A Report on the Feasibility of Using
Aerial Surveys and Mark and Recapture Techniques
to Conduct a Population Survey
of the West Indian Manatee.
Jane M. Packard
H. Franklin Percival
Florida Cooperative Fish and Wildlife Research Unit
School of Forest Resources and Conservation
117 Newins-Ziegler Hall
University of Florida
Gainesville, FL 32611
Purchase Order No. 40181-0414
U.S. Fish and Wildlife Service
75 Spring Street, Atlanta, Georgia 30303
Citation should read: Eberhardt, L. Lee. 1982. Censusing Manatees.
Prepared for U.S. Fish and Wildlife Service, P.O. No. 40181-0414.
Manatee Population Research Report No. 1. Florida Cooperative Fish
and Wildlife Research Unit. Gainesville, FL. 18pp.
RECOMMENDATIONS . . .
INTRODUCTION . . .
BACKGROUND AND ISSUES . .
PROSPECTIVE APPROACHES . .
DEVELOPING AN INDEX OF ABUNDANCE
ESTIMATING ADULT SURVIVAL .
DIRECT COUNTS . . .
CAPTURE-RECAPTURE METHODS .
LITERATURE CITED . .
LIST OF FIGURES
1. Counts of manatees at warm water refuges . .
2. Counts of manatees at the Riviera Beach Power Plant
compared with those in adjacent waters, December 1980
through February 1981 . . . .
3. Recorded manatee mortalities by county, 1979 through
1981 . . . . . .
LIST OF TABLES
1. Maximum aerial counts of manatees at power plant
sites, 1977 to 1982 . . . . .
. . . . 1
. . . . 2
. . . . 2
. . . . 3
. . . . 5
. . . . 11
. . . . 14
. . . . 15
. . . . 17
(1) Development of tagging methodology should be initiated immediately,
due to the time that will be needed to produce a suitable system.
Since manatees are so long-lived, tag loss must be minimal.
External tags seem most desirable.
(2) Since movements are poorly known, but may be extensive seasonally,
a limited number of pedunclee" tags should be deployed as soon
as possible to provide the data needed for planning more extensive
tagging, censusing, and for many other purposes.
(3) Counts at warm water refugia, available for the last 5 years,
should be developed into an index of abundance by further analysis
of the counts and auxiliary information (most importantly, tempera-
tures). Other refugia should be incorporated into the index
system as soon as possible. Differences in water turbidity
and other factors should be considered in devising units or
strata for index counts.
(4) It does not seem prudent, or cost-effective, to attempt a state-
wide census in the immediate future since so little is known of
movement patterns and because various aspects of census methodology
need to be worked out. Nonetheless, the proposed indices should
be used as the basis for developing a census method. A careful
review of the available data should be published to pave the
way for revised population estimates, which will quite certainly
exceed the number (1000) now widely accepted.
1~1_~ ~ __ ____ _
This report results from an invitation to review the needs and
prospects for capture-recapture and aerial census studies of the
manatee (Trichechus manatus) in Florida. The author visited Florida
on May 2 through 6, 1982, under auspices of the U.S. Fish and Wildlife
Service. Arrangements for the visit were made by Dr. Franklin Percival,
of the Cooperative Wildlife Research Unit at the University of Florida
in Gainesville. Dr. Galen Rathbun and the staff of the Gainesville
Field Station of the Denver Wildlife Research Center provided both
data and guidance, as did P. M. Rose of the Florida Audubon Society
(currently manatee recovery activities coordinator for the Fish and
Wildlife Service). Three aerial reconnaissance flights provided a
first hand view of manatee habitats, as follows: May 3, Suwannee
River to Kings Bay and Crystal River (Rathbun, Eberhardt), May 4,
Vero Beach to Ft. Lauderdale and Ft. Myers by way of Whitewater Bay
(Rose, Percival, Eberhardt), and May 5, Cape Canaveral to Jacksonville,
St. Johns River and Blue Spring (Rose, Kinnaird, Eberhardt).
This report takes a somewhat broader view of the census and
marking questions than was originally requested, in an effort to
provide a better perspective for research planning and coordination.
