journal of flerpetolWgy, Vol 25, No 4, pp 419-424, 1991
Copyright 1991 Society for Ihe Study of Amphibians and RepHles
Long-term Changes in a Population of
Alligator mississippiensis in South Carolina
LAURA A. BRANDT'
Savannah River Ecology Laboratory, P.O. Drawer E, Aiken, South Carolina 29802, USA, and
Department of Biological Sciences, Florida Internaononal University, Miami, Florida 33199, USA
ABSTRACT.-The status of the American alligator in a man-made reservoir in South Carolina was deter-
mined through the use of mark-recapture and mark-resighting techniques. Results were compared to
studies conducted during the 1970s to determine changes in population size, population structure, and
number of nests. Over the last 14 years the number of alligators in Par Pond has more than doubled (from
an estimated 110 to 266), and the size structure of the population has shifted from one with a high proportion
of large adults to one with a high proportion of juveniles (-1.8 m). The sex ratio of animals captured from
1986-1989 (2.6 male: 1 female) was similar to that found in the 1970s (3.2:1; Murphy, 1977). The number
of nests and the number of successful nests was variable among years, ranging from no nests in 1986 to 8
nests in 1988 for an average of 4.0 nests per year, an increase from the 2.3 observed during the 1970s.
Overall, the alligator population in Par Pond appears to be quite healthy, showing an overall average
annual exponential rate of increase (r.p.a) of 6% and a juvenile r.p.a. of 10%. It is expected that the population
will continue to increase in size, but that the average annual exponential rate of increase will decrease as
the carrying capacity of Par Pond is reached.
The American alligator,, .' mississippien-
sis, occurs i .. .. .. ...i the southeastern United
States and is the best studied of the 21 extant
species of crocodilians (Brisbin et al., 1986).
However, most studies have concentrated on
Louisiana (Mcllhenny, 1935; Chabreck, 1966;
Joanen, 1969; Joanen and McNease, 1970, 1972,
1975; McNease and Joanen, 1974, 1978) and
Florida populations (Hines et al., 1968; Fogarty,
1974; Goodwin and Marion, 1978, 1979; Deitz
and Hines, 1980). Many of the studies from oth-
er areas (i.e., North and South C ....i:.. are
short-term studies conducted in coastal areas
and are in the form of .. -*. i. or
theses (Bara, unpubl.; Murphy, 1977; Fuller,
1981; Wilkinson, 1 These studies have
shown that there is seasonal and yearly varia-
tion in aspects of '::'. ecology (such as in-
dividual growth rates and clutch size) that cause
changes in population size and structure. Un-
fortunately, few studies on alligators have re-
ported on **.. in population characteristics
One of the reasons for the paucity of long-
term studies on changes in ., . .i
is that alligators are slow-growing and long-
lived, so that changes in population size and
structure occur over decades. At Par Pond, South
SPresent Address: Department of Wildlife and
Range Sciences, University of Florida, Southwest
Florida Research and Education Center, P.O. Drawer
5127, Immokalee, Florida 33934.
( .. I had the opportunity to study an al-
ligator population in the northern portion of
the alligator's range for which data were avail-
able on the size and structure of the population
10-15 years ago i .i : :., 1977). Studies con-
ducted from 1972-1978 ... !./, 1977, 1981)
indicated that the population of -::,'-... in
Par Pond consisted of ..... :.. '. 110 alli-
gators (95% C.I. 48-215; 2.1 ::'. ** 'km) with
a 3.2:1 (male: female) sex ratio, a high
tion of adults :- relatively few juveniles
(51.8 m total length, TL) and low reproduction
(2.3 nests/yr). Murphy (1981) estimated that Par
Pond could support a : .!: of approxi-
mately 500 individuals (9.4 i-... The purpose
of this study was to determine the status of the
Par Pond population and to compare the cur-
rent population size and structure to that of the
Par Pond ..: .. during 1972-1978.
Par Pond is an 1120 ha cooling reservoir lo-
cated on the D : .. ..... of Energy's Savannah
River Site in Barnwell County, South Carolina
(Gibbons and Sharitz, 1981). Par Pond received
thermal, :r:... .: from R-reactor (into the North
Arm) from 1961 to 1964 and from P-reactor (into
the Hot Arm) from 1959 to 1987. 7: ...
schedules varied from year to year and only the
Hot Arm'.-.. :... '. 10% of Par Pond) was
affected when P-reactor was operational. The
remaining 90' .: -.... : Arm, West Arm and Main
S) is .. ::. a warm, monomictic lake.
