while fish are foraging and this attempt to maintain constant growth lik ely results in linear in crease in mortality with increasing juvenile density. Prey such as juvenile finfish can hide from predators unless the prey density is so high that it forces that prey to spend more time foraging ou tside of their preferred habitat, resulting in higher mortality. M unch and Conover (2003) also found fast growing Atlantic silversides Menidia menidia suffered higher mortality from predation despite being 40% larger than the slow-growing population (Munch and Conover 2003) Consumption of larger meals by fast growth fish may be detrimenta l to swimming ability and predator evasion (Billerbeck et al. 2001). Furthe rmore, the same physical and biological dynamics that lead to accumulated larval fish and prey may also accumu late their predators (Fernndez-Delgado et al. 2007; Govoni et al. 1989) although the number of pr ey eaten may not depend on the number of predators (Walters and Martell 2004; Munch and Conover 2003). Cannibalism associated with high abundance is another major source of mortality common in early life stages that should be considered (Grimes and Finucane 1991). Thus, my results indicate that fast er growth associated with high river discharge might result in higher mortality fo r juvenile fishes. Benefits of increased production associated with river plumes (Gillanders and Kingsford 2002; Grimes and Finucane 1991) may or may not out weigh the disadvantage of increased mortality. For example, Grimes and Kingsfo rd (1996) showed higher instantaneous natural mortality rates in the vicinity of the Mississippi river plume than away from the plume for larval

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36 Spanish mackerel Scomberomorus maculatus and king mackerel Scomberomorus cavalla, despite enhanced growth. Similar results for striped anchovy Anchoa hepsetus suggest that natural mortality in the plume front and within the river plume was higher than that experienced in shelf waters (Day 1993). Connel (1998) reporte d a positive relationship between juvenile reef fish abundance and mortality rates due to predation. If conditions in the vi cinity of river plumes support higher growth or fish abundances, but al so aggregate predators and increase mortality rate, enhanced recruitment into the fishery may no t occur. In my study, red drum, sand seatrout, and pinfish all suffered from higher mortality de spite either rapid grow th or higher relative abundance during high-discharge years. Differences in mortality rates based on ch anges in relative abundance with fish size should be interpreted with caution because the met hod assumes that fish have equal vulnerability to capture by the sampling gear. Faster growth rates may lead to biased mortality estimates, because large fish become less vulnerable to capture and would be under-represented, thus leading to inflated mortality rates. However, any inherent biases in the mortality estimates would be present during all years. Moreover, the data used in calculating instantaneous mortality rates was within the bounds of gear selectivity of each species analyzed. Fish captured during initial recruitment months in which they appeared to be partially vulnerable the gear, as well as later months when larger fish would be expected to either leave the area or avoid the gear were not included in mortality estimates. By only including fish captured + 5-mm around the cohort mode when calculating cohort abund ance, I further reduced the chance of gear selectivity biasing mortality estimates. This approach appeared to be valid based on the observed and predicted changes in the length freque ncy distributions through time.

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37 Results of this study could assist managers in the development and implementation of strategies to compensate for poor year-class production as a result of changes in river discharge. In areas where water control devices (i.e., dams and levies) currently exist along major tributaries, mangers could work with water cont rolling authorities to manage water levels for better fish production. This study has broad im plications for assessi ng how river discharge influences recruitment of ecol ogically and recreationally importa nt estuarine-dependent species in an estuary.

