Title: Status of the American crocodile (crocodylus acutus) at a power plant site in Florida, USA
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Permanent Link: http://ufdc.ufl.edu/UF00066437/00001
 Material Information
Title: Status of the American crocodile (crocodylus acutus) at a power plant site in Florida, USA
Physical Description: Book
Creator: Brandt, Laura A.
Mazzotti, Frank J.
Wilcox, J. Ross
Barker, Paul Douglas Jr.
Hasty, Gerald Luke Jr.
Wasilewski, Joe
Affiliation: University of Florida -- Department of Wildlife Ecology and Conservation
University of Florida -- Department of Wildlife Ecology and Conservation
Publisher: International Herpetological Symposium, Inc.
Publication Date: 1995
General Note: Drawn from Herpetological Natural History, Vol. 3, No. 1, pp. 29-36, 1995
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Bibliographic ID: UF00066437
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.


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Herpetological Natural History, 3 (1), 1995, pages 29-36.
1995 by the International Herpetological Symposium, Inc.


Laura A. Brandt Frank J. Mazzotti
Department of Wildlife Ecology and Conservation, University of Florida, P.O. Box 110450,
Gainesville, Florida 32611-0450, USA

J. Ross Wilcox
Department of Environmental Affairs, Florida Power & Light Company, P.O. Box 088801,
North Palm Beach, Florida, 33408, USA

Paul Douglas Barker, Jr. Gerald Luke Hasty, Jr. Joe Wasilewski
Florida Power & Light Company Land Utilization, ii: ... : : Florida, 33030, USA

Abstract. Surveys for distribution and nesting of American crocodiles (Crocodylus acutus) were con-
ducted from 1983-1993 at Florida Power & Light Company's Turkey Point power plant site in Homestead,
Florida, USA. The number of crocodile nests, hatchlings, and non-hatchlings observed per survey have
increased over the 11 year period. The percentage of animals in each size class fluctuated from year to year.
It is estimated that 24-30 non-hatchling crocodiles reside on the site. First year survival of hatchlings
averaged 8..:' and varied from year to year.

Key Words. American crocodile; Crocodylus aculus; Florida; Population ecology; Power plant.

The American crocodile (Crocodylus acutus) is a
primarily coastal crocodilian that occurs in parts of
M6xico, Central and South America, the Caribbean
and, at the northern extent of its range, in southern
Florida, USA. Historically in Florida, crocodiles
occurred as far north as Ft. .1 on the west coast
and Lake Worth on the east coast (Fig. 1). Currently,
there are three primary crocodile nesting areas in
southern Florida: northeastern Florida Bay within
Everglades National Park TI P), North Key Largo
(NKL), and Florida Power & Light Company's
(FPL) Turkey Point power plant site (TP) in Dade
County, Florida (Fig. 1). This paper discusses the
status of crocodiles at TP and includes data on
survival, nesting, and habitat use based on eleven
years (1983 through 1993) of monitoring.


Turkey Point is a 2388 ha site occupied primarily
by cooling canals serving four electrical generating
units. These 60 m -. :.!. ::' -.'..: canals comprise
a closed-loop system and alternate with 40 m wide
earthen berms. Canal water temperature averages

38 C and ranges from 34 to 42 C in summer. Salinity
inthecooling .: : .. ,:: 36 parts per thousand
(ppt) and ranges from 15 to 42 ppt depending on
seasonal rains (Mazzotti et al. 1986). Eight other
non-connecting canals of varying width, depth, and
salinity are located adjacent to the cooling canals
(Fig. 1; see Gaby et al. 1985 for detailed descrip-
tion). The cooling canal system (CCS), Interceptor
Ditch (ID), return canals (RC), Sea-Dade Canal
(SDC), and C107 are the areas primarily used by
crocodiles, though crocodiles also have been ob-
served in L-31, the test canals, and the moat.
Adult crocodiles were first observed at TP in
1976. In 1978, nesting was .: .. ...:. when
hatchling crocodiles were observed and captured in
the cooling canals. Gaby et al. (1985) reported on
the status of Crocodylus acutus at TP for the period
1978-1981. It was estimated that 17-19 non-hatchling
crocodiles occurred at TP ,- adult > 2.25 m, and
75% ::: :::i and subadult), with a male to female
sex ratio of 1:1.3. Four nests were located at TP from
1978-1981, and Gaby et al. (1985) estimated that
nests at TP produced nearly 25 hatchlings each year,
approximately 10% of the total annual number for

Herpetological Natural History, Vol. 3 (1), 1995

Turkey Point Power Plant

Biscayne Bay

Ft. K

0.5 KM

Model Land
Canal North


Sea Dade Canal

Figure 1. The Turkey Point power plant site, Dade County, Florida. =North pump station; 2- South pump station.



