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RELATIONSHIPS BETWEEN ZOOPLANKTON ABUNDANCE AND AGE-0
BLACK CRAPPIE ABUNDANCE AND SIZE AT THREE PRODUCTIVE FLORIDA
KEVIN J. DOCKENDORF
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
Kevin J. Dockendorf
To my parents, Tom and Helen.
I would like to thank my advisor, Dr. Mike Allen, for his assistance and mentoring on
this project. I am grateful for the opportunity to complete a research project with his
guidance and motivation.
I would also like to thank the members of my graduate committee, Dr. Tom Frazer,
Dr. Debra Murie, and Mr. Gary Warren, for their input and advice on this project from
the proposal to the thesis defense.
Thanks go to M. Bledsoe, T. Bonvechio, P. Cooney, T. Glancy, J. Greenawalt, K.
Henry, A. Hester, C. Horsburgh, M. Hale, M. Hoyer, B. Hujik, G. Kaufman, J. Kline, C.
Mwatela, E. Nagid, D. Parkyn, B. Pine, M. Rogers, B. Tate, K. Tugend, and P. Wheeler
for their assistance on this project. I greatly appreciated their time and efforts in the field,
lab, and/or presentation practices.
Funding for this project was provided by Florida Fish and Wildlife Conservation
Commission and Pinellas County Department of Environmental Management.
I am grateful for the love and support from my family and fiancee, Pamela. Their
patience and reassurance helped generate the confidence and motivation to complete this
TABLE OF CONTENTS
A C K N O W L E D G M E N T S ................................................................................................. iv
L IST O F T A B L E S ........ ................................................................................. vi
L IST O F FIG U R E S .... ....... ................................................ .... ..... .. ............. vii
A B S T R A C T ......... .................................. ................................................... v iii
INTRODUCTION .............. ................................... ..............
M E T H O D S .......................................................................... . 6
Study Sites ........................ .............................. .................. 6
A ge-0 B lack Crappie C ollection................................................ ........................... 6
L ab W ork-up ..................................................................................... . 8
A dult A abundance .................................................................................................. ....... 8
Zooplankton Density.......................................... 9
D iet A n aly sis................................................................. 9
A n a ly se s ..................................................................... 1 0
R E S U L T S ................................................................................12
D IS C U S S IO N ................................................................................................... 3 1
M ANAGEM ENT IM PLICATIONS ....................................................... 38
L IST O F R E FE R E N C E S .............................................................................. 40
B IO G R A PH IC A L SK E T C H ........................................................................................ 46
LIST OF TABLES
1 Annual mean levels of chlorophyll a (Chl a), total phosphorus (TP), total nitrogen
(TN), and secchi depth (cm) of Lakes Lochloosa, Tarpon, and Wauburg for 2000
and 2001 ..................................... ......................... .. ... .... ......... 29
2 Pearson correlations between fall juvenile black crappie abundance (October age-
0 CPM) and both larval density (fish/1000 m3) and juvenile abundance (May
CPM ) in the three study lakes (N). ............................................. ............... 30
LIST OF FIGURES
1 Location of study lakes in Florida. ............................................. ............... 19
2 Mean ( 1 SE) larval densities (a and b panels) as estimated from neuston net
catches and small trawl catch per minute (CPM) (c and d panels) across sample
months in 2000 and 2001 at Lakes Lochloosa, Tarpon, and Wauburg..................20
3 Large trawl catch per minute (mean 1 SE) of juvenile black crappie during fall
of 2000 and 2001. Sample size (n = number of trawls) is indicated...................21
4 Relative length frequency distributions for age-0 black crappie in Lakes Wauburg,
Tarpon, and Lochloosa in 2000 and 2001............................. ................22
5 Regression between logo adult black crappie catch-per-minute (CPM) prior to
spawning and loglojuvenile black crappie CPM during May 2000 and 2001 in
Lakes Lochloosa, Tarpon, and W auburg. ................................... ............... 23
6 Logio crustacean zooplankton copepodss and cladocerans) densities ( 1 SE ) in
Lakes Lochloosa, Tarpon, and Wauburg for each sample month in 2000 and 2001.
Mean densities with no error bars indicate low variance in those months. ...........24
7 Loglo crustacean zooplankton (> 0.6 mm) mean densities ( 1 SE) in Lakes
Lochloosa, Tarpon, and Wauburg during sample months in 2000 and 2001.......25
8 Age-0 black crappie selectivity (mean 1 SE; Chesson 1978) for zooplankton
taxa in 2000 on Lakes Lochloosa and Wauburg. The solid line represents neutral
selection n ........................................................ ................ 2 6
9 Loglo number of large crustacean zooplankton (LC) in diets of age-0 black
crappie in May 2000 (top panel) and 2001 (bottom panel). Points indicate
individual fish. Lines represent regression equations and the fish size range for
e a c h la k e ...................................................................... 2 7
10 Proportion of age-0 black crappie diets containing crustacean zooplankton,
macroinvertebrates, and fish during May, August, and October 2000 and 2001.
No age-0 black crappie were sampled during August 2000 from Lake Tarpon....28
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
RELATIONSHIPS BETWEEN ZOOPLANKTON ABUNDANCE AND AGE-0
BLACK CRAPPIE ABUNDANCE AND SIZE AT THREE PRODUCTIVE FLORIDA
Kevin J. Dockendorf
Chair: Dr. Micheal S. Allen
Major Department: Fisheries and Aquatic Sciences
Crappies Pomoxis spp. are valuable sportfish but are often difficult to manage due
to highly variable recruitment. I investigated relations between zooplankton abundance
and larval and juvenile black crappie Pomoxis nigromaculatus abundance and size at
three Florida lakes. Age-0 black crappie were collected at Lakes Wauburg, Lochloosa,
and Tarpon using surface and bottom trawls during spring, summer, and fall 2000 and
2001. Zooplankton abundance was measured concurrent with trawl sampling. In both
fall seasons, juvenile crappie were larger at Lake Tarpon than at Lakes Lochloosa and
Wauburg. However, age-0 black crappie abundance and summer crustacean zooplankton
densities were higher at Lakes Lochloosa and Wauburg than at Lake Tarpon in both
years. The lack of a relationship between crustacean zooplankton density and black
crappie size across lakes suggests that age-0 black crappie abundance and size may not be
related to zooplankton abundance in natural productive Florida lakes. In this study, fall
abundance of age-0 black crappie was highly correlated with early summer abundance,
suggesting that year-class strength was set by early summer in the study lakes. Early
summer juvenile abundance predicted age-0 black crappie fall abundance more
effectively than larval fish density in the spring. Adult abundance and abiotic variables
(e.g., water clarity) influenced age-0 black crappie abundance and size more than
crustacean zooplankton abundance.