It is difficult to find an example of a marine mammal census that
is not subject to a number of uncertainties as to validity of the
estimates. Some of the many difficulties have been reviewed by Eberhardt,
Chapman and Gilbert (1979). In the absence of much assurance that
"good" census techniques are readily available, it is prudent to
look rather carefully at circumstances and purposes before choosing
a course of action. It should be anticipated that the procedures
selected may require 5 years or more for development and implementa-
BACKGROUND AND ISSUES
On the basis of a 1976 survey (Irvine and Campbell 1978), size
of the Florida manatee population is frequently reported as being
about 1000 individuals. Evidence from that work and subsequent studies
have provided evidence that the survey estimate was quite certainly
well below the actual population size. The chief difficulty in an
aerial survey of the type used is that many of the manatees would
not be counted in consequence of the large and varied areas to be
covered, overhanging vegetation, dark and turbid waters, and the
bottom-resting habits of the species. The census difficulties are
well known to the scientists involved. Repetition has nonetheless
led to a widespread perception that the population is in the neighbor-
hood of 1000 animals. It can thus be expected that an improved estimate
will lead to some confusion and controversy, since reported abundance
could very well increase over the years, while actual numbers might
Packard (1981) and Shane (1981) have reported population estimates
or counts on the order of double the counts in warm-water refugia in
their respective study areas, while the corresponding power plant
counts in those areas are substantially higher than the tallies recorded
by Irvine and Campbell (1978). The Canaveral-Orlando counts (Table
1) have averaged 126 manatees in recent years, where Irvine and Campbell
show 75. Similarly, Irvine and Campbell (1978:615) used a count of
62 manatees at the Riviera Beach plant, whereas the subsequent 5 year
average maximum count (Table 1) is 116 manatees. Direct comparisons
are thus 242/137 = 1.77 for power plant counts, with a further mul-
tiplier up to double that ratio. However, questions as to seasonal
movements and methodology may require some conservatism in applying
yet another factor of two -- but, on the other hand, estimates based
on aerial counts are often substantially under true totals. Hence
an accurate estimate of the manatee population could quite conceivably
triple the number in current use. Hartman (1974:207) believed that
1000 was the "conceivable maximum" population size. His Figures
33 and 34 should, however, be contrasted with more recent estimates
for specific locales. A much more detailed and thorough review of
the available data is essential, and the calculations given above
are intended only to indicate the need for such a review.
An effective manatee "salvage" program has resulted in tallies
of more than 100 mortalities in a number of years. Certainly not
all dead manatees are found. If the prevalent population estimate
is accepted, the observed mortality may be more than the low reproduc-
tive potential of the species can support. Not enough is known about
some of the essential population parameters for an accurate determina-
tion, but a rough assessment of population dynamics will be given
The difficulties in providing accurate (unbiased) estimates
of both absolute abundance and of total annual numerical losses are
such that both goals may not be achieved for many years, if at all.
Alternatives are feasible, and need to be considered, especially
in short-term planning. Since manatees are quite vulnerable to several
features of expanding human populations in Florida (boating and habitat
alterations in particular), it does seem likely that long-term planning
should include development of reliable methodology for estimating
Since manatees have low reproductive rates, assurance of long-
term stability of the population likely requires monitoring of adult
survival rates. Unless these rates remain high, the population cannot
persist (Eberhardt and Siniff (1977) give an analysis of the relative
importance of various population parameters in influencing population
growth rates). It is thus important to consider monitoring survival
rates through a marking program. The existence of an efficient carcass
recovery program provides the necessary machinery of recovering tags.
Table 1. Maximum aerial counts of manatees at power plant sites, 1977
to 1982. 1977 to 1979 data, are from Rose and McCutcheon
(1980), 1980 data from Raymond (1981), and 1981 data are from
P. M.; Rose (pers. comm.).
Power plant 1977-78 1978-79 1979-80 1980-81a 1981-82a
Riviera Beach 68 99 108 102(141) 201(168)
Hobe Sound 56 97 38 39(50) 35(64)
Total 124 196 146 141(191) 236(232)
Cape Canaveral 83 82 75 102 120
Orlando Utilities 42 31 41 36 16
Total 125 113 116 138 136
Ft. Lauderdale 36 27 36 52(54) 19
Pt. Everglades 114 125 86 110 57(80)
Total 150 152 122 162(164) 76(99)
Ft. Myers 271 172 142 136 127
aEditorial Note: Figures in parentheses are corrections that are consistent
with 1980-81 data on pg. 22, 32 of Raymond (1981) and 1981-82 data on pg.
