• TABLE OF CONTENTS
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 Title Page
 Abstract
 Introduction
 Study area
 Methods
 Results and discussion
 Acknowledgement
 Literature cited






Group Title: Manatee population report no. 3.
Title: Correction factors for observability of manatees during aerial surveys
CITATION PAGE IMAGE ZOOMABLE
Full Citation
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Permanent Link: http://ufdc.ufl.edu/UF00073820/00001
 Material Information
Title: Correction factors for observability of manatees during aerial surveys
Series Title: Manatee Population Research Report
Physical Description: 10 p. : map ; 28 cm.
Language: English
Creator: Packard, Jane M
Summers, Robert C
Barnes, Lindsay B
U.S. Fish and Wildlife Service
Florida Cooperative Fish and Wildlife Research Unit
Publisher: Florida Cooperative Fish and Wildlife Research Unit, University of Florida
Place of Publication: Gainesville FL
Publication Date: [1983]
 Subjects
Subject: Manatees -- Florida   ( lcsh )
West Indian manatee -- Florida   ( lcsh )
Aerial photogrammetry -- Florida   ( lcsh )
Genre: federal government publication   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Bibliography: p. 10.
Statement of Responsibility: Jane M. Packard, Robert C. Summers, and Lindsay B. Barnes; prepared for the Florida Cooperative Fish and Wildlife Research Unit and supported by the U.S. Fish and Wildlife Service, Atlanta, Georgia.
General Note: "Prepared for the U.S. Fish & Wildlife Service under cooperative agreement no. 14-16-0009-1544."
General Note: Research work order no. 2.
General Note: "September 1983"
General Note: "Florida Cooperative Fish and Wildlife Research Unit for U.S. Fish and Wildlife Service" -- Cover.
Funding: This collection includes items related to Florida’s environments, ecosystems, and species. It includes the subcollections of Florida Cooperative Fish and Wildlife Research Unit project documents, the Sea Grant technical series, the Florida Geological Survey series, the Coastal Engineering Department series, the Howard T. Odum Center for Wetland technical reports, and other entities devoted to the study and preservation of Florida's natural resources.
 Record Information
Bibliographic ID: UF00073820
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: aleph - 001893043
oclc - 11062186
notis - AJW8298

Table of Contents
    Title Page
        Title page
    Abstract
        Page 1
    Introduction
        Page 2
    Study area
        Page 2
        Page 3
    Methods
        Page 4
    Results and discussion
        Page 5
        Page 6
        Page 7
        Page 8
    Acknowledgement
        Page 9
    Literature cited
        Page 10
Full Text







CORRECTION FACTORS FOR OBSERVABILITY


OF MANATEES DURING AERIAL SURVEYS

September 1983


Jane M. Packard, Robert C. Summers,
and Lindsay B. Barnes



Prepared for:
Florida Cooperative Fish and Wildlife Research Unit
117 Newins-Ziegler Hall
University of Florida
Gainesville, FL 32611






Supported by:
U.S. Fish and Wildlife Service
75 Spring St. S.E.
Atlanta, GA 30303


Cooperative Agreement No. 14-16-0009-1544

Research Work Order No. 2



Citation should read: Packard, J.M., R.C. Summers, and L.B. Barnes. 1983.
Correction factors for observability of manatees during aerial surveys.
Manatee Population Research Report No. 3. Technical Report No. 8. Florida
Cooperative Fish and Wildlife Research Unit. University of Florida, Gainesville
Florida. 10pp.









Abstract: The use of radio-telemetry to obtain correction factors for aerial
counts of manatees was investigated in a river where the actual number of
manatees present was known from counts of individuals identified at a
warm-water refuge. About 33 to 57% of the manatees present in the area were
sighted during each aerial survey. Estimated abundance based on the
ratio-of-radios-sighted or the mean ratio-of-manatees-sighted did not differ
significantly from known counts of individuals on five surveys. Three
methods of calculating corrected counts did not differ significantly.
Sightability of manatees with radios did not differ significantly from
sightability of unmarked manatees. However, the ratio-of-radios-sighted
differed among three habitat types. Manatees were more easily sighted in
lake and creek habitat than in river habitat. Because the distribution of
manatees in the three habitats changed between surveys, corrected counts
based on a correction factor obtained for each survey were closer to total
counts than corrected counts based on the mean ratio-of-radios-sighted over
all surveys.