The effect of thermal effluent on :: has
LAURA A. BRANDT
E 40 -
I-lT l. iK ill
<2 3 4 5 6 7 8 9 10 10
Size class (feet)
FIG. 1. Size class distribution (grouped by foot in-
tervals to correspond with previous studies) for 186
, captured in Par Pond 1986-
1988 (this study), and 95 .'- .. captured from
1974-1976 (Murphy, 1977).
been examined by Murphy and Brisbin (1974)
and Murphy (1977) and will not be discussed
here in detail.
Alligators 51.25 m TL were captured at night
by hand from an airboat. Animals >1.25 m TL
were captured using a self-locking wire noose
attached to PVC pipe, or by baited trip snares
(see below). Total ., :,. snout-vent length
(SVL; +0.1 cm) and mass (+1 g for animals <5
kg; +100 g for animals <10 kg; +200 g for
animals <35 kg and + I kg for animals >35 kg)
were recorded for each animal. Animals >35
cm TL were sexed by cloacal examination and
all animals were given an individual mark by
clipping scutes. A.::. .: . >1.8 m TL were fit-
ted with color-coded collars with either red
(male) or yellow (female) numbered cattle ear
Captures of animals 1.8 m TL were classi-
fied as pre-census, prior to July 1987, and then
by capture period (period 1, July 1987; period
2, March 1988; period 3, July 1988; period 4,
September 1988). Capture periods consisted of
three consecutive nights of capturing animals.
Attempts were made to capture animals of all
sizes. All of Par Pond was searched during each
capture period, and .i... .captured on nights
1 and 2 were held until the third night when
all animals were released.
Baited trip snares (Murphy and Fendley,
Murphy and Fuller, 1982) were set during three
separate trapping periods: September and Oc-
tober, 1987 (162 trap nights); April and May
1988 ' trap nights); and September 1988 (75
trap nights), Traps were set throughout Par Pond
in locations where animals had been observed
during night surveys. Traps were baited with
fish or chicken and checked once a day (early
morning). Because few animals were recaptured
using baited trip snares, and the trapping de-
sign did not meet the *** *:.: of the Pe-
tersen estimator (Seber 1973, 1986), aerial sur-
veys were used to calculate the number of
*. **.. >1.8 m TL. Aerial surveys were con-
ducted around the entire perimeter of Par Pond
once each month during March, April, May,
July, and October 1988. A Hughes 300C heli-
copter with one observer and a pilot was used.
All surveys were flown at an altitude of 10-25
m and an airspeed of 20-30 knots between 0900
and 1600 h and lasted approximately 1.5 h. Lo-
cation, size of animals, and presence of collars
were recorded. Animals were counted as
"marked" if they had a collar.
The Petersen (Seber, 1973, 1986) estimator was
used to calculate the number of animals > 1.8
m TL and the number of animals :1.8 m TL.
In addition, estimates of the number of : .
tors <1.8 m TL were made using the Schnabel
(Schnabel, 1938) and Jolly-Seber I :i 1965;
Seber, 1965, 1986) estimators.
Data from night counts conducted during the
fall of 1985-1989 (Brandt, :'"'-. and data from
previous studies (Bara, unpubl.; Murphy, 1981)
were used to calculate the average annual ex-
ponential rate of increase for the population
from 1971-1989 by regressing the natural log
of the relative density of :.. -' .i .. .
km) against year (C 1 ., 1977).
A total of 186 different non-hatchling and 157
hatchling alligators was .:. .... 1 from May
1986-October 1988. Thirty-nine of the non-
hatchlings '. .... :.. : in size from 1.50 to 3.85 m
TL) were captured by baited trip snares during
519 trap nights.
Of the 186 non-hatchlings captured 41
were adults (>1.8 m TL) and 145 (78%) were
juveniles (<1.8 m TL). This is different from the
adult to ....- ratios reported by Wilkinson
(1983) for a coastal South Carolina population,
and by Chabreck (1966) for a Louisiana marsh
population (16% adult, 84% ; .. .. :, x' = 4.618,
df = 1, P < 0.05 for both). The distribution by
size class (grouped at foot intervals to corre-
spond with previous studies) was significantly
different from that found by Murphy (1977) for
the same population in 1972-1976 (Fig. 1;
G-Test, G = 222.058, df = 2,6, P < 0.001), and
different than that reported by Chabreck (1966)
for a i : ..... inhabiting a coastal marsh in
Louisiana (G-Test, G = 75.84, df = 2,6, P < 0.001).