Recruitment can be defined as an addition of young individuals to an adult fish
population (Everhart and Youngs 1981). Abundance of young fish is subject to
environmental factors such as habitat variation and food availability, along with biotic
interactions including predation and competition that may affect recruitment (Sigler and
Sigler 1990). Due to highly variable recruitment and subsequent fishing quality, fisheries
managers have found it difficult to manage crappies, Pomoxis spp. (Allen and Miranda
1995). Studies that reveal factors influencing recruitment may allow fishery managers to
predict year-class strength and improve fisheries via harvest restrictions, particularly with
important sport fish such as black crappie P. nigromaculatus and white crappie P.
annularis (Maceina et al. 1991).
Previous investigations have evaluated variability of crappie recruitment due to
abiotic factors such as water-level fluctuations, turbidity, wind, and temperature in lakes
and reservoirs (Beam 1983; Guy and Willis 1995; Mitzner 1995). Maceina and Stimpert
(1998) found that short winter (<10 days) and long post winter (> 11 days) retention
times in Alabama reservoirs interacted to produce strong crappie year classes. High
water level increased crappie year-class strength in a Kansas reservoir (Beam 1983).
Coves may have provided shelter from the wind for larval crappies (Fisher 1997; Pope
and Willis 1998), thus creating "nursery" areas. Wind and subsequent sediment
resuspension increased turbidity (Bachmann et al. 2000), thereby reducing visibility and
foraging success of many larval fish including crappie (Mitzner 1995; Claramunt and
Wahl 2000). Variable water temperatures may have also influenced recruitment. Cool
water temperatures (-17 C) during spring reduced growth rates of early-hatched black
crappie within a Florida lake (Pine and Allen 2001). Similarly, high water temperatures
(>27 C) may constrain crappie growth during summer, likely due to increases in
metabolic rates (Hayward and Arnold 1996).
Stock-recruitment dynamics may also explain recruitment variation among fish
populations (Myers 1997). Generally, as adult stock abundance increases, the number of
recruits produced increases and then may reach an asymptote (Beverton-Holt model) or
begin to decline (Ricker model). Often, this relationship is subject to variability and not
clearly defined (Royce 1996). Allen and Miranda (1998) found significant Ricker-type
stock-recruit relations between the number of adult crappie and the number of juvenile
crappie in four southern U.S. reservoirs. Adult yellow perch Perca flavescens abundance
was related to the abundance of age-0 yellow perch, but no stock-recruit relationship was
determined in Crystal Lake, WI (Sanderson et al. 1999).
Food availability, predation, and competition also influence recruitment (Garvey
and Stein 1998). Foraging success of larval fish is dependent upon availability of
zooplankton that are equal or less than larval gape for many fish including crappies
(Miller et al. 1988; Schael et al. 1991; DeVries et al. 1998). Suitable prey availability for
larval and juvenile fish can strongly influence growth and survival to adulthood (Welker
et al. 1994). For example, greater condition was observed in larval striped bass Morone
saxatilis reared in aquaria with high-prey densities than larvae reared in low-prey density
containers (Chick and Van Den Ayvle 2000). In general, larger larvae may catch more
prey due to increases in swimming speed and gape size, thus contributing to survival and
subsequent growth (Miller et al. 1988). Therefore, a lack of adequate prey may directly
lead to starvation of larval fish or may indirectly lead to mortality of larval fish subject to
a longer exposure to predators (Partridge and DeVries 1999). Limited food resources
may occur when larval and juvenile fish have high diet overlap in food selection,
potentially reducing larval survival (Welker et al. 1994; Garvey and Stein 1998). Subtle
differences in growth and survival, as a consequence of variations in food resources, can
have potentially profound effects on fish recruitment (Houde 1989).
Early life history and diet habits of crappie have been previously documented.
Black crappie and white crappie spawn in water temperatures ranging between 15 C and
20 C (Siefert 1968; Mitzner 1991), which generally occur from February to early May in
Florida lakes (Hoyer and Canfield 1994; Pine and Allen 2001). Swim-up is initiated in
the littoral region between 2 and 4 days post-hatch depending on water temperature
(Siefert 1968). After swim-up, larval crappies move from the littoral region to the
pelagic zone (O'Brien et al. 1984). Following absorption of the yolk sac, larval crappie
up to 8-mm total length (TL) feed primarily on copepod nauplii, whereas crappie 8-14
mm TL consume both copepods and cladocerans (Siefert 1968; Pope and DeVries 1994).
Larval crappie greater than 10 mm TL are usually not limited by gape, but selected prey
size may be less than maximum gape width due to foraging and digestion efficiency
(Schael et al. 1991; DeVries et al. 1998). Crappies primarily forage during daylight
hours (Hanson and Qadri 1984) using a saltatory search method (i.e., swim and pause;
Browman and O'Brien 1992). Juvenile black crappie (15-40 mm TL) feed primarily on
large zooplankton such as Daphnia pp. and calanoid copepods (Pine and Allen 2001).
Crappies between 60 and 160 mm TL continue to select for large zooplankton, but also
consume macroinvertebrates, such as dipteran larvae, ephemeropterans, and odonates
(Siefert 1968; Tucker 1972). Crappies above 160 mm TL continue to forage on
macroinvertebrates and may switch to small fish as prey (Siefert 1968; Ellison 1984).
Ontogenetic diet shifts often regulate fish survival through early life. For example,
largemouth bass Micropterus salmoides with higher feeding efficiency on zooplankton
and invertebrates exhibited faster growth rates and shifted to piscivory earlier than fish
with poor feeding efficiency on zooplankton and invertebrates (Olson 1996).
Consequently, Olson (1996) suggested that an early shift to piscivory was important for
juvenile largemouth bass growth and survival. Ludsin and DeVries (1997) described
complex largemouth bass recruitment dynamics with factors such as hatching duration,
ontogenetic shifts, lipid accumulation, and first winter survival interacting to influence
survival. Ludsin and DeVries (1997) emphasized the importance of following a species
throughout developmental stages to describe variables influencing recruitment.
Recently, trawls have been used in reservoirs and natural lakes to sample larval
(Pope and Willis 1996; Sammons and Bettoli 1998) and juvenile and adult crappies (Pine
2000). Where applicable, trawls allow sampling crappies through larval and juvenile life
stages, which until recently have not been evaluated (Pine and Allen 2001). In addition,
no previous studies have compared multiple natural lakes to assess factors influencing
black crappie recruitment.
I investigated the early life history of black crappie at three natural, productive
Florida lakes and evaluated factors that potentially influence their recruitment. The
objectives of this study were fourfold: 1) to compare the abundance and size distributions
of larval and juvenile black crappie at each lake, 2) to assess the relationship between
adult stock abundance and juvenile black crappie catch rates, 3) to quantitatively assess
and compare the abundance of potential zooplankton prey within each lake, and 4) to
investigate the relationship between prey abundance and relative year-class strength
among the three study lakes. I expected age-0 black crappie size and abundance to
increase with density of preferred-size zooplankton and black crappie stock abundance to
be related to age-0 black crappie abundance among lakes.