Brandt et al. Status of the American Crocodile, Crocodylus acutus

Florida. Because of TP's importance as crocodile
nesting habitat, FPL established a long-term croco-
dile monitoring program to insure that the operation
and maintenance of the site was compatible with
:. : survival.
Nesting success, juvenile growth, and survival
were the factors chosen for monitoring at TP. Popu-
lation trends were determined by locating and fol-
lowing the fate of nests and hatchlings and by
examining the distribution and abundance of ': :. -
ent size classes of. : ..:: on the site.
Nest surveys were conducted within the cooling
canal system from April: ....... 1. August 1984-1993.
Berms were searched by day on foot for signs of
crocodile activity (trails, tail drags, and .... ..
Areas of past nesting and recent crocodile activity
were searched in an attempt to locate nests before
.. :: .:..! however, because :.. :ili at TP are
primarily hole nesters, most nests were found only
after -.. i::..: had been located during night sur-
veys. Night surveys for i. :- !A.. were conducted
five nights per week during July and August. If
hatchlings were located in an area that had not been
identified as a nest site, that area was surveyed on
.:. I : . ... day to locate the nest. Once a nest
was located, it was examined for failed and hatched
eggs and the area surrounding the nest was described
(soil type, vegetation, distance from shore, and nest
.i mound or hole).
: :.:... were captured by hand or landing net,
measured to the nearest mm (snout-vent length
[ .. i j. total length [TL], and head length [HL]),
weighed,givenan individual mark .. ....
S, ,., i , and released either at the site of
capture or in a designated area nearby.
To monitor the distribution of crocodiles at TP
and to collect data on hatchling and -: ...... : sur-
vival, night surveys were conducted by airboat, jon
boat, canoe, or truck once or twice per week through-
out the study area during all seasons from 1983-
1993. Animals were located by their eyeshines
using .-::: ::: : + candle power :i; .::: :-witha4
volt Wheat cap lamp. Location, size estimate, and
behavior were recorded when an animal :.!.K..
Crocodiles< 1 mTL were captured by .... : i ....:..
net, or pilstrom .: Those 1-2 m TL were cap-
tured using i'.: i:::, wire nooses. No attempts
were made to capture animals > 2 m. Captured
animals were measured, sexed (when possible), in-
dividually marked, and released at the capture site.
Temperature and .*. : were recorded for each
capture site.

Day surveys for animals in the -. !:-- system
and adjacent canals were conducted :......:.. .:
Surveys of the ID were used to document seasonal
..... ;....: use of this canal and to look for tail drags
and i *: ::: :i::: 1 ::i ** ::: ::: ofanim als from
one area to another (crossings). Surveys were con-
ducted by truck before noon :.. .: ::: < 3 h after
sunrise). Estimated size, location, and behavior
were recorded for each .. ....:.. sighted; number
and : . .. : ....: of tail .A. :.. also were recorded.
Foranalysis of changes i:: : : : ::
all sightings and captures were coded as: ...
<0.7m,:. .::,. 0.71-1.50 m, subadults 1.51-2.25
m, adults >2.25 m, or unknown. To correct for
'.:: ... in survey effort and biases due to differ-
ences in habitat use, results are expressed as croco-
. ... when ..::. ::... the number of croco-
diles : :::'. within the same area, or average
number of crocodiles/survey when comparing the
number of crocodiles observed in an area among
years. C. ... in relative abundance were exam-
ined by. .. :..:.., the average exponential rate of
increase (r), which is the slope of the least-squares
line of:

In (density ::: : :.::: : + r(time)

where the density indices are number of nests/year
or average number of ........ ..... (:. .
1987). I i: .. :I. :: : : :. : :: :: using
data only for ... ...: > 0.70 m.