The overall sex ratio was 2.6:1 (Fig. 2). There
was no difference between the .;- ..i.. (2.7:1)
and adult sex ratios (2.6:1, X2 0.0046, d.f. = 1,
P > 0.05) or between the current adult sex ratio
and the 3.2:1 sex ratio reported by Murphy in
1977 (X2 = 1.223, df = 1, P > 0.05), or the 2.3:1
ALLIGATOR POPULATION CHANGES
60 70 80 50 40 30 20 10 0 10 20 3 o0 4 50 60 70 80
FIG. 2. Size and sex ratio of 186 non-hatchling
:: captured in Par Pond 1986-1988.
ratio reported in 1981 1981; x7 =
0.1544, df = 1, P > 0.05).
A test of the assumption of equal probability
of capture was not significant (x2 = 5.7280, df
= 3, P = i : * and no tag loss was observed
as all animals were marked by clipping scutes;
also, no animals with collars were missing col-
lars on subsequent captures. The number of al-
ligators > 1.8 m TL estimated using the Petersen
estimate ranged from 94 + 72 (estimate + 95%
confidence interval) to 122 + 45 with a :. !.t
ed average (calculated using the equation ([ 1 /
var,X,]/1l/var,; Snedecor and Cochran, 1967)
for April to October 1988 of 108 + 18 (Table 1).
Estimates for the number of .::.. : <1-8
m TL ranged from 127 + 19 to 209 + 66 using
the Petersen estimator, from 127 + 36 to 150
417 using the Schnabel estimator, and from 152
+ 19 to 51 + 22 using the Jolly-Seber estimator
(Table 2). An overall population estimate of 266
38 .ii:; .:... was calculated by combining the
TABI.E 2. Population estimates for juvenile alli-
gators in Par Pond.
Month Petersen Schnabel Jolly-Seber
160 + 36
127 + 36
Average 158 20 142 + 16
51 + 22
Petersen estimate for adults with the Petersen
estimate for juveniles.
The average exponential rate of increase for
the population (excluding hatchlings) based on
changes observed in night counts was 0.06, and
was ,.* :,f different from zero (F = 72.93,
df = 1,10, P < 0.0001, r2 = 0.879; Fig. 3). Esti-
mates of rates of change based on captures were
0.10 for .. .. 0.03 for adults, and 0.06 for
the total population.
Sixteen nests or pods of young were located
from 1985-1988, five in 1985, none in 1986, three
in 1987, and eight in 1988. Based on nest and
pod locations this represents the reproductive
output of at least eight females and an average
of 4.0 nests per year.
The number of alligators inhabiting Par Pond
has more than doubled in the last 14 years, as
shown by both capture data and the calculation
of the average exponential rate of increase based
on late summer night-time eyeshine counts.
:' ...'.: .: estimates indicate that the popula-
tion has increased since the 1970s from 110, or
2.1 alligators/km, to 266, or 5.0 .i .., /km.
It is possible that differences in population es-
timation techniques between the two studies
TABLE 1. Population estimates for juvenile and adult alligators in Par Pond, 1987-1988.
Number Number Number marked estimate Confidence
Month captured marked in population (Petersen) Standard error interval
Mar 88 72
Apr 88 64 11 21 122 23.1 45
May 88 48 12 25 100 18.0 35
Jul 88 15 4 25 94 36.8 72
Oct 88 10 3 35 117 53.9 106
Average 108 5.7 18
Jul 87 35 27 98 127 10.0 19
Mar 88 27 15 116 209 33.5 66
Jul 88 28 26 128 138 6.4 13
Sep 88 16 13 130 160 18.2 36
Average 158 10.0 20
Average total population 266 38
LAURA A. BRANDT
ob~T The lower rate of increase calculated for adults
0.4 D in this population may be a result of the gap in
0.. the number of males between 213 and 305 cm
9 40 TL that were captured. This could represent sev-
eral bad reproductive years in a row in the mid-
-a.2 late 1970s (animals in this size class would have
-o.4- hatched in 1975-1978; Brandt, unpubl.). How-
-o.60. ever, this pattern has been observed in Croco
0.00 dylus porosus in Australia (Messel and Vorliceck,
_-1.0 1986) and probably represents the movement
197[07o i5 a980 1985 !99o of subadult males into inaccessible areas to avoid
Y or aggressive interactions with large adults.