Lake location and physical and chemical parameters are shown in Figure 1 and
Table 1, respectively. Lake Wauburg is a 150-ha natural, hypereutrophic lake that
exhibits relatively stable and high black crappie recruitment (M. Allen, unpublished
data). Lake Lochloosa is a 2,286-ha natural, hypereutrophic lake that had variable black
crappie recruitment that was lower in the mid 1990's than later years (M. Allen,
unpublished data). Lake Tarpon is a 1,030-ha natural, eutrophic lake and has historically
sustained a quality black crappie fishery, yet abundance of recruits is often low (M.
Allen, unpublished data). Lake trophic state was determined with criteria developed by
Forsberg and Ryding (1980).
Age-0 Black Crappie Collection
Larval (< 15 mm TL) and juvenile (16 to 190 mm TL) black crappie were sampled
at Lakes Wauburg, Lochloosa, and Tarpon from February to October in 2000 and 2001 to
determine abundance and size structure. Five open-water sites were marked with a
Garmin III GPS unit and used for each sampling event. On all three lakes, crappie larvae
were collected with a neuston net (1x2-m2 mouth, 4-m bag, 1-mm mesh; Sammons and
Bettoli 1998) towed from a 6.7-m boat equipped with a 70-HP outboard motor. Neuston
net samples were conducted between 0900 and 1400 hours three times per month (every
10 days) in 2000 and twice per month (every two weeks) in 2001. At each site, the
neuston net was pulled once at a speed of -1.0 m/s for three minutes to minimize net
clogging. The trawl path was offset from the boat path in an attempt to reduce net
avoidance by larval fish due to prop wash. A General Oceanics flowmeter was mounted
in the net mouth to estimate the water volume sampled to determine larval crappie
density (fish/1000 m3). At the end of each tow, samples were rinsed from the cod end,
fixed in 10% formalin, and returned to the laboratory for analysis. Neuston netting for
larval black crappie (< 15 mm TL) was initiated in late February of each year and
continued until black crappie larvae were no longer caught in the trawl for three
consecutive sampling trips. Surface water temperature and dissolved oxygen were
measured once at the first site on each sample date.
When black crappie > 15 mm TL were collected in the neuston net, I used a small
otter trawl to collect juvenile black crappie (3.7 m mouth, 4.6 m body, 8.4 mm bar mesh
throat, 6.4 mm bar mesh bag; Pine and Allen 2001). At each fixed site, the small otter
trawl was deployed from the same vessel and towed once for three minutes at 1.0 m/s.
Sampling was conducted twice a month in April, May, and June of both years and once a
month in August 2000 and July, August, and September 2001. Age-0 black crappie
collected with the small otter trawl were preserved in 10% formalin and transported to the
In October of both years, juvenile black crappie were sampled at the same sites
using both the small trawl and also a larger otter trawl (4.6 m mouth, 4.9 m long, 38.1
mm bar mesh body, 31.8 mm bar mesh bag; Pine 2000) each pulled once at each site for
three minutes. Both trawls were pulled because black crappie less than 100 mm TL may
not be effectively collected with the large trawl (Pine 2000). The number of juvenile
black crappie caught per minute (CPM) was used as an index of abundance for both
bottom trawls. In all lakes, the number of large trawls pulled was increased to obtain
adequate samples for size structure in October of both years, especially at Lake Tarpon.
Black crappie were placed on ice and brought to the lab to extract otoliths. Black crappie
collected in October were verified as age-0 by examining otoliths for lack of annuli.
At the laboratory, larval and juvenile black crappie were counted, measured to the
nearest mm TL, and transferred to 95% alcohol for storage. Length-frequency
distributions of age-0 black crappie across dates were used to describe size of black
crappie through time at each lake. Up to 30 black crappie were subsampled in proportion
to the length frequency distribution each month (Ludsin and DeVries 1997) and their
stomachs were removed (O'Brien et al. 1984) and opened under a dissecting microscope.
Cladocerans were identified to genus whereas copepods were identified as calanoid,
cyclopoid, or nauplii according to Pennak (1987). Macroinvertebrates were identified to
family (Merritt and Cummins 1996) and undigested fish identified to lowest possible
taxon (Holland-Bartels et al. 1990). All organisms were counted and up to twenty
individuals of each taxon from each stomach were measured to the nearest 0.01 mm
using a standardized ocular micrometer.
Adult black crappie (> 200 mm TL) were collected with the large trawl at all three
lakes in October 1999 and 2000 to relate to juvenile abundance in 2000 and 2001. Fall
CPM of black crappie > 200 mm TL in 1999 and 2000 was used as an index of stock fish
abundance prior to the production of the 2000 and 2001 year classes.
Zooplankton were collected concurrent with each sampling event to assess potential
prey abundance. Samples were taken using a Wisconsin-style plankton net (80-itm mesh)
raised vertically from the lake bottom to the surface at three of the five fixed trawl sites.
Samples were concentrated and stored in 10% formalin. At the laboratory, samples were
adjusted to a known volume and three 1-ml subsamples taken (Miranda and Gu 1998).
Each 1-ml subsample was placed in a Sedgewick-Rafter counting cell and examined
under a compound microscope equipped with an optical micrometer at 100X
magnification (Pine and Allen 2001). All zooplankton were counted and up to 20
individuals of each taxon were measured to the nearest 0.01 mm. Cladocerans were
identified to genus, copepods identified as calanoid, cyclopoid, or nauplii, and rotifers to
genus using Pennak (1987). We determined zooplankton density (D) for each sample by:
where n is the total number of organisms counted in the subsample, vs is the total volume
of that sample in ml, dis the station depth (m), and 0.0113 is the mouth area (m2) of the
zooplankton net (Tugend and Allen 2000).
Selectivity analysis was performed to provide insight on the relationships between
zooplankton availability and diet composition in black crappie on each date. Chesson's
alpha (Chesson 1978) was used to assess prey selectivity:
a- m (2)
where r, is the proportion of the prey type i in the diet sample (in proportion to the total
number of prey types in the diet sample), p, is the proportion of prey type i in the
environment as determined from zooplankton counts, m is the number of prey types in
the environment, and a is the index of taxon selectivity. Alpha ranges from 0 to 1 where
neutral selection is equal to 1/m, positive selection is greater than 1/m, and negative
selection is less than 1/m.