23, 29 of McGehee, M. A. (1982,Manatees (Trichechus manatus): Abundance
and distribution in and around several Florida power plant effluents during
the winter of 1981-82. Prepared for Florida Power and Light Co. Miami,
FL. 67pp.). The maximum counts at adjacent sites were not necessarily
made on the same day, so if there was movement between sites, some manatees
may have been counted twice in the totals. JMP 5/9/83
Methods for estimating survivalarates from tags or bands taken from
dead animals have been rather extensively developed in applications
in ornithology, wildlife management, and in fisheries. In most ap-
plications, both marking and recovery over a number of years is required
if acceptable accuracy (unbiased estimates) and precision (small
variance) are to be achieved. Some further discussion on these points
will be provided below.
Marking manatees will have further benefits in various ways,
including possibilities for direct estimation via capture-recapture
methodology, "correction factors" for aerial surveys, and delineation
of population units and of movement patterns. Since the major mortal-
ity factors quite certainly are more intense in some areas than others,
knowledge of local populations and of movements is of substantial
importance. Because an efficient and economical marking method is
not currently available, an appreciable development time may be required.
An enumeration of some prospects appears below.
Because an extensive manatee marking program will undoubtedly
be essential in the long term, steps should be taken now to develop
suitable methods. Results cannot, however, be expected for some
years. Meanwhile, there is a need for current information and for
an indication of trends in manatee populations. The possibility
of developing a census method without permanent marking will be discussed
below. At best, however, such a program might be started in the
near future, and will provide little or no information on past abun-
DEVELOPING AN INDEX OF ABUNDANCE
The main prospect for assessing manatee populations in the recent
past is provided by the extensive counts in warm water refugia.
There have been a variety of such counts, but the longest and most
extensive consistent series is that made by the Florida Audubon Society
under contract to Florida Power and Light Company, starting in 1977
and continuing to the present. A detailed analysis is beyond the
scope of the present study, so that only a few points are covered
The counts were made at a series of sites around Florida, and
can be supplemented by at least two other sequences of similar data.
Aerial tallies were made at approximately weekly intervals from fall
to spring, and thus cover the entire period in which warm water refugia
are utilized. A simple approach to using the counts is to use the
maximal annual count at each site as an index. The resulting data
appear in Table 1, and in Figure 1 (which also includes two other
sites, Blue Spring and Crystal River). That the counts represent
an important potential source of information about the manatee popula-
tion is evident from the totals (Table 1). Clearly, a substantial
fraction of the population is represented in the totals.
77 78 79 80 81
78 79 80 81
78 79 80 81
77 78 79 80 81
Counts of manatees at warm water refuges. Blue Spring Run
data are total identified individuals, all others are maximum
aerial counts. Blue Spring and Kings Bay counts are from
staff of Sirenia Project, USFWS, Gainesville, Florida. Other
data are from Table 1.
Biases very likely exist in the simple index suggested here,
and various such prospects are discussed in the several reports on
the original data collection. In view of the potential utility of
an index constructed from these counts, it would appear worthwhile
to devote a substantial effort to seeking ways to compensate for
possible biases. One of the major known influences is, of course,
temperature. Using temperature as an auxiliary variable, possibly
"lagged" in time, may provide a route to an improved index. It is
also possible that such an adjustment might make it feasible to extend
the index to include some earlier counts, including those of the
1976 survey, made over a short time period. This is well worth the
trouble, inasmuch as the winter of 1976-77 was reported to be the
severest on record, and some cold-related mortality occurred then
Various investigators have remarked that manatees appear to
somehow anticipate cold weather, or may follow a seasonal cycle,
possibly related to photoperiod. If this is true, the direct asso-
ciation with temperature may show some seasonal differences, depending
on the annual variation in temperature patterns. One useful analytical
technique will be to regress counts on temperature and then plot
the residuals (deviations from regression) against time, and contrast
these plots between sites in the same year and between years for
the same site. Non-linear least-squares fits to various models may
also be useful, and data that have been classified as to counting
conditions may be most useful. While a general seasonal pattern
may emerge, it should not be expected that the regression relation-
ship will be the same at all sites, due to local conditions and lat-
In the absence of unbiased population estimates, with which
index data might be "calibrated", one cannot be sure that a particular
kind of adjustment does in fact improve a "crude" index, such as
the maximum count suggested above. About all that can be done is
to search for procedures that have a basis in logic, and for which
some sort of internal consistency can be demonstrated. Even so,
development of an index should very likely be regarded as the first
step in a series aimed at ultimately producing an unbiased population
estimate. In the present example, one such step may be to make a
series of counts well away from the refugia. If the time trend at
warm water refugia is accurately mirrored by an inverse pattern away
from the refugia, and both are correlated to temperature change, one
may feel justified in relying on the index. Figure 2 shows the data
for counts at Riviera Beach power plant (Packard 1981) contrasted
with counts in the surrounding area. It seems evident that there
was an influx of manatees to the region in mid-December. Hence the
counts "away from" a refugium may well need to encompass a very large
area. Other prospects for checking on an index will likely become
evident as increasing numbers of marked animals become available
in the population.