Fifty-five nests were found from 1984 through
1993. All were hole nests dug into oxidized peat
substrates in or near vegetated areas, at an average of
8.1 m from the shorelines (range 3-15 in) of the
cooling canals. Median hatching date was 15 July,
range was 27 June to 15 August.
The number of nests found per year has in-
creased from two in 1978 (Gaby ct al. 1985) to a
maximum of 12 in 1992 (Fig. 2). There was no
significant change in the number of nests located
from 1978-1985. There was a significant increase in
the number of nests located from 1986-1993. The
average, annual, i ..: .:!:. rate of increase was
significantly different than zero and was: or
23% per year (f=54.67, P<0.01, df=7). At least 25
i::.i : :::.: : i ::. *: have been used. The number
of ...:: '. captured each year varied with the
number of nests and .:.- :? from 0 -1": during the

Herpetological Natural History, Vol. 3 (1), 1995

Figure 2. Number of Crocodylus acutus nest sites found
and hatchlings marked at Turkey Point 1978-1993. 1978-
1981 data are from Gaby et al. (1985).

period of 1983-1993.
Hatchling size analyses were based on measure-
ments from 786 hatchlings captured within five days
of hatching during 1984-1993. Hatchlings averaged
and 59.4+9.9 g (n=785). TL of hatchlings differed
significantly among 27 different nests with > nine
hatchlings (Kruskal-Wallis, h=358.2, P<0.01,
df=488) as did SVL (h=361.1, P<0.01, df=488).
Mean TL ranged from 23.0+0.41 to 30.5+0.42.
First year minimum known alive survival of
hatchlings varied from 0% in 1988 to 33% in 1986
(.= 8.5%). Minimum known alive survival to 36
months averaged 5.9% for 1984-1990 hatchlings,
with a range of 0-16.7% (Table 1). Low survival was
associated with high salinity. This is illustrated by
two groups of hatchlings from 1984, when a nest was
found in the return canals (Fig. 1). Because this nest
was close to the intake of the generating units, the
hatchlings (n=18) were relocated to an area in the
southwestern portion of the system adjacent to the
freshwater ID. Twelve of the 18 relocated hatchlings
survived for at least six months. During the same
year, 16 hatchlings were left near their nest site in the
hypersaline cooling canal system. These animals
had the lowest growth rate of crocodiles at TP (pers.
obser. 1993), and none were captured more than
three months after hatching.
The 1847 surveys (860 day and 987 night) con-
ducted from 1983-1993 resulted in 4658 sightings of
crocodiles. Of these,2691 were made during theday
and 1967 at night. The majority (58%) of day

60 A




1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993
-m- Juveniles -A Sub-adults Adults

Figure 3. Percentage of three size classes of Crocodylus
aculus observed during day and night surveys 1983-1993
(juveniles=0.71-1.50 m; subadults=1.51-2.25 m;

surveys were of the ID while the majority (60%) of
the night surveys were of the CCS and RC. Other
areas were surveyed nearly equally during the day
and night. The average number of crocodiles ob-
served per night survey was highest in March and
June and lowest in October and January. The aver-
age number of crocodiles observed during day sur-
veys was highest in January and February. Adults
and subadults were observed more often during day
surveys. Adults were observed at a rate of 0.69 +
0.27 crocodiles/survey during the day compared to
0.07 + 0.04 crocodiles/survey at night (paired t-test,
t=7.98,P<0.01,df= 10). Subadults were observed at
a rate of 2.13 + 1.08 crocodiles/survey during the
day and 0.48 + 0.16 crocodiles/survey at night
(t=5.48, P<0.01, df 10). Hatchlings were observed
significantly more at night (0.87 + 0.69 crocodiles/
survey at night compared to 0.07 + 0.09 crocodiles/
survey during the day; paired t-test, t=-3.67, P<0.01,
df-10). There was no significant difference in the
number of juveniles observed during the day than
observed at night. The percentage of sightings in
each size class has fluctuated from year to year
(Fig. 3).
The average exponential annual rate of increase
(r) for the mean number of non-hatchling crocodiles
observed per survey for day and night surveys from
1983-1993 combined, was significantly different
from zero (f=52.79, P<0.05, df=10; Fig. 4) and was
0.09 or 9%. This indicates that, based on all surveys,
the average number of non-hatchlings observed per
survey has increased since 1983. The regression of

Brandt et al. Status of the American Crocodile, Crocodylus acutus

TABLE 1. Number of Crocodylusaculus known to be alive at 1-4 years of age from clutches from 1984-1992. Percent
is percent of the hatchlings surviving to that age class.