A repeated-measures analysis of variance (ANOVA) was used to test for
differences between mean larval black crappie density (fish/1000 m3) in the neuston net
and mean catch-per-minute (CPM) with small otter trawl across months and lakes in both
years. Data were logio-transformed after adding 0.001 to all values to homogenize the
variances. In the analyses, lakes were considered fixed factors, sample months were
treated as a time variable, and sample sites were treated as subjects. A least-squares
means procedure was used to separate means if the lake, month, or lake*month
interaction was significant in the repeated measures ANOVA. A one-way ANOVA was
used to test for differences in mean CPM among lakes for age-0 black crappie sampled in
October with the large otter trawl in both 2000 and 2001. The size structure of age-0
black crappie was described with length frequency histograms from bottom trawl
collections made from April through October. In October, fish from the small and large
trawls were combined to determine size structure of age-0 black crappie in the fall,
because the large trawl may ineffectively sample juvenile black crappie less than 100 mm
TL (Pine 2000). A Kolmogorov-Smirnov (KS) test was used to assess differences in the
length frequencies of black crappie during October of each year. Correlations between
larval density, early juvenile abundance, and fall age-0 abundance were determined for
each year across lakes. Regression analysis was used to assess relations between adult
black crappie (fish greater than 200 mm TL) abundance in October 1999 2000 (i.e., the
fall prior to production of the 2000 and 2001 year classes) and early summer (May)
juvenile abundance in both years at each of the three study lakes.
To assess differences in prey abundance among lakes, repeated-measures ANOVA
was used to test for differences in loglo-transformed crustacean zooplankton density
(org/L) using lakes and months as factors and the sites as subjects. Density of
zooplankton taxa that were common in the diets of juvenile black crappie were also
compared among lakes and sample months using repeated-measures ANOVA. An
analysis of covariance (ANCOVA) was used to determine differences in the number of
large zooplankton consumed by age-0 black crappie from each lake during May of both
years. Most age-0 black crappie collected in the small trawl in May were
zooplanktivorous in all lakes. Age-0 black crappie diets containing zooplankton,
macroinvertebrates, and fish were analyzed using frequency of occurrence to assess
potential diet shifts among lakes. All statistical analyses were conducted with SAS
(1997) and statistical tests were considered significant when P < 0.05.
The timing of larval black crappie (i.e., < 15 mm TL) first occurrence varied by
lake and year. Larval black crappie were first collected at both Lakes Wauburg and
Lochloosa on March 6, 2000. No larval black crappie were collected at Lake Tarpon in
2000. Catch of larval black crappie declined to zero by May 2000 at Lake Wauburg, but
larval black crappie were collected with the neuston net at Lake Lochloosa through May
of the same year (Figure 2a). In 2001, larval black crappie were first collected on
February 19 at Lake Lochloosa, February 28 at Lake Wauburg, and only on March 2 at
Lake Tarpon. Larval black crappie were collected until March 10 at Lake Wauburg, and
until April 23 at Lake Lochloosa (Figure 2b). Thus, hatching duration may have been
longer at Lake Lochloosa in both years as indicated by the extended catch of larval black
crappie in the neuston net.
Larval black crappie abundance varied among lakes in both years. The lake*month
interaction on larval densities was significant in 2000 (F6,36 = 3.27, P = 0.008) and 2001
(F4,24 = 3.27, P = 0.0114). No larval black crappie were collected at Lake Tarpon in
2000. Larval densities were greater at Lake Lochloosa than Lake Wauburg in April 2000
(Ismeans P < 0.001, Figure 2a). Larval densities were greater at Lake Lochloosa than
Lakes Wauburg and Tarpon in February and March 2001 (Ismeans P < 0.02, Figure 2b).
Therefore, Lake Lochloosa generally had the highest larval density in the spring of both
years, but differences were not consistent across months.
The small otter trawl was used at all lakes starting in early April 2000 and 2001.
Repeated-measures ANOVA revealed a significant lake*month interaction in CPM
during 2000 (F7,34 = 2.82, P = 0.02). Lake Wauburg generally had a greater mean CPM
in the small otter trawl than Lakes Lochloosa and Tarpon in 2000 (Figure 2c). Lake
Tarpon had the lowest black crappie abundance as only 4 fish were collected during
summer 2000 with the small otter trawl (N= 2 in April and May), resulting in mean CPM
values below 0.2. Thus, summer abundance of juvenile black crappie was generally
highest at Lake Wauburg and lowest at Lake Tarpon (Figure 2c). In 2001, juvenile black
crappie were collected in all sample months beginning in April and continuing through
October except at Lake Tarpon, where no fish were collected in June 2001. In 2001, the
lake*month interaction was significant (F11, 70 = 2.52, P = 0.0098) indicating that
differences existed but were not consistent through time. For example, in May 2001,
mean CPM was highest at Lake Wauburg and lowest at Lake Tarpon (Figure 2d).
Conversely, mean CPM was highest at Lake Lochloosa, intermediate at Lake Wauburg,
and lowest at Lake Tarpon in June 2001 (Figure 2d).
Results from October sampling with the large otter trawl were indicative of the
same differences in black crappie abundance among lakes as summer samples with the
small otter trawl, i.e., Lake Wauburg exhibited a higher mean CPM than either Lake
Lochloosa or Tarpon in both 2000 and 2001 (Figure 3). The one-way ANOVA revealed
significant differences in mean black crappie CPM among all three lakes in both years
(2000 F2,64= 159.8, P = <0.0001, 2001 F2,83 = 69.21, P = <0.0001). Least squares means
showed significant differences among lakes in both years (2000 P < 0.032, 2001 P <
0.001). Thus, fall abundance of age-0 black crappie was highest at Lake Wauburg and
lowest at Lake Tarpon in both years (Figure 3).
Size of juvenile black crappie was generally largest at Lake Tarpon and smallest at
Lake Lochloosa during 2000 and 2001. Size structure of age-0 black crappie at Lake
Tarpon was not evident during summer 2000 due to low sample size (Figure 4). During
June 2000, modal lengths were 7 and 5 cm at Lakes Wauburg and Lochloosa,
respectively (Figure 4). This trend continued, and by October 2000, the modal lengths
were 10 and 11 cm at Lake Wauburg and 9 cm at Lake Lochloosa (Figure 4). Thus, age-
0 black crappie at Lake Wauburg appeared to be larger in fall 2000 than those at Lake
Lochloosa (Figure 4). Size distribution was more variable in October at Lake Tarpon
compared to the other two lakes, with age-0 black crappie ranging from 8 to 19 cm
(Figure 4). Modal length of age-0 black crappie at Lake Tarpon was 13 cm. Thus, Lake
Lochloosa had the smallest age-0 black crappie in the fall and Lake Tarpon the largest.
In October 2000, age-0 black crappie were larger at Lake Tarpon (K-S = 4.16, P < 0.001)
than Lake Wauburg and age-0 black crappie at Lake Lochloosa were smaller than those
at Lake Wauburg (K-S = 3.97, P < 0.001).