INTRODUCTION


Manatees (Trichechus manatus) have been censused by aerial surveys
(Irvine and Campbell 1978, Leatherwood 1979, Odell 1979, Hartman 1979, Shane
1981, Irvine et al. 1981). However, results have been difficult to interpret
because neither the proportion of the population sighted nor variation in
observability by habitat type has been determined. For example, estimates
obtained from ground counts of manatees at a winter aggregation were twice as
high as those obtained from aerial surveys (Shane 1981).

Factors influencing aerial counts have been investigated for a number of
terrestrial species (Caughley 1974). Caughley et al. (1976) suggested that
the specific biases involved in each survey situation should be measured to
correct estimates and, by this empirical approach, general models for improv-
ing accuracy of aerial surveys may be developed.

Correction factors have been obtained by comparing aerial counts with a
known number of animals in an area (LeResche and Rausch 1974, Caughley et al.
1976). Where the actual density is not known, the observed proportion of a
known number of animals carrying radio-transmitters has been determined
(Floyd et al. 1979).

We investigated the use of radio-telemetry to obtain correction factors
for aerial surveys of manatees in a freshwater system. To compare corrected
counts with the actual number of manatees present, surveys were conducted
during the winter when manatees congregated at a warm water refuge and all
individuals occupying the area could be identified and counted.

STUDY AREA

The study was conducted on the St. Johns River from Lake Monroe to the
southern end of Lake George in northeastern Florida (Figure 1). The ecology
of the area has been described by Bengtson (1981). The waterways within this
region vary in width and depth, and were classified in this study as three
habitat types: lakes, river channel and creeks. Lake habitat comprised
approximately 66% of the water surface area surveyed, and was generally
characterized by widths greater than 100 m and depths ranging from 1 to 2 m.
About 21% of the water area was river habitat, characterized by channels
approximately 60 to 100 m wide and 4 to 8 m deep. Creek habitat was narrow,
meandering waterways or canals, which were often obscured by overhanging
vegetation. Creeks represented about 13% of the water area surveyed, and were
approximately 20 to 60 m wide and 2 to 3 m deep.

Submerged and floating vegetation consumed by manatees is located in the
shallow portions of creeks and lakes and along the edges of the river where
the vegetation is not disturbed by boat traffic. The St. Johns River is a
travel route for recreational boats and commercial barges.

The study area consisted of the winter range of manatees that are known
to aggregate at Blue Spring, a natural warm-water refuge (Bengtson 1981). The
spring is located 23.5 km south of the northern boundary of the study area
and 13 km north of Lake Monroe. Fourteen manatees, whose movements were









INTRODUCTION


Manatees (Trichechus manatus) have been censused by aerial surveys
(Irvine and Campbell 1978, Leatherwood 1979, Odell 1979, Hartman 1979, Shane
1981, Irvine et al. 1981). However, results have been difficult to interpret
because neither the proportion of the population sighted nor variation in
observability by habitat type has been determined. For example, estimates
obtained from ground counts of manatees at a winter aggregation were twice as
high as those obtained from aerial surveys (Shane 1981).

Factors influencing aerial counts have been investigated for a number of
terrestrial species (Caughley 1974). Caughley et al. (1976) suggested that
the specific biases involved in each survey situation should be measured to
correct estimates and, by this empirical approach, general models for improv-
ing accuracy of aerial surveys may be developed.

Correction factors have been obtained by comparing aerial counts with a
known number of animals in an area (LeResche and Rausch 1974, Caughley et al.
1976). Where the actual density is not known, the observed proportion of a
known number of animals carrying radio-transmitters has been determined
(Floyd et al. 1979).

We investigated the use of radio-telemetry to obtain correction factors
for aerial surveys of manatees in a freshwater system. To compare corrected
counts with the actual number of manatees present, surveys were conducted
during the winter when manatees congregated at a warm water refuge and all
individuals occupying the area could be identified and counted.

