Herpetologica, 42(2), 1986, 191-196
1986 by The Herpetologists' League, Inc.
FIELD AND LABORATORY OBSERVATION ':- ON THE
EFFECTS OF HIGH T' :-.iPERATURE AND SALINITY ON
IIATCIHLING CROCODYLUS ACUTUS
FRANK J. MAZZOTTI', BARBARA 'I :: 2
MARK P. MCMAHON2, AND J. ROSS WILCOX'
'Graduate Program in Ecology, 213 Ferguson
Pennsylvania State U ... .' .. .. Park, PA 16802, USA
'Connell Associates, P.O. Box 341934, Coral Gables, FL 33134, USA
'Environmental Affairs, Florida Power and Light Company,
P.O. Box 14000, Juno Beach, FL 33408, USA
ABSTRACT: I" : thermal effluents raise the temperature and salinity of the canal
system at the Turkey Point power plant in southern Florida. Field and laboratory studies were
conducted to assess the responses of hatchling Crocodylus acutus to elevated temperatures and
salinities. Laboratory experiments that hatchling crocodiles should occur in the cooler,
less saline portion of the i.'. system, which field observations confirmed. Temperatures record-
ed at capture sites were similar to those at which crocodiles gained mass in laboratory studies.
Salinities recorded at capture sites were greater than those at which hatchlings could maintain
mass in laboratory experiments. Hatchling crocodiles also grew faster in the cooling canal system
than in cooler, less saline canals adjacent to the cooling system. Apparently hatchling C. acutus
have all of the requirements for survival in the human-altered habitat.
Key words: Crocodylus acutus; ** :' Temperature; i. Growth; Survival
THE effects of thermal effluent from rimental to the biota. Gibbons et al. (1980)
electric power plants on the quality and concluded that thermal effects on aquatic
stability of the environment have long systems could not be automatically clas-
been a matter of concern. -1. :... 1 tem- sified as detrimental or beneficial and that
peratures were at first thought to be det- the response of a system was species-spe-
HERPETOLOCICA [Vol. 42, No, 2
cific. Hence basic biological studies are
necessary before the consequences of
thermal alterations can be understood.
These consequences are particularly im-
portant when dealing with an endangered
species such as the American crocodile
Crocodylus acutus in Florida are at the
northern limit of their range and probably
near the limit of certain ecological toler-
ances (Kushlan, 1980). The Florida pop-
ulation of C. acutus was placed on the
Federal Endangered Species list in 1975.
In 1978, Florida Power and Light Co. ini-
tiated a study of American crocodiles at
the Turkey Point power plant near Home-
stead, Florida. In July 1978, hatchling
crocodiles were found in the power plant
cooling canal system. The cooling canal is
not only thermally enriched but is also hy-
persaline (Gaby et al., 1985).
Crocodilians occur in aquatic habitats
with varying temperatures and salinities
('.i :il, 1971). In recent years, the ability
of crocidilians to osmoregulate and to
thermoregulate under different environ-
mental conditions has received increasing
attention. In general, crocodilians have
preferred body temperatures between (30-
35 C) and voluntary critical, or lethal
maxima at temperatures between 35-38 C
... i :t et al., 1946; Johnson et al., 1976,
1978). The little data available suggest a
similar trend for C. acutus (Lang, 1979).
Crocodylus acutus, like Alligator missis-
;;'i-.:, : ; has a relative lack of physio-
logical specialization for life in saline water
(Dunson, 1970, 1982; Ellis, 1981; Evans
and Ellis, 1977; Mazzotti and Dunson,
1984). .......'::.: porosus is apparently
more : .: -1 for marine conditions
and can grow and survive in sea water
without access to fresh water (Grigg et al.,
1980). Mazzotti and Dunson (1'. i con-
cluded that growth of C. acutus can occur
in saline habitats if brackish water is pe-
riodically available, based on laboratory
The purpose of this study was to deter-
mine (1) the range of temperatures and
salinities in the cooling canal system when
hatchling crocodiles were present, and (2)
the potential effects of these temperatures
and salinities on the survival and growth
of hatchling crocodiles under field and
Field ( :: .: .. I
The Florida Power and Light Co. Tur-
key Point power plant is located in south-
eastern Dade County, Florida. The 2388
ha site is occupied primarily by a closed-
loop cooling canal system serving four
electrical generating units. The cooling
canal system is a series of alternating 60
m wide canals and 40 m wide earthen
berms and serves to circulate water to cool
the plant's condensers. Eight other canals
are located adjacent to the cooling canals;
nesting has only been documented in the
cooling canal system (Gaby et al., 1985).