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38 LIST OF REFERENCES Baird, D., and J. J. Heymans. 1996. Assessment of ecosystem changes in response to freshwater inflow of the Kromme River Estuary, St Fr ancis Bay, South Africa: a network analysis approach. Water SA 22(4):307-318. Barrera-Guevara, J. C. 1990. The conserva tion of Totoaba macdonaldi (Gilbert), (Pisces: Sciaenidae), in the Gulf of California, Me xico. Journal of Fish Biology 37:201-202. Bebars, M. I., and G. Lasserre. 1983. Analysis of the Egyptian marine and lagoon fisheries from 1962-1976, in relation to the construction of the Aswan Dam (completed in 1969). Oceanologica Acta 6:417-426. Billerbeck, J. M., T. E. Lankford, and D. O. Cono ver. 2001. Evolution of intrinsic growth and energy acquisition rates. I. Tradeoffs with swimming performance in Menidia menidia. Evolution 55:1863-1872. Binett, D., L. L. Reste, and P. S. Diouf. 1995. The influence of runoff and fluvial outflow on the ecosystems and living resources of West African coastal waters Effects of riverine inputs on coastal ecosystems and fisheries resource s. FAO Fisheries Technical Papers. 349:89118. Bishara, N. F. 1984. The problem of prawn fisheries in Egypt. Archiv f r Hydrobiologie. 101:577-586. Browder, J. A. 1991. Watershed management and the importance of freshwater flow to estuaries, p. 7-22. In S. F. Treat and P. A. Clark (eds.). Proceedings, Tampa Bay Area Scientific Information Symposium 2, 1991 Fe bruary 27-March 1, Tampa, Florida. Caddy, J. F., and A. Bakun. 1995. Marine catchment basins and anthropogenic effects on coastal fishery ecosystems, Effects of riveri ne inputs on coastal ecosystems and fisheries resources. FAO Fisheries Technical Papers 349:119-133. Champalbert, G., A. Bourdillon, C. Castellon, D. Chikhi, L. Le Direach -Boyrsier, C. MacquartMoulin and G. Patriti. 1989. Determinisme de s migrations des larves Et uveniles de soles. Ocanis 15:171-180. Champalbert, G. and C. Koutsikopoulos. 1995. Be havior transport and r ecruitment of Bay of Biscay sole (Solea solea): laboratory and field studies. Journal of the Marine Biological Association of the United Kingdom 75:93-108. Connell, S. D. 1998. Effects of predator s on growth, mortality and abundance of a Juvenile reef-fish: evidence fr om manipulations of predator and prey abundance. Marine Ecology Progress Series 169:251-261.

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39 Costa, M. J., R. Vasconcelos, J. L. Costa, and H. N. Cabral. 2007. River flow influence on the fish community of the Tagus estuary (Portugal). Hydorbi ologia 587:113-123. Crivelli, A. J., M. C. Ximenes, B. Gout, G. Lasse rre, F. Freon, and T. Do Chi. 1995. Causes and effects of terrestrial runoff and riverine outflow on brackish/coastal marine fisheris ecosystems in the northern Mediterranean region. Effects of riverine inputs on coastal ecosystems and fisheries resources. F AO Fisheries Technical Papers 349:59-88. Day, G. R. 1993. Distribution, abundance, growth and mortality of striped anchovy, Anchoa hepsetus, about the discharge plume of the Mi ssissippi River. M.S. Thesis, Univ. West Florida, Pensacola, Florida. Deegan, L. A. 1990. Effects of estuarine e nvironmental conditions on population Dynamics of young-of-the-year gulf menhaden. Ma r. Ecol. Prog. Ser. 68:195-205. Deegan, L. A., and J. H. Day, J. G. Gosselink, A. Yanez-Arancibia, G. S. Chavez, and P. Sanchez. 1986. Relationships among physical characteristics, vegeta tion distribution, and fisheries yield in Gulf of Mexico estuaries. In D. A. Wolfe (ED.), Estuarine variability (pp. 83-100). New York: Academic Press. Drinkwater, K. F. and K. T. Fra nk. 1994. Effects of river regulati on and diversion on marine fish and invertebrates. Aquatic Conservation. Freshwater and Marine Ecosystem 4: 135-151. Fernndez-Delgado, C., F. Bald, C. Vilas, D. Garca-Gonzalez, J. A. Cuesta, E. GonzlezOrtegn, and P. Drake. 2007. Effects of th e river discharge Mana gement on the nursery function of the Guadalquivir river estuary (SW Spain) Hydrobiologia 587:125-136. Finucane, J. H., C. B. Grimes, and S. P. Naughton. 1990. Diets of young king and Spanish mackerel off the southeast United States. Northeast Gulf Science 11:145-153. Garcia, A. M., J. P. Vieira, and K. O. Wine miller. 2003. Effects of 1997-1998 El Nio on the dynamics of the shallow-water fish assembla ge of the Patos Lagoon Estuary (Brazil). Estuarine, Coastal and Shelf Science 57:489-500. Gillanders, B. M., and Kingsford, M. J. 2002. Im pact of changes in flow of freshwater on estuarine and open coastal habitats and th e associated organisms. Oceanography and Marine Biology: An Annual Review 40: 233-309. Govoni, J. J. 1997. The association of th e population recruitment of Gulf menhaden, Brevoortia patronus, with Mississippi River discharge. J. Mar. Syst. 12:101-108. Govoni, J. J., D. E. Hoss, and D. R. Colby. 1989. The spatial distribution of larval fishes about the Mississippi River plume. Limnology and Oceanography 34:178-187. Grange, N., A. K. Whitfield, C. J. De Villiers, and B. R. Allanson. 2000. The response of two South African east coast estuarie s to altered river flow regi mes. Aquatic Conservation. Marine and Freshwater Ecosystems 10:155-157.