STUDY AREA

The study was conducted on the St. Johns River from Lake Monroe to the
southern end of Lake George in northeastern Florida (Figure 1). The ecology
of the area has been described by Bengtson (1981). The waterways within this
region vary in width and depth, and were classified in this study as three
habitat types: lakes, river channel and creeks. Lake habitat comprised
approximately 66% of the water surface area surveyed, and was generally
characterized by widths greater than 100 m and depths ranging from 1 to 2 m.
About 21% of the water area was river habitat, characterized by channels
approximately 60 to 100 m wide and 4 to 8 m deep. Creek habitat was narrow,
meandering waterways or canals, which were often obscured by overhanging
vegetation. Creeks represented about 13% of the water area surveyed, and were
approximately 20 to 60 m wide and 2 to 3 m deep.

Submerged and floating vegetation consumed by manatees is located in the
shallow portions of creeks and lakes and along the edges of the river where
the vegetation is not disturbed by boat traffic. The St. Johns River is a
travel route for recreational boats and commercial barges.

The study area consisted of the winter range of manatees that are known
to aggregate at Blue Spring, a natural warm-water refuge (Bengtson 1981). The
spring is located 23.5 km south of the northern boundary of the study area
and 13 km north of Lake Monroe. Fourteen manatees, whose movements were








Lake George /





























0 I0 Kilometers
Lake Woodruff

Lake Dexter----"'"







\





I 1
-N-**



l 0 o


Lake Beresford



Blue Spring Run





Lake Monroe
:-..,


Figure 1. Study area on the St. Johns River.


Florida









monitored by radio telemetry, did not leave the study area during winters
1978-1980 (Bengtson 1981). During the winter, manatees gather in Blue Spring
when the river water temperature drops below 200C, and leave the spring
periodically to feed during the warmest part of the day (afternoon) or during
warm weather (Powell, O'Shea and Rathbun, unpublished data, Hartman 1979,
Bengtson 1981). In contrast to the river (secchi disk readings of 60-90 cm),
water clarity is excellent in the Blue Spring run, facilitating accurate
counts of manatees.

METHODS

The number of manatees known to be in the study area was determined by
counts of individuals in Blue Spring run. Individuals were identified by
characteristic scar patterns (Hartman 1979), a technique that has been used
to monitor manatee attendance at Blue Spring since 1970/71 (Powell, O'Shea
and Rathbun, unpublished data). Manatees in Blue Spring were counted from
the ground at the same time as the plane passed over the run. Because Blue
Spring was not included in aerial surveys, the number of manatees in the
survey area was estimated to be the total identified at the spring during one
to two days prior to the survey, minus the number in the spring run during
the survey.

To obtain correction factors for the proportion of manatees observed
during aerial surveys, we conducted five surveys during February and March
1983 when the number of manatees in the study area could be determined.
Radios were attached to the peduncles of seven manatees, as described by
Bengtson (1981). Two ratios were calculated for each survey: (1) the ratio
(C m= O/E) of total manatees observed from the air (0) to total identified
individuals expected to be in the survey area (E) and (2) the ratio (C r=
Or/Er) of observed radioed manatees (Or) to the number of radioed manatees
verified by telemetry to be in the survey area (Er).

The same pilot and two passengers (junior authors) conducted each
survey. The pilot was experienced and passengers had three training flights
prior to collection of the reported data. The back-seat passenger (referred
to as Observer) searched visually for manatees and the front-seat passenger
(referred to as Tracker) monitored the radio signals. The Observer recorded
location, cumulative duration visible, portion of body observed, behavior,
and presence of a peduncle attachment for each manatee sighted. When a
radioed manatee was not sighted, the Tracker informed the pilot and Observer
and the plane circled back to determine the location and possible reason why
the radio was not sighted.

Corrected counts were calculated for each survey and compared with the
actual number of identified manatees known to be in the survey area (E).
Three methods for calculating corrected counts were compared: (1) the mean
ratio of manatees observed (Cm) over the five surveys was divided into the
number of manatees observed on each survey (0), (2) the ratio-of-radios-
sighted (Cr) for each survey was divided into the number of manatees observed
on each survey (0), and (3) the mean ratio-of-radios-sighted (Cr) over all
surveys was divided into the number of manatees observed on each survey (0).
The Friedman two-way analysis of variance (Conover 1971) was used to compare
the three methods of calculating corrected counts (0/Cm, O/Cr, 0/Cr).









Differences between the known number of manatees (E) and corrected counts,
paired for each survey, were tested by Wilcoxin's signed ranks test (Conover
1971).