Hatchling crocodiles were hand cap-
tured at night from a canoe or airboat us-
ing head lamps during 1978, 1979 and
1981. Hatchlings were permanently
marked by clipping tail scutes. Body mass
was obtained using 100 g, 300 g, or 1 kg
Pesola spring scales. All individuals were
released within 1 h at their capture site.
Surface water temperature and salinity
were measured at each capture location
with a Scientific Products thermometer
( 0.5 C) and an American Optical tem-
perature-compensated refractometer ( 1
ppt). In addition, salinity and minimum/
maximum water temperatures were mon-
itored weekly during 1980 at 17 stations
located in the cooling system and outside
Eight captive-reared hatchling C. acu-
tus of Jamaican stock were obtained from
Cecil Clemmons at Gatorama, in Palm-
dale, Florida, in July 1981. A 3 x 3 m
environmentally controlled room was used
to house hatchlings and to conduct exper-
iments. Room temperature was main-
tained at 25-30 C using a 1500 W space
heater. Photoperiod was set at 14L:10D
initially, then adjusted to 12L:12D for the
duration of the experiments. Hatchlings
were kept in separate 75 liter aquaria and
were provided with a choice of land or
water. Fresh water was provided during
[Vol. 42, No. 2
the first week; salinity was then increased
and maintained at 10 ppt (Instant Ocean
artificial sea water) for 2 wk prior to ex-
perimentation. Crocodiles were fed live
mosquito fish (Cambusia sp.) (and occa-
sionally . :.. sp.) ad lib. daily.
Critical thermal stress.-Critical ther-
mal maximum (CTM) experiments are one
method used to determine upper temper-
ature limits (Gibbons et al., 1980; Holland,
1974; Hutchinson, 1961). In ( .'i exper-
iments, individuals in tanks of water are
heated rapidly and the behavior and exact
! ... -. o :'ure at which each loses coordi-
nation are recorded. Because C. acutus is
an endangered .. !'.. .. experiments
were modified : '.. ......... critical ther-
mal stress (CTS) rather than CTM tem-
peratures. The criteria used for determin-
ing CTS were panting, pupil dilation,
rapid eye ::.i .::. jerky body move-
ments, and :. "i':. to climb out of the
Hatchling crocodiles were kept in sep-
arate 75 liter aquaria at either 10 ppt or
40 ppt (randomly c( ....... for 24 h.
Hatchlings were fed during this time. No
access to dry land was provided. After the
24 h period, hatchlings were placed in a
water bath ( i. #220) at 10 or 40 ppt
salinity and 26.2 + 0.5 C temperature.
The crocodile was allowed 10 min to ad-
just to the bath before the heat was turned
on. Water temperature was increased at
0.5 C/min. The hatchling's behavior was
closely monitored; when the animal ap-
peared stressed, it was removed from the
bath and its body :, i :.:) temperature
was recorded with a Schultheis thermom-
eter. The hatchling was cooled slowly to
room temperature in a water bath before
returning it to its 75 liter ; -: :: 1::...
Mass gain trials.-A simple but : -
tive means of estimating the ability of a
reptile to survive under different environ-
mental conditions is to monitor changes in
body mass in response to simulated envi-
ronmental conditions in the laboratory
(Dunson, I'' ', Mazzotti and Dunson,
1': i To simulate the effects of thermal
effluent, submersible aquarium heaters
(Hagen Corp., Mansfield, MA) were used
to heat the water portion of each 75 liter
aquarium to 30, 35, or 40 C. Salinity was
maintained at 20 or 40 ppt. The sequence
of temperatures and salinities was ran-
domly determined. Each trial ran for 10
days, during which each hatchling was
. i::. 1 and fed Cambusia sp. and Poe-
cilia sp. ad lib. 1 .1.. Trials were termi-
nated when mass loss exceeded 10% of ini-
tial body mass. To allow recovery, the
crocodiles were returned to 30 C, 10 ppt
and were fed for five days between each
Condenser cooling water at Turkey
Point is discharged into the northeastern
corner of the . .:::. canal system and
flows west, then south,: .: .:: :' in a north-
south temperature gradient (Gaby et al.,
1985). Crocodile nests and '. 1. :. oc-
cur in the cooler southwestern area of the
S1::, system. Mean maximum water
temperatures in the southwestern area
range from 34-42 C during the summer.