Age-0 black crappie size structure in 2001 exhibited trends similar to those in 2000.
In June 2001, Lake Wauburg modal lengths were 5 and 7 cm whereas Lake Lochloosa
age-0 black crappie were mostly 5 cm. No age-0 black crappie were collected at Lake
Tarpon during June 2001 (Figure 4). In October 2001, the modal lengths were 9 and 10
cm (range 8 to 12 cm) at Lake Wauburg and 8 cm (range 7 to 13 cm) at Lake Lochloosa
(Figure 4). Lake Tarpon had a modal length of 10 cm with age-0 black crappie size
ranging from 8 to 17 cm (Figure 4). In October 2001, age-0 black crappie were larger at
Lake Tarpon (K-S = 3.56 P < 0.001) than Lake Wauburg, and age-0 black crappie at
Lake Wauburg were larger than Lake Lochloosa (K-S = 7.96 P < 0.001). Thus, age-0
black crappie size was generally largest at Lake Tarpon and smallest at Lake Lochloosa
in both years.
Mean CPM of age-0 black crappie in May (calculated from small bottom trawl
catches) effectively predicted fall age-0 black crappie abundance, but larval densities (as
indicated in neuston samples) were not related with fall age-0 black crappie CPM.
Juvenile black crappie CPM in May was highly correlated with fall age-0 black crappie
CPM in both years (Table 2). However, larval densities were not correlated to fall
abundance among lakes in either year (Table 2).
The relationship between adult black crappie (> 200 mm TL) abundance prior to
spawning and early juvenile abundance was marginally significant (P = 0.0593) among
years. In both years, Lake Tarpon had low early summer juvenile CPM and low fall adult
CPM whereas Lake Wauburg had relatively high early summer juvenile CPM and high
fall adult CPM (Figure 5). Therefore, mean CPM of age-0 black crappie in early summer
was positively related to CPM of adult black crappie the previous fall.
Crustacean zooplankton abundance varied among lakes and sample months in both
years (Figure 6). There was a significant lake*month interaction in 2000 (F11,34 = 9.65, P
< 0.001) and 2001 (F15,46 = 2.05, P = 0.032) indicating that zooplankton abundance
differed among lakes but differences were not consistent through time. For example,
Lake Wauburg had the highest crustacean densities in February 2000 (331 org/L),
whereas Lake Lochloosa had the highest crustacean zooplankton abundance during June
2000 (1,821 org/L) (Figure 6). Zooplankton data were not collected from Lake Tarpon
during August 2000 (Figure 6) because black crappie catches were nil during summer. In
2001, Lake Lochloosa patterns of crustacean zooplankton densities in April (310 org/L)
were higher than the other two lakes, however, crustacean zooplankton abundance were
generally similar across lakes across months (Figure 6). In general, Lake Lochloosa had
the highest crustacean zooplankton densities, followed by Lake Wauburg, then by Lake
Tarpon in both years.
Large crustacean zooplankton copepodss and cladocerans > 0.6 mm) (LC
zooplankton) density varied among lakes and sample months in both years (Figure 7).
The lake*month interaction was significant in 2000 (F9,28 = 7.67, P < 0.001). For
example, Lake Wauburg had the highest LC zooplankton densities in February 2000 (62
org/L), whereas Lake Lochloosa had the highest LC zooplankton density during June
2000 (251 org/L) (Figure 7). In 2001, there was a significant lake*month effect on LC
zooplankton density (F14,30 = 3.08, P = 0.0047). Lake Lochloosa LC zooplankton
densities in April 2001 (33 org/L) were higher than Lake Wauburg (10 org/L) and Lake
Tarpon (6 org/L). However in August 2001, LC zooplankton densities were higher at
Lake Wauburg (36 org/L) than Lake Lochloosa (7 org/L) and Lake Tarpon (< 1 org/L)
(Figure 7). In general, Lake Lochloosa had the highest LC zooplankton densities during
summer, Lake Wauburg had intermediate LC zooplankton densities, and Lake Tarpon LC
zooplankton densities were generally lower than the other two lakes, especially during
Diet selection of zooplankton taxa by age-0 black crappie varied by season in both
years. In 2000, age-0 black crappie diet selectivity was diverse in early summer
compared to later summer months where LC zooplankton were highly selected (Figure
8). Lake Tarpon yielded few fish for diet analysis in 2000 (N=5), so selectivity indices
could not be determined. In 2001, selection of zooplankton was similar to 2000 and by
late summer, age-0 black crappie selection for LC zooplankton was pronounced in all
three lakes. Thus, age-0 black crappie exhibited similar selection trends among lakes,
with variable selection of zooplankton taxa in spring (e.g., March), followed by strong
selection for large zooplankton by late summer and fall (i.e., August October) in both
Consumption of LC zooplankton by age-0 black crappie in May varied among
lakes. In May 2000, juvenile black crappie at Lake Wauburg were larger than those
found at Lake Lochloosa, making comparisons of consumption invalid. However, in
May 2001 the number of LC zooplankton consumed by age-0 black crappie differed
among lakes (ANCOVA, test for difference in elevation, P < 0.001). Age-0 black crappie
at Lakes Wauburg and Tarpon consumed more LC zooplankton than similar-sized fish at
Lake Lochloosa (Figure 9).
Most age-0 black crappie contained crustacean zooplankton in their diets in May
and throughout the summer in all lakes in 2000 and 2001, but occurrence of
macroinvertebrates and fish in the diets varied among lakes and years (Figure 10). In
2000, zooplankton was found in 100% of black crappie in May, but by August over 80%
of fish in Lochloosa contained macroinvertebrates compared to about 20% of fish at Lake
Wauburg. Low numbers of age-0 black crappie collected at Lake Tarpon precluded diet
analysis for fish in this lake in summer 2000. In October 2000, fish were found in about
60% of Lake Tarpon black crappie diets whereas most fish at Lakes Lochloosa and
Wauburg contained zooplankton and macroinvertebrates (Figure 10). In 2001, over 90%
of age-0 black crappie at Lake Lochloosa contained macroinvertebrates in May, whereas
most age-0 black crappie in all lakes contained zooplankton and macroinvertebrates
during August and October (Figure 10). Thus, most age-0 black crappie at Lake
Lochloosa consumed macroinvertebrates earlier in summer than the other two lakes in
both years (Figure 10), yet age-0 black crappie size was not larger by October (Figure 4).
Occurrence of fish in diets of age-0 black crappie was higher at Lake Tarpon than the
other two lakes in both years (Figure 10) and age-0 black crappie size was larger by
October (Figure 4).
Figure 1. Location of study lakes in Florida.