Radios were attached to the peduncles of seven manatees, as described by
Bengtson (1981). Manatees carrying radios are henceforth referred to as
"radioed manatees."

Factors known to influence aerial counts were relatively constant during
the surveys. Surveys were conducted in the afternoon, with durations from
1.8 to 3.1 hrs. Weather conditions were sunny to partly cloudy, wind
velocity ranged from 7 to 16 knots, water temperature in the river near Blue
Spring was 17 to 180C and water surface conditions ranged from small ripples
to a steady chop without whitecaps. Water clarity in the river was at least
60 cm as measured by a secchi disk and monitored by objects submerged at
known depths and visible from the airplane. Surveys were flown in a Cessna
172 at heights of 85 to 95 m and speeds of 70 to 80 knots/hr.

Surveys of the study area followed a consistent route from south to
north. The plane flew along the west bank of the river, looping back along
the east bank at approximately two-km intervals. Creek habitat was usually
covered in one pass, unless the observer judged that the water was obscured
by vegetation and requested a loop. The perimeters of large lakes were
searched and transects were flown over the centers.

To determine factors likely to influence correction factors obtained
from radio-telemetry, an additional 10 surveys were conducted at times when
the number of manatees in the study area could not be determined with
certainty. The data from all 15 surveys were pooled to determine if
sightability of radios differed by habitat type (river, lake, creek), the
frequency distribution of sighting duration, and relative visibility of three
portions of the body (anterior third, middle third and posterior third).

To examine the visibility of passive marks, vinyl flags (15 cm diameter
half-circles) were attached to three manatees by peduncle straps. The
threshold at which flags were visible from the air was monitored by sub-
merging flags on a structure at depths of 60, 80 and 100 cm.

RESULTS AND DISCUSSION

About 43-67% of the manatees present in the St. Johns River were not
sighted during aerial surveys. The proportion of total manatees sighted (Cm)
ranged from 0.33 to 0.57 with a mean of 0.47 (Table 1). The proportion of
radioed manatees sighted (Cr) was lower than the proportion of total manatees
sighted; Cr ranged from 0.0 to 0.75 with a mean of 0.38. All methods for
correcting counts yielded estimates that did not differ significantly from
the known number of manatees present (Table 1). Results of the three methods
of correcting counts did not differ significantly (X= 2.8, 2 degrees of
freedom, p = 0.75).

The use of radio-telemetry to obtain observability correction factors is
most valid when the ratio-of-radioed-manatees-sighted on a survey is used to
correct the count from the same survey. Because of the mean ratio-of-radios-










Table 1. Comparison of corrected and actual counts of manatees.


TOTAL MANATEES


CORRECTION
FACTOR (C)


CORRECTED COUNTS


SURVEY Observeda
DATE (0)


In Survey
Area (E)


% manateesc
(Cm)


% radios
(Cr)


m/C


0/Cr 0/C,


23 Feb

25 Feb

3 Mar

4 Mar

16 Mar


0.33

0.48

0.53

0.45

0.57


Mean


Cm=0.47


0.75

0.00

0.33

0.33

0.50


Cr=0.38


Te=
p =



aNumber of marked and unmarked manatees sighted on survey
(excludes Blue Spring).


17 11 21


- 29


21 30 26

19 27 24

17 16 21


19 21 24

0 -.36 -1.78

0.50 0.36 0.04


of study area


Number of marked and unmarked manatees identified in Blue Spring prior to
survey minus the number of manatees in Blue Spring during the survey.
C0 divided by E

dNumber of radioed manatees sighted in study area divided by number of
radioed manatees located in study area.
eWilcoxins ranked signs test with pairwise comparison between E and each
corrected count over all surveys. For a two-tailed test, p less than 0.025
would indicate a significant difference.









sighted (er) was lower than the mean ratio-of-manatees-sighted (Em),counts
corrected by Cr would tendto overestimate abundance. Using a one-tailed
test, counts corrected by Cr were significantly higher than the known number
of manatees (E) (p < .05). However, counts corrected by the ratio (Cr)
obtained for each survey were not significantly higher than expected counts
(E) using a one-tailed test (p > .05).