IG. 3. Average annual exponential rate of in- The skewed sex ratio (2.6:1) observed in the
se for the Par Pond alligator population from 1971- current population is not different from that
9 (1971-1978 data are from Bara, 1971, 1972, 1973, reported by Murphy (1977) and does not seem
4; Murphy, 1981). to be unusual for alligator N. .! ..... Nichols
and Chabreck (1980) reviewed data from wild
ount forsomeof theobserved .- . For populations in Louisiana and found 11 of 14
mple, Murphy did not use aerial surveys, a populations to have sex ratios biased toward
hnique that may have increased the total area males. Wilkinson (1983) reported a 1.5:1 male-
veyed, but which is biased against certain biased sex ratio for populations in coastal South
e classes and probably represents an under- Carolina from 1979-1983, and Fuller (1981) re-
imate of the population (Caughley, 1974). ported the same ratio for both juveniles and
wever, the night-time eyeshine counts adults in Lake Ellis Simon in North Carolina.
)wed the same average yearly increase of 6% It is possible that the bias toward males in the
the overall population estimate indicating adult population is a result of capture technique
t the differences in population estimation rather than a true indication of the animals uti-
hniques are not substantial. lizing Par Pond. Males are known to favor open
he overall annual exponential rate of in- water habitats (Joanen and McNease, 1972) and
ase (r.p.a.) observed in this population is sim- females marsh or swamp habitats (Joanen and
Sto rates of change (5-7%) reported for pop- McNease, 1970). Males favoring the more open
tions of Crocodylus porosus recovering from areas would have been more likely to be trapped
controlled hunting in the Northern Territory in this study, as traps could not be set equally
Australia (Bayliss, 1987). In addition, the dif- in open water and marshy habitats (such as the
ent rates of increase observed for the adult backs of coves). The June aerial survey data also
d juvenile segments of the population is con- indicate the presence of animals that may be
tent with data on other increasing popula- untrappable and unsightable during the rest of
ns. the year because of a preference for more se-
The differences observed in the r.p.a. of adult cluded areas (i.e., females inhabiting the back
d juvenile alligators in Par Pond reflects the portions of coves).
nges that have occurred in the structure of Alligators <1 m do not seem to be segregated
population, going from a population pre- by sex, but rather stay together in pods. Thus,
minantly made up of large adults to a pop- the juvenile sex ratio is probably not biased by
tion with a high proportion of juveniles. The capture technique. The juvenile sex ratio was
rease in the number of alligators and the not different from the adult sex ratio, indicating
ft in the size distribution is what would be that either more males are produced (as a result
pected for a colonizing population. Par Pond of nest temperatures >33 C; .. -..-.. and Jo-
now 30 years old. The alligators that origi- anen, 1982) or that females suffer higher mor-
ly colonized the reservoir were probably tality than males, particularly during the first
ults inhabiting the creeks and ponds that were year. The observed juvenile sex ratio is more
aimed to create the reservoir, and subadults likely to be a result of the former, because males
it moved in from surrounding areas. Initially produced at high temperatures are smaller than
re were probably only a few mature females, females produced at low temperatures (Fergu-
ich might not have nested every year. In son and Joanen, 1982), and smaller animals, un-
74 there were approximately 15 adult females less they have faster growth rates, would be
... 1977); currently there are approxi- susceptible to predation or mortality for a lon-
tely 28, indicating that despite what ap- ger time. This would result in a female-biased
ared to be low reproduction in the mid-1970s population rather than the observed male-bi-
urphy, 1977), the extent of reproduction and ased population.
ruitment over the last 14 years has been ad- Based on the current population structure and
uate to increase the adult population, the rate of increase over the last 14 years the
ALLIGATOR POPULATION CHANGES
alligator population in Par Pond appears to be
quite healthy and should continue to increase;
however, the overall rate of increase in Par Pond
is expected to decrease as the carrying capacity
is reached. As this happens the number of nests
in the reservoir should stabilize, and it is likely
that some females will move out of Par Pond
into nearby suitable unused areas for nesting.
Acknowledgments. -This work was supported
by the Savannah River Ecology Laboratory
through a Department of Energy contract to the
University of Georgia (DE-AC09-76SR00-819),
the Graduate Student Education F.... and
Oak Ridge Associated Universities Graduate
Student Travel Contract (T-417). I would like
to thank the ': i :.. people for their assis-
tance: R. Estes, M. Fulmer, R. Fischer, J. Greene,
M. Jackson, D. Kling, J. .. i.. B. Maddox, T.
Mills, T. Owens, D. Scott, R. Kennamer, W. Har-
vey, P. Dixon, S. Busa, R. Nodell, and L. Hasty.
G. H. Dalrymple, J. W. Gibbons, M. Tracey, F.
J. Mazzotti, P. Moler, P. Bayliss, B. Magnuson,
and R. Seigel provided helpful comments on
earlier versions of this manuscript. This paper
is part of a thesis submitted in partial fulfillment
of the requirements for the Master of Science
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