I_I _~~ ~L~l_
20 40 60
Days from December 1
Counts of manatees at the Riviera Beach power plant compared
with those in adjacent waters, December 1980 through February
1981. Data are from Packard (1981).
The mortality data and the 1976 statewide survey give some further
indications as to possible population status and survey needs. Total
mortalities by county from 1979 through 1981 (C. Beck, letter of 13,
May 1982) appear in Figure 3. The high loss in Brevard County is
well known and under study. As yet, it does not appear to have in-
fluenced counts at the nearby power plant sites (Figure 1). It is,
of course, possible that these losses may influence populations asso-
ciated with much of the eastern part of the state. Mortality in
Lee, Collier, and Monroe counties is also substantial and is associated
with a declining trend at the Ft. Myers power plant, plus the large
loss in Lee County in 1982 (41 manatees through May 10, 1982). It
can thus be supposed that detailed population studies in this south-
western region very likely should have high priority. It is apparently
not known how well the Ft. Myers counts represent populations further
south, since these "southern" animals may have less reason to seek
winter refuge at power plants.
The Collier-Monroe County area is clearly a difficult region
to census, due to the opacity of the water there, extent of the water
areas, and convoluted shoreline. Nonetheless, in the 1976 survey
(Irvine and Campbell 1978), 163 manatees were observed (Jan. 29-
Feb. 3), while Irvine et al (1981) tallied 149 in November of 1979.
In the 1976 survey, 91 manatees were counted in the Ft. Myers area
on January 30. The high counts in the Ten Thousand Islands White-
water Bay area in spite of difficult counting conditions suggest
that the manatee population there is probably a large one. Against
this should be set the declining trend at Ft. Myers and the high
loss in that area this year. Next winter's count at Ft. Myers is
thus a very important datum in evaluating this situation. Very likely
a high priority should be assigned to marking manatees at Ft. Myers,
and monitoring for marked animals throughout the southwestern area.
Mortality records in the Tampa Bay area (10) plus the 1976 survey
count there (43) suggest that some consideration might be given to
establishing a monitoring system there. Similarly, the St. Johns
River and Jacksonville area appear to need more attention, with 33
mortalities and little data on the warm-water refugia in the Jacksonville
area. Opacity of the water is again a major problem, and the main
indication of trend in the region (counts at Blue Spring Run) shows
evidence of a general upward trend. Surveys of various refugia on
the upper St. Johns River (Anon. 1981) suggest little prospect of
substantial numbers wintering outside Blue Spring Run, so the remaining
questions may have to do with numbers wintering in the Jacksonville
area and with possible migration southwards along the Atlantic coast.
Bengtson (1981) telemetered 15 manatees in Blue Spring Run and did
not observe any indication of these animals travelling to the Atlantic.
However, he estimated the manatee population of the St. Johns River
to be 50-75 animals, a figure that is not consistent with the observed
recent average mortality (11 per year) and the increasing trend of
numbers at Blue Spring. Very possibly the main population at Blue
Spring is rather permanent and largely independent of the animals
in the Jacksonville area. Some "transients" are regularly reported
at Blue Spring (Anon. 1981).
... 2.n., "
-u j~~.E O LON
IT -l V -
1| k 1
) ( t -_ -.r1- -
_- -- ----- --
3 I I i *tr
f r--.J IG .V ur"
Figure 3. Recorded manatee mortalities by county, 1979 through 1981.
Data from Sirenia Project, USFWS, Gainesville, Florida.
Several other sites, including Lake Okeechobee and Blue Lagoon
may need to be incorporated into an "index" system.