During summer 1980, mean maximum
water temperatures in the cooling system
ranged from 1.9-7.4 C warmer than in
the adjacent canals outside the :::!
system. Temperature did not differ sub-
stantially among these outside canals (Ta-
ble 1; Student-Newman-Keuls multiple
range test, P < 0.0005).
Salinity varies from fresh to hypersaline
at Turkey Point. During summer 1980, sa-
linities in the cooling canal system were
higher than in any other canal (Table 1;
Kruskal-Wallis test, P < 0.05). Salinities
within the cooling system were higher in
the northern end (Kruskal-Wallis test, P <
Nests and .:. '.1... crocodiles occurred
in the southwestern portion of the 1..
canal system. Some .,.: 1:i:::,: : i
not only grew and survived in the Turkey
Point cooling canal system, but continued
to do so after moving into the outside ca-
nals. Three of 18 ::! 1:1::.. marked in July
1978 moved from the ."...; system to
the outside canals between September and
December. One of these :.... was
recaptured 1 yr later. In 1979, four of 26
TABiLE 1.-Mean maximum water temperature and mean salinities in the cooling canal system and outside
canals from June through October 1980.
Sea Dade canal
Model I.and canal east
(n = 73)
34.0 + 2.0
(n = 91)
34.7 + 3.1
(n = 45)
(n = 72)
(n = 32)
ippt i SD)
(n = 63)
11.0 4 8.8
(n = 63)
0.0 1 0.31
marked hatchlings survived for at least 1
yr. In 1981, two of 30 marked hatchlings
were recaptured over 1 yr later. --
it is not possible to ,'. :...-. 1: between
death and dispersal, this represents a min-
imum number known to survive.
Regression of linear plots of mass against
age (for 90 days after first animal cap-
.ii; gave M = 1.18A + 51.47 (P <
0.001) for 18 hatchlings in 1978, M =
I 1' -4 + *.. : (P < 0,001) for 26 hatch-
i.... in 1979, and M = 0.76A + 56.52
(P < 0.001) for 30 hatchlings in 1981.
Twenty-five hatchlings recaptured in the
system increased mass at 2.7 +
1.2% of initial body mass per day. Salini-
ties at the capture sites ranged from 32
ppt to 44 ppt (: = 36.8 + 5.8 SD, n = 48),
Water temperatures ranged from 32-35 C
(f = 33.5 + 1.1 SD, n = : Sixteen
hatchlings recaptured in the outside ca-
nals gained mass at 1.46 1 of initial
body mass per day. Salinities ranged from
1-26 ppt (x = 11.1 + 7.6 SD, n = 37) and
water temperature ranged from 22-32 C
(i = 27.6 2.6 SD, n = 37). The only
instance of a hatchling losing mass -y
of initial body mass per day) was in one
of the outside canals after tropical storm
Dennis in 1981.
Laboratory r .: ..:.
Critical thermal stress.--- ii . ..... C.
acutus showed obvious signs of CTS when
cloacal temperature exceeded 38 C (x =
38.6 + 0.6 SD) and the bath temperature
exceeded 40 C (x = 40.0 0.8 SD). The
response was the same whether !. !.!:...