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40 Grimes, C. B. 2001. Fishery production and the Mississippi River discharge. Fisheries 26:1726. Grimes, C. B., and J. H. Finucane. 1991. Sp atial distribution and abundance of larval and juvenile fish, chlorophyll and macrozoopl ankton around the Mississippi River discharge plume, and the role of the plume in fish recruitment. Marine Ecology Progress Series 75:109-119. Grimes, C. B., and M. J. Kingsford. 1996. How do riverine plumes of di fferent sizes influence fish larvae: do they enhance recruitmen t? Mar. Freshwater Res. 47:191-208. Hansen, D. J. 1969. Food, growth, migra tion, reproduction, and abundance of pinfish, Lagodon rhomboides, and Atlantic croaker, Micropogon undulatus, near Pensacola, Florida. Fishery Bulletin 68:135-146. Livingston, R.J. 1984. Trophic response of fish es to habitat variabil ity in coastal seagrass systems. Ecology 65:1258-1275. Livingston, R. J. 1991. Histori cal relationships between resear ch and resource management in the Apalachicola River Estuary. Ecological Applications 1:361-382. Livingston, R. J. 1997. Trophic response of estuar ine fishes to long-term changes of river flow. Bulletin of Marine Science 60:984-1004. Loneragan, N. R. and S. E. Bunn. 1999. Rive r flows and estuarine ecosystems: implications for coastal fisheries from a review and a case study of the Logan river, Southeast Queensland. Australian Journal of Ecology 24: 431-440. Martins, I., M. A. Pardal, A. I. Flindt, and J. C. Marques. 2001. Hydrodynamics as a major factor controlling the occurrence of green macroalgal blooms in a eutrophic estuary: a case study on the influence of precip itation and river management. Estuarine, Coastal and Shelf Science 52:165-177. Mattson, R. A., and M. E. Rowan. 1989. Th e Suwannee River estuary: An overview of research and management needs. Specia l Publication 89-4:1731. American Water Resources Association, Bethesda, Maryland. McMichael, R. H. Jr. and K. M. Peters. 1989. Early life history of the spotted seatrout, Cynoscion nebulosus, Pisces: Sciaenidae, in Tampa Ba y, Florida. Estuaries 122:98-110. Meyers, R. H., 1990. Classical and modern regression with applications. PWS-Kent, Boston, Massachusetts, USA. Molles, M. C. and C. N. Dahm. 1990. A perspective on El Nio and La Nia; global implications for stream ecology. Journal of the North American Benthological Scociety 9:68-76. Munch, S. B., and D. O. Conover. 2003. Rapid growth results in increa sed susceptibility to predation in Menidia menidia. Evolution 57:2119-2127.

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41 Nemeth, D.J., J.B. Jackson, A.R. Knapp, and C.P. Purtlebaugh. 2006. Age and growth of sand seatrout (Cynoscion arenarius) in the estuarine waters of the eastern Gulf of Mexico. Gulf of Mexico Science 1:45-60. North. E, W., and E. D. Houde. 2001. Reten tion of white perch and striped bass larvae: Biological physical interactions in Chesapeake Bay estuarine turbidity maximum. Estuaries 24:756-769. North, E. W., and E. D. Houde. 2003. Linki ng ETM physics, zooplankton prey and fish earlylife histories to striped bass Morone saxatilis and white perch M. Americana recruitment. Marine Ecology Progress Series 260:219-236. Peterman, R. M. 1990. Statistical power an alysis can improve fisheries research and management. Canadian Journal of Fisheries research and management. Canadian journal of Fisheries and Aquatic Sciences 47:2-15. Peters, K. M., and R. H. MicMichael. 1987. Early life history of the red drum, Sciaenops ocellatus (Pisces: Sciaenidae), in Tampa Bay, Florida. Estuaries 10:92-107. Rooker, J. R., S. Holt, J. Holt, and L. Fuiman. 1998. Spatial and temporal variability in growth, mortality, and recruitment poten tial of postsettlement red drum, Sciaenops ocellatus in a subtropical estuary. Fish. Bull. 97:581-590. SAS Institute Inc. 1989. SAS/STAT users guide, Version 6, Fourth Edition, Volume 2, Cary, NC. Sinclair, M. 1988. Marine populations: An essay on population regulation and speciation. (University of Washi ngton Press: Seattle). Shoji, J., and M. Tanaka. 2006. Influences of spring river flow on the recruitment of Japanese seaperch lateolabrax japonicus into the Chikugo estuary, Japan. Scientia Marina 70S2:159-164. Staunton-Smith, J., J. B. Robins, D. G. Mayer, M. J. Sellin, and I. A. Halliday. 2004. Does the quantity and timing of fresh water flowing into a dry tropical estuar y affect year-class strength of barramundi (Lates calcarifer)? Marine and Freshw ater Research 55:787-797. Strydom, N. A., and A. K. Whitfield. 2000. The effects of a single freshw ater release into the Kromme Estuary. Water SA 26. 3:319-328. Strydom, N. A., A. K. Whitfield, and A. W. Pate rson. 2002. Influence of altered freshwater flow regimes on abundance of larval and juvenile Gilchristella aestuaria (Pisces:Clupeidae) in the upper re aches of two South African es tuaries. Mar. Freshwater Res. 53:431-438. Tsou, T., and R. E. Matheson Jr. 2002. S eason changes in the nekton community of the Suwannee River estuary and the potential impact s of fresh water withdrawal. Estuaries 25:1372-1381.