ESTIMATING ADULT SURVIVAL
The essential features of the dynamics of manatee populations
are not known with any accuracy. Two individuals, observed since
birth, were in late pregnancy at age 5 at Crystal River. For adult
animals, the interval between births is generally between 2 and 3
years, with a current average of about 2.5 years (Rathbun pers. comm.).
Female births per female would thus be about .5(1/2.5) = .2, assuming
an even sex ratio, as has largely been observed thus far. Referring
to Fig. 2 of Eberhardt and Siniff (1977), it can be inferred that
the annual survival rate required for an equilibrium (constant) popula-
tion level quite likely is appreciably greater than 0.90. From the
figure, age of first reproduction is likely not to have a major impact
on this estimate.
Survival for the first year or two is generally not known for
marine mammals (in the graph referred to above,survival to age 3
was assumed to be half of the adult rate). No doubt continued close
observation at Blue Spring and Crystal River will yield an estimate
of calf survival in time, and thus a basis for some improved calcula-
tions. However, calf mortality is no doubt substantially higher in
other areas, where populations are in more jeopardy from boats and
Some further calculations of likely rates are worthwhile, using
the fairly widely observed fraction of calves in the population (about
9 or 10 percent in much of the data), and possibly the model suggested
in Eberhardt et al (1982), which has been revised to take senescence
into account. Although there are no observations on senescence in
manatees (or in most marine mammals), an arbitrary truncation at some
advanced age may be preferable to using the assumption of constant
adult survival regardless of age assumed in the graphs given by
Eberhardt and Siniff (1977).
A detailed review of the methodology for estimating survival
is beyond the scope of the present report. As noted above, the proce-
dure suggested for manatees involves recovery of tags or marks in
carcasses obtained through the "salvage" program. Since the carcasses
could be examined radiographically, an externally visible mark is
not necessary, and it has been suggested that a mark operating on
the principal of the magnetic fish tag might be somehow injected
in the caudal appendage ("paddle"). This will involve some complex-
ities in the form of detection equipment and possibly in encoding
tags for identity. Application to free-swimming animals requires
a simple and rapid method. Multiple marks may need to be used due
to prospects of loss of marks, and the best approach may be to use
compressed gases to inject a number of marks into the caudal appendage.
At a minimum, marks need to be distinguishable by year of applica-
tion, and there would be various advantages to identifying individuals.
However, the best prospect for applying substantial numbers of marks
is in the concentrations of animals observed in warm-water refugia.
If marks cannot be identified visually, the observations will consist
of a time and place of recovery, plus either a time and place of
marking, or only a time (year) of marking. Some further analysis
is worthwhile to examine the likely utility of place of marking with
respect to an auxiliary program in which externally visible marks
are used, thus providing opportunities for multiple observations
In any case, the marks need to be identified to at least year
of application, and a number of years of application will be needed
(see below). The question of coding by place of application may
thus be considered with respect to the number of codes available.
One of the prospects for a mark may be steel shot, coded by using
different alloys might then be a limitation. An alternative that
has been suggested is to use numbered "fingerling" tags, in which
case many individuals can be identified, possibly at the cost of
poorer ballistic performance of the marks.
Another problem that needs consideration is the prospect of
double-marking of individuals, if there is no external evidence of
a mark. This needs to be guarded against in a given year, possibly
by incorporating some kind of marking substance in the application
process, so as to leave a surface mark. Multiple marking over succes-
sive years would likely be unavoidable, but may in fact yield some
useful information in itself. This is one of several questions needing
further attention, with respect to the underlying models for mark
and recovery processes.
The immediate need, of course, is for development of marking
technology. Presumably, initial efforts will be devoted to tests
on freshly dead specimens, followed by work in oceanaria and at sites
where close observation of known individuals is possible. Experience
under the Sirenia Project and a series of photographs collected over
the years indicate that marking in or through the caudal appendage
("paddle") is likely to be a safe and reliable procedure.
As has been suggested above, results will not be available for
some years and will improve in precision with the passage of time,
as more and more tags are recovered. If an implanted mark (not visible
from the surface) is used, the data will be similar to that obtained
from a variety of marking schemes. Possibly the closest parallel
is with bird-banding of non-hunted species. The data will differ
from the usual such situation in that manatees are much more long-
lived than most birds. There are, however, parallels in some of
the larger oceanic birds. For example, Cormack (1964) gives annual
survival estimates of 0.94 to 0.97 for the fulmar petrel (Fulmarus
glacialis). In many of the bird-banding studies, there are questions
about the consistency of the recovery effort, and the degree of non-
reporting of bands. In the present situation, minimization of problems
of this nature will depend on maintaining the present manatee "salvage"
effort at a high and consistent level of efficiency. Such a scheme
cannot be shut down and then re-activated as it is needed, due to
the many difficulties of achieving agency and public cooperation.