100 T 2000 larval
0.1 I I I I I I
100- 2001 larval b
0.1 I -I I I I
Feb Mar Apr
May Feb Mar Apr May
Feb Mar Apr May
1000 T 2000 small trawl
1000T 2001 small trawl
Apr MayJun Aug Oct Apr May Jun Aug Oct Apr May Jun Aug Oct
Figure 2. Mean ( 1 SE) larval densities (a and b panels) as estimated from neuston net
catches and small trawl catch per minute (CPM) (c and d panels) across sample
months in 2000 and 2001 at Lakes Lochloosa, Tarpon, and Wauburg.
I I I I
n = 20 trawls
n = 7 trawls
n = 6 trawls
n = 54 trawls
WAUBURG LOCHLOOSA TARPON
Figure 3. Large trawl catch per minute (mean + 1 SE) of juvenile black crappie during
fall of 2000 and 2001. Sample size (n = number of trawls) is indicated.
n = 23 trawls
n = 43 trawls
0 2 4 6 8 10 12
E Apr l May f Jun l Aug M Oct
Figure 4. Relative length frequency distributions for age-0 black crappie in Lakes
Wauburg, Tarpon, and Lochloosa in 2000 and 2001.
-,V- I I1
ogloMay=1.5572 + 1.6377 (logloAdult)
r2 = 0.6303
P = 0.0593
Stock black crappie CPM
Figure 5. Regression between logo adult black crappie catch-per-minute (CPM) prior to
spawning and loglojuvenile black crappie CPM during May 2000 and 2001 in
Lakes Lochloosa, Tarpon, and Wauburg. Symbols indicate lake
(L=Lochloosa, T=Tarpon, W=Wauburg) and year (00=2000, 01=2001).
Feb Mar Apr May June July Aug Sep Oct
Figure 6. Loglo crustacean zooplankton copepodss and cladocerans) densities (+ 1 SE) in
Lakes Lochloosa, Tarpon, and Wauburg for each sample month in 2000 and
2001. Mean densities with no error bars indicate low variance in those months.
No samples were collected from any lakes in July 2000 or from Lake Tarpon
in August 2000.
Feb Mar Apr May June July Aug Sep Oct
Feb Mar Apr
July Aug Oct
Figure 7. Loglo crustacean zooplankton (> 0.6 mm) mean densities ( 1 SE) in Lakes
Lochloosa, Tarpon, and Wauburg during sample months in 2000 and 2001. Mean
densities with no error bars indicate low variance in those months. No samples were
collected from any lakes in July 2000 or from Lake Tarpon in August 2000.
-- --I- -I- -z
Feb Mar Apr May June July Aug Oct
I WAUBURG 2001
-I r I
RO NA EU BO AL D CY DA CA
RO NA EU 80 AL DI CY DA CA
0 ,, I ,
0 -3 -
o E I i- I -
RO NA EU BO AL DI CY DA CA
Figure 8. Age-0 black crappie selectivity (mean 1 SE; Chesson 1978) for zooplankton
taxa in 2000 on Lakes Lochloosa and Wauburg. The solid line represents
neutral selection. Taxa (RO rotifers, NA nauplii, EU Eubosmina, BO -
Bosmina, AL Alona, DI Diaphanosoma, CY cyclopoid copepods, DA -
Daphnia, and CA calanoid copepods) are listed left to right from small to
large. N equals the number of fish diets examined.
0 1 -
-f -: :
Wauburg LC =-49.01 + 2.64 (TL)
Lochloosa LC =-8.29 + 0.92 (TL)
Wauburg LC =-6.88 + 2.54 (TL)
LC = 14.21 + 0.59 (TL)
Lochloosa LC = 1.32 + 0.22 (TL)
I I I I I I
0 10 20 30 40 50 60
Age-0 black crappie total length (TL)
Figure 9. Logio number of large crustacean zooplankton (LC) in diets of age-0 black
crappie in May 2000 (top panel) and 2001 (bottom panel). Points indicate
individual fish. Lines represent regression equations and the fish size range for
I I I
TARPON WAUBURG L
30 27 29
28 30 20
Figure 10. Proportion of age-0 black crappie diets containing crustacean zooplankton,
macroinvertebrates, and fish during May, August, and October 2000 and 2001.
No age-0 black crappie were sampled during August 2000 from Lake Tarpon.
The number above the columns indicates the number of diets analyzed in each
Table 1. Annual mean levels of chlorophyll a (Chl a), total phosphorus (TP), total
nitrogen (TN), and secchi depth (cm) of Lakes Lochloosa, Tarpon, and
Wauburg for 2000 and 2001. Lakes Lochloosa and Wauburg data were from
Florida Lakewatch (2001) and Lake Tarpon data were from Pinellas County
Department of Environmental Management (unpublished data). Monthly
samples (Jan-Dec) were collected from 3 sites at Lake Tarpon and 4 sites from
Lake Wauburg in both years. In Lake Lochloosa, samples in 2000 were
collected monthly at 4 sites except February secchi and November and
December for all parameters. Due to low water levels in Lake Lochloosa in
2001, sampling protocol was reduced to two monthly water samples in
February and August and one secchi depth reading in February. Percent of lake
volume inhabited by macrophytes (PVI) was determined in Lochloosa, Tarpon,
and Wauburg during May, October, and June of 2001, respectively. Dynamic
ratio estimates are from Bachmann et al. (2000).
Lochloosa Tarpon Wauburg
Surface area (ha) 2286 1030 150
Mean water depth (m) 2.0 2.5 3.0
Chl a (pg/L)
2000 254 33 61
2001 138 29 83
2000 96 40 94
2001 90 43 115
2000 5275 1222 1445
2001 3824 1114 1793
Secchi Depth (cm)
2000 26 82 72
2001 40 85 61
PVI (%) 5.0 4.3 0.6
Dynamic ratio (km/m) 2.39 1.26 0.41
Table 2. Pearson correlations between fall juvenile black crappie abundance (October
age-0 CPM) and both larval density (fish/1000 m3) and juvenile abundance
(May CPM) in the three study lakes (N).
Large bottom trawl
October age-0 CPM
Surface trawl r 0.122 -0.206
Larval fish/1000 m3 P 0.922 0.868
Small bottom trawl r 0.999 0.992
May age-0 CPM P 0.008 0.082
Age-0 black crappie abundance and size during fall was not related to crustacean
zooplankton abundance through summer among the three lakes. Age-0 black crappie at
Lake Lochloosa were small compared to the other two lakes, but zooplankton density was
highest at this lake in both years. Conversely, Lake Tarpon had the largest age-0 black
crappie, but low zooplankton densities relative to the other two lakes. Lake Wauburg had
higher abundances and intermediate size of age-0 black crappie and intermediate
zooplankton abundance. Pope and Willis (1998) found no relation between crappie
abundance and crustacean zooplankton size or abundance in two South Dakota systems
over a three-year period. However, a weak correlation between zooplankton abundance
and larval crappie density in Normandy Reservoir, Tennessee was reported by Sammons
et al. (2001). Crustacean zooplankton densities (range 44 89 org/L) in Normandy
Reservoir were lower than in South Dakota (50-700 org/L, Pope and Willis 1998). The
lakes I evaluated were eutrophic/hypereutrophic and contained high crustacean
zooplankton densities (50-1300 org/L). Thus, crustacean zooplankton densities did not
appear to limit age-0 black crappie abundance or size in this study. Systems with greater
crustacean zooplankton densities (i.e., more than 50 org/L) may not be zooplankton
limited compared to systems that contain low abundance of crustacean zooplankton.