Year # Hatchlings 1 yr (%) 2 yr (%) 3 yr (%) 4 yr (%)

1984 34 2(5.9) 2(5.9) 2(5.9) 2(5.9)
1985 21 3(14.3) 2(9.5) 2(9.5) 0(0)
1986 6 2(33.3) 1(16.7) 1(16.7) 1(16.7)
1987 30 4(13.3) 2(6.7) 2(6.7) 1(3.3)
1988 49 0(0) 0(0) 0(0) 0(0)
1989 100 3(3.0) 0(0) 0(0) 0(0)
1990 101 5(5.0) 1(1.0) 1(1.0)
1991 108 2(1.8) 0(0)
1992 154 1(0.6) -

Mean % 8.5% 5.0% 5.9% 4.3%

the average number of non-hatchling crocodiles
observed per night survey and year was not signifi-
cantly different from 0, indicating no significant
increase from 1983-1993 based only on night sur-
veys, while the average exponential rate of increase
for day surveys was significant (f=25.95, P<0.05,
df 10; r =0.18).
During the 1983-1993 period, 66 individual non-
hatchling crocodiles (17 male, 29 female, 20 un-
known sex) were captured. The total number of
individuals captured per survey increased up to
1986 and has fluctuated since then. Based on the
number of individuals captured, number sighted in
the ID in one day, and the number of nests, it is
estimated that 24-30 non-hatchling crocodiles re-
side on the Turkey Point site.
The sex ratio for 1983-1993 was 1:1.7
(male:female, 64% female) which does not differ
from a 1:1 sex ratio (X2, P<0.05). The percentage of
females captured has fluctuated between 33% of all
sexed animals (1990) and 100% (1984 and 1993).
Juveniles were observed in the cooling canals in all
years and in the return canals in 1986 and 1988-
1993. Sightings of juveniles in the ID, C107, and
SDC are less consistent and vary from year to year.
The number of sightings of juveniles in the CCS
decreased up until 1992 and then increased in 1993.
The percentage of all juvenile sightings that have
been in the CCS has remained approximately the
same (Figs. 5, 6). Subadults were observed consis-
tently in all areas. Adults were observed most often
in the ID, followed by the cooling canals, primarily

during nesting (April-August).


The number of crocodile sightings at TP has
increased since Gaby et al.'s (1985) 1978-1981
study. Part of this reflects an increased awareness of
the crocodiles' presence on the property, and part is
due to an actual increase in the population. Gaby et
al. (1985) estimated the population to consist of 17-
19 individuals in 1978-1981. By 1985 the popula-

1983 19B4 1985 1986 1987 1988 198-) 19 1991 1992 1993
SDay A survey A Nig

Figure 4. Natural log average number of non-hatchling
Crocodylus aculus observed per survey in all areas of
Turkey Point from 1983-1993. Regressions of Ln number
vs. year for day and all surveys were significantly different
from zero (see text).

Herpetological Natural History, Vol. 3 (1), 1995

R 12

Z 04

1983 1984 1985 1986 1987 1988 19B9 1990 1991 1992 1993
E IM C107 = SDC

Figure 5. Number of juvenile Crocodylus acutus ob-
served per night survey in the major canals at Turkey
Point 1983-1993.

tion was estimated at 18-22 non-hatchling croco-
diles with a seasonal peak of approximately 50
individuals immediately after hatching (pers. obser.
1985). Currently it is estimated that 24-30 non-
hatchling crocodiles occur at TP. At least 12 breed-
ing females have used the site (12 nests in 1992).
The increase in the number of nests observed could
be due in part, to a greater search effort over the
years, but it also indicates an increase in reproduc-
tive effort. In 1986 and 1987, nests containing small
infertile eggs were found, suggesting the presence of
first-time nesters (Ferguson 1985). This suggests
that the increase in the number of nests is in part, a
resultof younger animals becoming sexually mature
on the site. Animals from off-site also may be
locating and selecting this area as suitable nesting
habitat. Animals marked in 1978 and 1979 should
now be sexually mature and may be returning to the
site to nest.
The number of crocodiles resident at TP appears
to be leveling off, and this would be expected for a
population reaching stability, perhaps as a site ap-
proaches carrying capacity. Fluctuations in the per-
centage of sightings by sizeclass have been greatest
for subadults and smallest for adults. Given the
stable number of adults observed, the increase in
nesting suggests that some animals nesting at TP are
probably occupying other habitats during most of
the year. Gaby et al. (1985) speculated that croco-
diles may not use an adjacent fresh water canal (L-
31) because of the presence of alligators. An alter-
nate explanation is that prior to 1982, the low density
of crocodilians at TP allowed them all to remain in