There is now a substantial literature on survival estimation
from tag recovery data, much of which is concerned with bird-banding.
An extensive handbook (Brownie et al 1978) provides an array of struc-
tural models, and some guidelines for planning a study. It is partic-
ularly important to note that this extensive effort is based on the
assumption that no bands are lost between application and recovery.
Since manatees can be expected to have a high survival rate (over
90 percent per year, as noted above), even a highly efficient recovery
system will still only yield a few returns per year -- probably less
than 5 per 100 marks applied. It will thus require a substantial
number of years before enough marks are recovered to yield the kind
of information that may be needed. Hence the marking scheme needs
to be virtually "foolproof" in assuring that a marked animal is iden-
tified as such on recovery. However, all animals with major damage
at the marking site (e.g., the caudal appendage) can simply be excluded
from consideration and thus that aspect of reidentification probably
needs not be a matter of major concern. The critical issue is that
of possible unobtrusive loss of marks or of overlooking marks on
examination of dead manatees.
While references such as Youngs and Robson (1975) and North
and Cormack (1981) should be consulted for a basis for a detailed
evaluation of sample size requirements, a rough idea of the "variance
problem" may be obtained from results given by Chapman and Robson
(1960) by considering recoveries from a single year of marking (the
reference actually considers estimating survival from age structure
data, but the results apply also to band recoveries, given the usual
assumptions). Chapman and Robson (1960: eq. (5) or eq. (7)) give
a variance estimate appropriate when all of the tagged animals are
dead (i.e. a "complete" series of recoveries) and one for a truncated
series (eq. (20)), which might be applied during the course of the
recovery process (i.e., when only k years have passed, where k is
less than the life span of a manatee). The sample size (n) in the
incomplete series will be smaller, but can be adjusted for by comparing
the ratio of expected sample sizes (incomplete series/complete series).
A rather hasty examination of such a comparison suggests the
need for a more careful analysis before using internal tags, identifi-
able only on the death of the animal. Visible tags, if coded by time
(year) of marking, will provide an additional opportunity for estimating
survival (through resighting) as well as essential information on
movements and the prospect of estimating total numbers. A general
impression is that the human-caused mortality problem may be most
important in particular areas. It is consequently very important
to know whether there are fairly discrete sub-populations in these
areas, or whether there is a substantial movement of manatees over
large portions of the range. Some evidence of long distance move-
ments exists (Shane 1981).
As has been remarked above, the substantial variability in habitats
and visibility conditions argues against an expectation of successful
direct counting of manatees over large areas. The notable exception
is counting in warm-water refugia, where it may be feasible to devise
a workable scheme for censusing through direct counts with correction
factors. A key point is that it is known from studies at Blue Spring
and Crystal River that some individuals may be absent from a given
refuge at any particular time. Hence, although direct counts might
be corrected for animals not visible at the time of counting, there
may be another component of the population to be accounted for in
Very likely, the question of whether or not direct counts should
presently be attempted has to be resolved in terms of the urgency
attached to "getting a number". Since manatees are long-lived and
quite possibly follow a rather fixed annual cycle of movements, a
successful marking scheme, necessary for estimating survival, should
ultimately also'yield good data on numbers.
The sizable numbers of manatees counted through aerial surveys
over power plant effluent discharge areas suggest that these counts
mdy provide the starting place for attempts to develop a census method-
ology. Since sighting and flying conditions vary from site to site,
a detailed study of several such sites may provide the optimal first
step in developing a method. An initial question is that of the
fraction of the animals present that are actually seen from the air,
determined by some sort of "ground truth" operation. If locations
of manatees sighted from the air are mapped, then it may be feasible
for shore-based observers to locate, in advance, groups of manatees
in distinctive locations, and to keep them spotted while the aerial
count is made, as has been done for sea otters in California. Some
extensive experimentation will be required to determine whether such
a scheme is feasible for manatees and sufficiently efficient.