Factors potentially influencing observability of radios were examined.
The sightability of radioed manatees did not differ significantly from
unmarked manatees (Chi square = .54, 1 degree of freedom, p > 0.25). Radioed
manatees were sighted on 8 of 20 occasions that the radio signal was located,
and unmarked manatees were sighted in 35 of 71 potential occasions during the
five surveys when the known number of manatees could be determined.
The ratio-of-radios-sighted was influenced by habitat type. During the
15 surveys, radios were located 67 times. Locations of radioed manatees were
more frequent in lake habitat (n = 30) than in creek (n = 21) and river
habitat (n = 16). The proportion of times radios were sighted was lower in
river habitat (25%) than in lake (66%) and creek habitat (71%). The
frequency of sighting vs. no sighting of radioed manatees was significantly
non-random when all three habitat types were compared (Chi-square = 9.639, 2
degrees of freedom, p < .05). Differences among habitats were examined by
Freeman-Tukey deviates (Bishop et al. 1975). The number of manatees sighted
was lower than expected in river habitat (z = -1.94), and higher than
expected in lake (z = + 0.63) and creek habitat (z = + 0.81).

Variation in the ratio-of-radios-sighted may be partially attributed to
changes in manatee distribution among habitat types. Over the entire study
(15 aerial surveys), the ratio-of-radios-sighted ranged from 0.0 to 1.0 with
a mean of .54. As manatees moved away from Blue Spring when temperatures
rose, they gathered in groups in Lake Monroe and were less frequently sighted
in river habitat. The mean ratio for seven surveys conducted prior to March
19th was lower (Cr= 0.34) than the mean ratio for the remaining surveys
(Cr .70).
If activity of manatees varies seasonally, it could also influence the
ratio-of-radios-sighted. Reasons why radioed manatees were not sighted were
determined in 27 cases. In 33% of these cases, manatees were obscured by
vegetation, usually when they were feeding in mats of floating plants. In
59% of the cases, manatees were submerged and activity could not be
determined. Manatees that rest in a submerged position leave few clues to
their presence; when feeding or traveling occurs under water, mud plumes
usually indicate the activity.of the manatee (Hartman 1979).
Learning by the Observer probably influenced the ratio-of-radios-
sighted. Manatees tended to remain in one location for a few days after they
dispersed from Blue Spring following a rise in water temperature. The
Observer could anticipate sighting an individual where it had been located
the previous day. This source of error could be minimized by scheduling
surveys at two to three day intervals. Because the ratio-of-radios-sighted
increased from the beginning to the end of the study, the possibility of
improvement in observation skills of the Observer cannot be discounted.








Survey conditions could influence variation in the ratio-of-radios-
sighted. Although conditions were relatively constant for the five surveys
from which corrected counts were calculated, conditions varied during the
additional surveys. For example, on a day with whitecaps, no radios were
sighted (this survey was not included in analyses). Wind velocities of 12 to
15 knots were considered to be at the upper threshold for adequate visibil-
ity. Glare on sunny days and cloud reflections reduced visibility, but
overcast skies and rain were not a problem.

Non-verbal communication from the Tracker to the Observer may partially
influence the ratio-of-radios-sighted. Whenever the observer was aware of
cues from the Tracker, which indicated presence of a radio, the sighting was
not counted. This problem was difficult to avoid, because a minimal time
delay was necessary to reduce the chance that the manatee changed location or
activity. If the Observer sat in the front and the Tracker sat in the back,
the problem might be reduced, but communication between the Tracker and the
pilot would be more difficult.

A major problem with the use of radio-telemetry to develop correction
factors for manatee surveys is the attenuation of the radio signal in saline
water. A large portion of manatee habitat in Florida is brackish or saline
water (Irvine et al. 1981). To evaluate the potential use of passive marks to
obtain correction factors in brackish habitat, we collected some information
regarding the visibility of vinyl flags attached to the peduncle.

When manatees were at the surface, the vinyl flags were readily visible
from the air. However, the stationary flags were never visible below 80 cm,
and rarely visible below 60 cm. Visibility of stationary flags corresponded
closely to secchi disk readings, which varied from 60 to 90 cm. Blue was
less visible than red and yellow; it was never observed below 60 cm.