I found a correlation between abundance of age-0 black crappie in May and stock
abundance the previous fall among the three lakes. Similarly, Allen and Miranda (1998)
documented Ricker-type stock recruitment relations for crappies in four southeastern
reservoirs. Because my study only contained six lake-years of data (3 lakes and 2 years),
I was unable to develop a stock-recruitment curve. However, a positive relation existed
between stock and recruits in this study, and stock abundance during fall explained about
63% of the variation in early-summer age-0 black crappie abundance among lakes.
Population bottlenecks often regulate rates of mortality and growth in juvenile fish
(Diana 1995). Although bottlenecks in survival may occur during summer in juvenile
largemouth bass (Olson 1996; Ludsin and DeVries 1997) and bluegill Lepomis
macrochirus (Cargnelli and Gross 1996), I found no evidence of a summer bottleneck for
age-0 black crappie. Fall abundance of age-0 black crappie was strongly related to
juvenile abundance during early summer among the study lakes. Because early summer
catch rates were highly correlated to fall catch rates, zooplankton abundance during
summer appeared adequate for high survival. Thus, age-0 black crappie fall abundance
was set by early summer in both years.
Larval black crappie densities were not related to CPM of juvenile black crappie
during early summer (May) or fall (October). Larval black crappie densities at Lake
Lochloosa were higher than Lake Wauburg whereas juvenile black crappie were most
abundant at Lake Wauburg. Higher larval black crappie catch rates at Lake Lochloosa
may have resulted from the lake being shallow relative to Lake Wauburg (2 m vs 3 m)
i.e., the neuston net sampled a larger portion of the water column relative to Lake
Wauburg. Although I collected larval black crappie (7 to 15 mm) with the neuston net
throughout the spring, I did not assess the survival of larval crappie to the juvenile stage.
Therefore, mortality of larval black crappie may have occurred during spring of each year
prior to recruitment of fish to the small bottom trawl sampling gear. Alternately, larval
sampling during daylight hours may have been less effective as compared to night
sampling by Sammons and Bettoli (1998). It is possible that the neuston net may have
ineffectively collected larval black crappie and confounded the relationship between
larval and age-0 fall abundance at the three study lakes.
I detected differences in fall size structure of black crappie among lakes. However,
fall size structure may not have been indicative of growth rates because differences in
hatching duration may have contributed to the differences in size among age-0 black
crappie. Black crappie began hatching at similar times, however, larval crappie (< 15
mm) were present longer at Lake Lochloosa in both years. The extended occurrence of
larval fish could have resulted from a protracted hatching period at Lake Lochloosa. This
difference may be related to the resulting fall size structure of the study lakes (i.e., late-
hatched fish may represent the smaller fish in the sample). At Lake Tarpon, no larval fish
were caught in 2000, likely due to low abundance.
Age-0 black crappie size differences among lakes may be attributed to the amount
of large zooplankton consumed. Juvenile black crappie at Lakes Tarpon and Wauburg
consumed more large zooplankton in early summer and obtained a larger modal size by
fall than juvenile black crappie at Lake Lochloosa. In comparison, juvenile bloater
Coregonus hoyi that consumed large prey items achieved greater size than bloaters in
aquaria without large prey available (Miller et al. 1990). Therefore, juvenile black
crappie that consumed more large zooplankton in early summer may have had a size
advantage by fall.
Growth of young and adult crappies has been shown to be density dependent in
some lakes and reservoirs (Swingle and Swingle 1967; Allen et al. 1998). However, I
found little evidence of density-dependent growth due to zooplankton abundance among
lakes. Lake Tarpon had the largest age-0 black crappie and the lowest zooplankton
density in this study, but Lake Wauburg had higher zooplankton density and intermediate
age-0 black crappie size among the three lakes. Zooplankton abundance was generally
highest at Lake Lochloosa where age-0 black crappie were smallest in October. Thus,
my expectation that age-0 black crappie size would be greater at lakes with higher
zooplankton densities was not evident in this study.
Results of diet analysis of juvenile black crappie among the three lakes revealed
similar prey composition as found in previous studies (Siefert 1968; Tucker 1972, Schael
et al. 1991; DeVries et al. 1998, Pine and Allen 2001). Primarily large crustacean
zooplankton, especially calanoid copepods, were positively selected by juvenile black
crappie in the study lakes. However, preferred-size zooplankton densities were generally
higher at Lake Lochloosa where juvenile black crappie size was smaller by fall when
compared to the other two lakes. Pine and Allen (2001) found no significant differences
in calanoid copepod densities through summer at Lake Wauburg, yet there were different
growth rates between early and late age-0 black crappie cohorts, likely due to water
Differences in water clarity (i.e., secchi depth) may have contributed to the
observed variation in age-0 black crappie sizes by fall among the study lakes. Reduced
water clarity can be caused by phytoplankton production (Brown et al. 2000) and
sediment resuspension (Bachmann et al. 2000) and may negatively influence foraging
efficiency of larval fish, including black crappie (Mitzner 1991). Barefield and Ziebell
(1986) demonstrated that the number of large zooplankton (Daphnia pulex 2.0-2.5 mm)
ingested by crappies (100-150 mm) was reduced as turbidity increased in a laboratory
environment. Striped bass larvae ate fewer copepods as turbidity increased although the
size of copepods consumed did not vary (Breitburg 1988). Miner and Stein (1993)
suggested that larval bluegill growth may be negatively affected by turbidity. However,
Miner and Stein (1993) also demonstrated the importance of contrast in the background
to highlight transparent zooplankton. Thus, there may be a threshold in turbidity as
suggested by Mitzner (1995) that provides optimum foraging for larval crappie.