The difficult conditions will, of course, be those where turbid
water has to be dealt with. No doubt some kind of identification
of individual animals will be required in the most difficult sites,
and costs may then be excessive. However, an essential first step
is a sufficiently detailed inventory of sites and local conditions,
plus an appraisal of the likely sequence in which census operations
should be undertaken. That is, there is little prospect for producing
a fully satisfactory estimate of the entire manatee population in
a year or two. There may be, however, a good prospect for developing
a satisfactory index to abundance and for improving that index in
the direction of converting it to a census in the future.
Some work on a temporary sonic tag may be worthwhile. Quite
possibly some sort of "hedgehog" sonic tag would stay in place long
enough to be useful in correcting direct counts. Probably the thick
and tough integument of a manatee would not be damaged by the impact
of an attachment surface having many short and possibly barbed bristles.
Such a device would require an appreciable amount of development
and testing, but it appears that census work in the most heavily
obscured waters will require either an "active" tag or acoustic
Improving the accuracy of counts in warm water refugia does
not resolve the census problem if there are significant numbers of
manatees present away from these areas when the peak counts.are
made. Some data on this point are available, and more can be collected.
An important feature of the high counts in warm water refugia is
that they are temporary phenomena, and that a substantial fraction
of the population may be thus concentrated. Under such circumstances,
it may be possible to utilize the resulting change in counts away
from the refugia to provide an estimate of population size. A recent
description of a "removal" method for estimating abundance is available
in Eberhardt (1982). The circumstances here are different, but the
same principle may well apply. A critical issue may be the variability
in replicate counts away from the refugia when much of the population
is concentrated in the warm-water area. Since these counts will
presumably steadily decrease as the counts at warm water refugia
increase, some sort of regression model may be useful in an estima-
tion scheme using the "removal" principle.
Capture-recapture estimates may be attempted on the basis of
two marking methods. One would use the tags developed for survival
estimation, assuming that these are external and color-coded to at
least year of application. The second would be based on recognizable
scar patterns of individual manatees. Some experience is now avail-
able on use of scar patterns at Blue Spring, Crystal River (Rathbun,
pers. comm.), Brevard County (Shane 1981), and in Hobe Sound (Packard
1981). The experience at Blue Spring shows that a file of photographs
is an essential adjunct to use of scar patterns for identification,
due to the difficulty of producing unambiguous sketches of various
features (Rathbun pers. comm.). Effective use of scar patterns requires
both clear water and good access to individual manatees. Continued
participation by the same small group of experienced observers is
also essential. Rathbun (pers. comm.) notes the history of the Crystal
River and Blue Spring populations as a case in point. In the early
records of identities of individual manatees there appears to be a
considerable change in composition, while the recent records (kept
by a few people using the photo file) show little turnover.
Research on the utility of scar patterns as "tags" is needed,
particularly by way of verifying accuracy of identification. The
few tests done thus far have involved matching photographs. It would
be worthwhile to develop a series of photographs for that purpose
by taking sequences of photos of the same individuals, so that there
is no question that the photos are of the same animal. Photos of
tagged individuals might also be used, with the tags blocked out.
A considerable amount of attention should be paid to the statistical
design used in presenting such a series of photos to investigators,
and opportunities should be sought to test scar pattern identification
in a "closed" population (i.e., in an area with a confined population).
Tests in the field are of course to be preferred to work with photos.
Planning for a large scale capture-recapture study will depend
on a better understanding of manatee movement patterns. As remarked
above, it may well be desirable to do some immediate marking with
pedunclee tags" to obtain preliminary data on movement patterns and
exchange between regions. If such work were initiated now, results
could be available by the time a suitably efficient tag has been
developed. Also, the movement data obtained from a limited number
of peduncle tags might make it feasible to do more work on local
populations based on scar pattern recognition.
Marking presumably would most efficiently be done in warm-water
refugia. Resighting in the same areas raises the question of biases
due to non-representative sampling of the population. Assessing the
likely nature and extent of such biases will require a better under-
standing of seasonal movement patterns (and hence requires marking).
Most likely, population estimates will have to be built up over a
number of years and, in common with survival estimates, will depend
on continuing introduction of marks. With such a scheme, there is
a prospect of estimating survival through the Seber-Jolly equations.
Some modifications will be required to allow for the fact that resighting
rather than recaptures provides the primary data, and new marks are
introduced in annual "batches". Some experience with such a scheme
is available in Siniff et al (1977).
~ ~IIII~ ~------~--l~--P--s~--
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