A mark attached to the peduncle is less likely to be visible than a mark
attached to the anterior two-thirds of the body of a manatee. Portion of the
body observed was recorded on 153 sightings of manatees. The posterior third
of the body was sighted less frequently (71%) than the anterior third (95%)
and the middle third of the body (98%).

When a manatee is sighted, it usually remains visible long enough to
identify a mark. In 63% of 151 sightings, manatees were visible for longer
than 90 seconds. Only 11% of the sightings were briefer than five seconds,
18% were between 5 and 30 seconds duration, and 8% were between 31 and 90
seconds duration.

Our results suggest that care should be taken in interpreting data from
manatee aerial surveys for which correction factors were not determined.
Radio-telemetry may be used effectively in freshwater to obtain specific
correction factors for aerial surveys, but additional research is needed to
control survey conditions such that a general correction factor could be
applied. If flags attached to the peduncle are used to determine sightabil-
ity in saline habitat, correction must be made for the probability of sight-
ing a manatee at the surface. Methods of attaching marks to the anterior
portion of the body should be developed.








Acknowledgements. We thank Tom O'Shea, Wayne Hartley, and Galen Rathbun for
their invaluable assistance during this study. Peduncle attachments were
placed on manatees by the U.S Fish and Wildlife Service, with the assistance
of Sea World, under the authority of Federal Fish and Wildlife permit
PRT 2-8430. Permission from Florida Department of Natural Resources to use
facilities at Blue Spring State Park is greatly appreciated. We are grateful
to Vic Johnson for his skills as pilot and to Deland Aviation for its flexi-
bility in scheduling surveys. The study was financed by the U.S. Fish and
Wildlife Service, under Cooperative Agreement No. 14-16-0009-1544, Research
Work Order No. 2, with the Florida Cooperative Fish and Wildlife Research
Unit. Administrative support was provided by the School of Forest Resources
and Conservation, Institute of Food and Agricultural Sciences, University of
Florida. The following persons contributed to design of the project and/or
review of the manuscript: D. DeMaster, R. Gregory, R. Hofman, H. Kochman, D.
Mech, T. O'Meara, T. O'Shea, F. Percival, K. Portier and D. Siniff. We thank
D. Stinson for typing the manuscript.








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Minnesota, Minneapolis, MN. 126pp.

BISHOP, Y.M.M., S. E. FIENBERG and P. W. HOLLAND. 1975. Discrete
Multivariate Analysis: Theory and Practice. MIT Press, Cambridge,
Massachusetts, 555pp.

CAUGHLEY, G. 1974. Bias in aerial survey. J. Wildl. Manage. 38:921-933,

R. SINCLAIR, and D. SCOTT-KEMMIS. 1976. Experiments in
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FLOYD, T. S., L. D. MECH and M. E. NELSON. 1979. An improved method of
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43(1):258-261.

HARTMAN, D. S. 1979. Ecology and Behavior of the Manatee (Trichechus
manatus) in Florida. Special Publication No. 5. The American Society
of Mammalogists. 153pp.

IRVINE, A. B., and H. W. CAMPBELL. 1978. Aerial census of the West Indian
manatee, Trichechus manatus, in the southeastern United States. J.
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Manatees and dolphins in western peninsular Florida: with notes on
sightings of sea turtles and crocodiles. U.S. Fish and Wildlife
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20pp.

LEATHERWOOD, S. 1979. Aerial survey of the bottlenosed dolphin, Tursiops
truncatus, and the West Indian manatee, Trichechus manatus, in the
Indian and Banana Rivers, Florida. Fish. Bull. 77:47-59.

LERESCHE, R. E. and R. A. RAUSCH. 1974. Accuracy and precision of aerial
moose censusing. J. Wildl. Manage. 38:175-182.

ODELL, D. K. 1979. Distribution and abundance of marine mammals in the
waters of the Everglades National Park. Pages 673-681 in R. M. Linn,
ed. Proceedings of the First Conference on Scientific Research in
National Parks. New Orleans, LA, 9-12 November 1976, U.S. Dep. Inter.,
Natl. Park Serv. Tran. Proc. Ser. 5(1).

SHANE, S. H. 1981. Abundance, distribution and use of power plant
effluents by manatees (Trichechus manatus) in Brevard County, Florida.
Report to Florida Power and Light Co., Miami. National Technical
Information Service PB81-147019.




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