preferred habitats. Since 1981, however, the num-
ber of crocodiles in L-31 has increased from zero to
at least four. The distribution of sightings of croco-
diles within the cooling canals also has expanded,
with more sightings occurring in the return canals
and northern portions of the system. It may be that
the cooling canals, with their higher salinities, are
only marginal crocodile habitat, to which subadults
are being displaced (Messel et al. 1982). The ob-
served fluctuations in subadult numbers may be the
result of animals searching for suitable habitat and
then being displaced by adults during the breeding/
nesting season. Gaby et al. (1985) observed that
crocodiles of different size classes used habitats
available on the TP site differently, with the majority
of adult sightings in the ID and the majority of
juvenile and subadult sightings in the brackish ca-
nals (C107 and SDC). The patterns of distribution
observed in this study were similar, with adults
being observed most often in the ID, subadults in all
areas, and juveniles primarily in the CCS,but also in
the surrounding canals. The decrease in the number
of juveniles observed throughout the site up to 1992
may be due to interactions with an increasing num-
ber of subadults, high juvenile mortality, or increase
in human activity within the system; or it might
reflect changes in the availability of suitable juve-
nile habitat within the cooling canals.
Crocodylusaculus prefer sheltered areas such as
undercut banks and mangrove snags and roots that
are protected from wind and wave action (Mazzotti
1983). The cooling canals at TP are protected from
wind and wave action, and during the early 1980s,

= ccs RC R B ID
rl[-D C107 = SDC

Figure 6. Percentage of all individual Crocodylus aculus
observed per night survey in the major canals at Turkey
Point 1983-1993 that were juveniles.

Brandt et al. Status of the American Crocodile, Crocodylus acutus

most of the system had vegetated canal banks. In
1984, however, a 10-year program was initiated to
restore the canals to the design efficiency. As part of
this canal maintenance operation, all vegetation was
removed from all berms not designated as crocodile
nestsite sanctuaries. This eliminated potential cover
sites for crocodiles. In addition to removing poten-
tial cover, these maintenance activities, along with
an increase in night surveys, increased the amount
of human disturbance within the cooling canals
(Fig. 7). Combined with the growth in the number
of subadult crocodiles, this increase in human activ-
ity may have displaced juveniles that would nor-
mally have resided in the system to suitable habitat
elsewhere. Maintenance activities are ending and it
will be interesting to see if the number of juveniles
observed in the cooling system changes.
Because of low growth rates and low survival of
hatchlings in the cooling system, it was decided that
the relocation of hatchling crocodiles from areas of
potentially adverse conditions (areas of high salinity
and/or high plant or maintenance activity) would
continue. The policy of moving hatchlings from the
hypersaline portions of the system to lower salinity
areas has probably increased hatchling survival and
may have contributed to the observed increase in
crocodiles at TP. Survival of hatchlings and year-
lings is variable among years, and survival of differ-
ent age/size classes has been impossible to deter-
mine, since hatchlings disperse off site and may not
be recaptured until they return to the system as
adults. Average annual survival for hatchlings (8.5%)
is lower than that reported for alligators in a cooling
reservoir in South Carolina (52%, Brandt 1989),
alligators in north-central Florida (19%; Woodward
et al. 1987), C. porosus (38-46%, Messel et al.
1982), crocodiles on NKL (20.4%, P. Moler, pers
comm. 1993), but higher than for crocodiles in ENP
(<5%, pers. obser. 1984). However, because it is
impossible to distinguish between death and dis-
persal, the low observed survival rates also may
reflect movement of juveniles and subadults to habi-
tats adjacent to those surveyed. There is some
evidence for this because two 1979 hatchlings were
recaptured in the study area after periods of 10 and
12 years. In addition, animals from NKL have been
captured at TP and in ENP (pers. obser. 1990), and
an animal from TP has been captured on the main-
land side of Barnes Sound (P. Moler, pers. comm.
1990), demonstrating that animals do move from
site to site. Since animals > 2 m have not been
regularly captured at TP, it is not possible at this time

83 84 85 86 87 88 89 90 91 92 93
+- Equipment hours Juveniles observed -- Crocodiles captured

Figure 7. Number of juvenile Crocodylus acutus ob-
served per survey, number of non-hatchlings captured
per survey and yearly hours of operation of equipment
used for cooling canal restoration.

to quantify long-term crocodile survival and use of
the site by non-nesting animals. However, we do
know that TP is important for the overall survival of
crocodiles in Florida. It provides nesting habitat for
approximately one third of the known nesting croco-
diles in Florida. Because of the abundance of
suitable nest sites (high ground in a relatively undis-
turbed area), it is expected that the number of croco-
diles nesting at TP will continue to increase.


We would like to thank S. Busa, M. D'Orazio, J.
Lindsey, the staff at Florida Power & Light
Company's, Land Utilization, and the numerous
other individuals who assisted with field work or
logistical support over the years. C.A. Abercrombie,
J.W. Gibbons, and P.E. Moler provided valuable
comments on earlier versions of this manuscript.
This work was funded by Florida Power & Light
Company. University of Florida, Agricultural Ex-
periment Station Journal Series Number R-04550.


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