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42 USGS (U.S. Geological Survey). 2006. Map of real-time stream flow compared to historical stream flow for the day of the year, Florida. URL: http://water.usgs.gov/cgibin/waterwatch?ma p_type=real&state=fl (July 2006). Walters, C. J., and J. D. Martell. 2004. Fisheries ecology and management. Princeton University Press, Princeton, New Jersey. Walters, S. L. 2005. Mapping Tampa Bay Cynoscion nebulosus spawning habitat using Passive acoustic surveys. M.S. Thesis. University of South Florida. 65pp. Weinstein, M. P. 1983. Population dynami cs of an estuarine dependent fish, Leiostomus xanthurus, along a tidal creek-seagrass meadow coenocline. Can. J. Fish. Aquat. Sci. 40:1633-1638. Whitfield, A. K. 1994. A review of the icthyofa unal biodiversity in Sout hern African estuarine systems. Annales des Sciences Zooloiques Muse Royal del Afri que Centrale 275: 149163. Whitfield, A. K. and T. D. Harrison. 2003. River flow and fish abundance in a South Africa estuary. J. Fish Biol. 62:1467-1472. Wilber, D. H. 1994. The influence of Apalachicola River flows on blue crab, Callinectes sapidus, in north Florida. Fi shery Bulletin 92:180-188. Winemiller, K. O. and M. A. Leslie. 1992. Fish assemblage across a complex, tropical freshwater/marine ecotone. Enviro nmental Biology of Fishes 34:29-50. Winner, B. L., and R.H. McMichael, Jr. 1997. Evaluation of a new type of box splitter designed for subsampling estuarine ichthyofauna. Trans. Am. Fish. Soc. 126: 1041-1047. Wooldridge, T. H., and C. Bailey. 1982. Eu ryhaline zooplankton of the Sundays Estuary and notes on trophic relations. South African Journal of Zoology. 17:151-163. Zar, J. H., 1996. Biostatistical analysis. Pr entice-Hall, Inc., Upper Sa ddle River, New Jersey.

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43 BIOGRAPHICAL SKETCH Caleb Hunter Purtlebaugh was born February 20, 1975 in Winchester, Virginia, the son of Michael and Sun Purtlebaugh. He graduated fr om James Wood High School, Virginia in 1993. He received his Bachelor of Science degree in Wildlife and Fisheries Resources from West Virginia University. He became interested in the outdoors while hunting and fishing with his father. He would often canoe and camp along th e Shenandoah River located in the western part of Virginia. After graduating from college with his Bachelor of Scie nce degree, he briefly attended graduate school at Fros tburg State University, however eventually moved and started a job working for the Delaware Division of Fish a nd Wildlife for a couple of years. It was there that he became interested in marine science. During his time in Delaware, he continued graduate school at Delaware State Universi ty while also working for the Delaware Division of Fish and Wildlife. It soon became apparent to him that he wanted to broaden his experience in marine science. In 2000, he accepted a job as a marine fi sheries research scientist with the Florida Fish and Wildlife Research Institute, located in Ce dar Key, Florida. After gaining extensive experience in the marine science field and after the birth of his son Hunter, he decided to finally finish what he had started. He contac ted the University of Florida and Dr. Mike Allen took him on as a graduate student in the fall 2005. After his enrollment into graduate school he continued to work full-time. After his graduati on in December 2007, he plans to continue his successful career in marine fisheries research.