The dynamic ratio (km/m) (Bachmann et al. 2000) indicates how likely shallow
lakes are susceptible to turbulence and sediment resuspension due to wind. Bachmann et
al. (2000) indicated that a lake with a dynamic ratio, (the square root of lake surface area
in square kilometers divided by mean depth in meters), greater than 0.8 km/m may be
prone to sediment resuspension. In addition, the percent of lake volume inhabited by
macrophytes (PVI) may alter the dynamic ratio (e.g., high macrophyte abundance may
buffer the turbulence generated by the wind, thereby overestimating the dynamic
ratio)(Bachmann et al. 2000). Calculating the dynamic ratio for each study lake, Lake
Wauburg had the lowest value at 0.41 km/m, Lake Tarpon was intermediate at 1.26
km/m, and Lake Lochloosa had the highest value at 2.39 km/m. Secchi depths were
lower at Lake Lochloosa than at Lakes Tarpon and Wauburg in both years and PVI
values in all lakes were relatively low. With low secchi depths and low PVI, it is
possible that reduced water clarity at Lake Lochloosa may have resulted from a high
dynamic ratio and associated sediment resuspension. In addition to potential wind
resuspension of bottom sediments, chlorophyll values at Lake Lochloosa (> 100 [tg/L)
were higher than Lakes Wauburg and Tarpon (70 and 30 ag/L, respectively). Thus, both
physical (i.e., dynamic ratio) and biotic (i.e., chlorophyll) characteristics indicate that
visibility and associated feeding efficiency of age-0 black crappie at Lake Lochloosa may
have been lower than at Lakes Wauburg and Tarpon.
There is a need to determine the feeding success of age-0 black crappie on
zooplankton prey in environments with varying water clarity (turbidity). Although
Barefield and Ziebell (1986) evaluated feeding rates at different turbidities, the size of
zooplankton prey (Daphnia pulex, 2.0-2.5 mm) they used was relatively large compared
to prey types in my study lakes. Results of such a study would increase our
understanding of the influence of turbidity on growth and survival of age-0 black crappie.
Many previous studies have shown that ontogenetic diet shifts can affect the growth
and survival of age-0 fish. For example, dramatic increases in age-0 largemouth bass
growth rates occur after a diet switch to macroinvertebrates whereas no diet shift causes
poor fish growth and survival (Olson 1996, Ludsin and DeVries 1997). Northern pike
Esox lucius require a shift from zooplankton prey to macroinvertebrates during early life
prior to their shift to piscivory (Bry et al. 1995). Conversely, studies that observed a diet
switch to macroinvertebrates by yellow perch Perca flavescens have shown conflicting
results. Cobb and Watzin (1998) observed faster yellow perch growth rates when
macroinvertebrates were consumed. However, juvenile yellow perch (> 30 mm TL)
growth rates decreased when the proportion of benthic prey items increased at Lake
Mendota, Wisconsin (Wu and Culver 1992). In this study, occurrence of
macroinvertebrates in diets of black crappie size increased with fish size consistent with
findings of previous studies (Tucker 1972; Ellison 1984). However, age-0 black crappie
were smaller by fall at Lake Lochloosa than Lake Tarpon although macroinvertebrates
occurred commonly in the diets of fish from both lakes. Additionally, crustacean
zooplankton occurred more often than macroinvertebrates in age-0 black crappie diets at
Lake Wauburg, yet modal fish size was intermediate. Thus, a diet switch to
macroinvertebrates by age-0 black crappie at all lakes did not translate to large size by
fall as observed in studies of other predators (Bry et al. 1995; Olson 1996). However,
macroinvertebrates and fish were present in age-0 black crappie diets at Lake Tarpon and
crappie size by fall was greater than the other two lakes. Therefore, it appears that some
age-0 black crappie at Lake Tarpon may have benefited from a diverse diet of
zooplankton, macroinvertebrates, and fish by October. However, I did not assess
macroinvertebrate or forage fish density among lakes and thus was unable to relate black
crappie size and abundance to macroinvertebrate or forage fish abundance.
Age-0 black crappie abundance and size was not related to crustacean zooplankton
density in highly productive natural lakes in Florida. Although crustacean zooplankton
abundance was not related to age-0 black crappie abundance or size, I observed large
differences in fall size and abundance of black crappie among the study lakes. Abiotic
factors (e.g., reduced water clarity) may have reduced successful capture of crustacean
zooplankton, especially at Lake Lochloosa, with negative consequences for growth of
larval crappie in this lake. Juvenile black crappie abundances were also influenced by the
abundance of adult black crappie the previous fall.
Age-0 black crappie abundance and size was variable at productive lakes in Florida.
For example, age-0 black crappie at Lake Tarpon exhibited larger size than Lakes
Lochloosa and Wauburg during both study years, some reaching 18 cm by October.
These sizes are more common in black crappie older than age-0 (Carlander 1977; Allen et
al. 1998), thus crappie in Lake Tarpon may attain a harvestable size faster than other
crappie populations. Harvest of these fish during a winter fishery at Lake Tarpon may
reduce the potential number of spawning individuals the following year (Webb and Ott
1991). Age-0 black crappie at Lake Tarpon might be protected by imposing a length
limit, potentially increasing their contribution to the reproductive population. Crappie
populations with fast growth rates and low abundance, such as Lake Tarpon, may benefit
from the implementation of harvest regulations. Conversely, implementing regulations
on lakes or reservoirs with a high crappie abundance and/or small size, such as Lakes
Wauburg and Lochloosa, may not be beneficial.
I found that mean CPM of age-0 black crappie in May was highly correlated with
fall abundance among the three lakes in both years. Thus, year class strength of fish
entering fall appeared to be set by May in both years. Larval fish densities, as determined
from neuston net samples during spring, were not a reliable index of year class strength
by fall as shown by Sammons and Bettoli (1998) in Tennessee. Thus, fishery managers
might be better served by conducting bottom trawls in early summer to assess black
crappie year class strength in Florida lakes.
Mean CPM of age-0 black crappie in May was strongly related to adult black
crappie abundance in the previous fall. Thus, fall adult crappie stock abundance may
depict juvenile crappie abundance the following summer. Hence, fall stock abundance
may partially explain abundance of juvenile black crappie in Florida lakes. However, the
interaction between stock densities and variable environmental conditions strongly
influences crappie recruitment (Allen and Miranda 2001). Future studies that evaluate
stock-recruitment relationships with abiotic effects are needed to further understand the
mechanisms that result in variation in crappie recruitment.
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Kevin J. Dockendorfwas born on February 15, 1973, in Iowa City, Iowa, the son of
Tom and Helen Dockendorf. He was raised in the rural community of New Hampton,
Iowa, with two brothers, Jeff and Brad. He graduated from Iowa State University with a
B.S. in fisheries and wildlife biology in May 1996. He was a fisheries technician with
the Iowa Department of Natural Resources and the Illinois Natural History Survey during
1996-2000. In February 2000, he began graduate work evaluating black crappie
recruitment in the Department of Fisheries and Aquatic Sciences at the University of
Florida. In December 2002, he will graduate with his Master of Science degree. His
future plans include getting married in June 2003 and pursuing a career as a fisheries
management biologist within a state agency.