were tested at 10 or 40 ppt. These results
....: I that temperatures observed inside
the Turkey Point... ::.. system could be
....:.... .'. stressful to hatchling croco-
Mass gain trials.--Hatchling C. acutus
could not maintain mass (Table 2) at ele-
vated temperatures (40 C) or salinities (40
ppt) similar to those recorded inside the
cooling system. The results were similar
to those of Dunson (1 : ', c:
gained mass fastest at about 10 ppt sea-
water and could not maintain mass at 40
ppt seawater. Although the amount of fish
eaten was not quantified, there were little
or no fish left in the low salinity trials at
30 and 35 C; most of the fish were not
eaten during the high salinity and/or high
temperature trials. Thus, whatever other
stresses are incurred, it appears that a di-
rect effect of high temperature and salin-
ity is that hatchling crocodiles reduce food
Hatchling American crocodiles can
grow and survive in the . 1:::., system
and other canals at the Turkey Point pow-
er plant site. i... .i from the laboratory
study .... :I .- that to survive, C. acutus
[Vol. 42, No. 2
TABLE 2.-Changes in body mass of hatchling C. aculus offered a choice between land and water and fed
live fish ad lib. daily Values are medians + I SD of the % change in initial mass/day for the 10 day trials
except for 30 C, 10 ppt trials which were five days and the 40 C, 40 ppt trials which were terminated
preimalurely; (n) = number of crocodiles,
J, lM 20 40
30 1.6 + 1.0 (14) 1.7 0.7 (6) -3.5 0.8 (4)*
35 1.0 .0 (5) 3.3 10 (5)
40 -- 2.0 1+ 2 (4)t -3.9 1.1 (3)t
(G,o t rates derc.,rsed .ignlfiecanti a,s alm Tncreased (Joinckhel i Test, P < 0 05)
(.ro th ntes decr( asd signmhcantll as tmperatre incie ed (J,,nekhnerr's Test. P < 0 05).
.. :. i'... need to occupy cooler, less sa-
line areas in the 'I::. canal system. In
fact, '. 1.:... that gained mass and sur-
vived were located in cooler and less sa-
line areas of the cooling system.
There was good agreement between the
water temperature at which 1.. r.... C.
acutus showed stress in the ( : experi-
ment (40 C) and -. ...:.. .- .... at which
they remained out of the water, ceased
and could no longer maintain
mass. The body temperatures of the C.
acutus were similar to those determined
to be critical thermal maxima (CTM) for
alligators (Colbert et al., '1i .. That
hatchling crocodiles only showed stress
while some .: .:... died at this temper-
ature might be related to the more tropi-
cal : :::1 ::: :. of : 1:1 resulting in
a tolerance of warmer temperatures.
Hatchling C. acutus were not found in the
warmer areas of the cooling system. The
range of water temperatures for 48 cap-
tures of up to 3 mo old : i:?i was 32-
35 C. T. .. -. of the hatchlings small size,
it is likely that their body temperatures
were closely tied to the water temperature
of- .. :. et al., 1972) and within the range
of preferred body temperatures of other
species of crocodilian (Colbert et al., 1946;
.' .:. :: et al., 1976; Johnson et al., 1978),
but 1.:, 1' than those reported for C. acu-
tus by Lang (1979).
Salinities recorded at the capture site
ranged from 32-44 ppt, : .. :.i:.. con-
ditions at which 1.=: i:1:::,; crocodiles
should not be able to maintain mass unless
exposed to periodic sources of fresh water
(Table 2; Dunson, 1982; Mazzotti and
Dunson, 1'.-' i' Hatchlings not only gained
mass inside the saline cooling system, but
they gained mass faster there than in the
less saline outside canals. C: -. et al.
("::- ::; showed that although hatchling C.
porosus could gain mass in salt water, the
growth rate of these hatchlings was less
than half that reported for C. porosus in
other parts of northern Australia (G.
Webb, personal communication). This
suggests that hatchling C. porosus could
be stressed by high salinities. No such di-
minished growth rate was shown in hatch-
lings of C. acutus. In fact, growth rates of
hatchling C. acutus from saline habitats
in southern i : :i are among the fastest
reported for any species of ii ::
' i , ; 1. ,1 .
i: : '; .;'o' of C. acutus gained mass
faster than in the cooling canal !, ... than
in the other canals at Turkey Point or un-
der any of the laboratory conditions. Ap-
parently access to fresh water is not a
problem for .. ... .i1, in southern Flori-
da. Dunson (1982) showed that animals
from :i::: areas of Everglades National
Park have similar osmotic and ionic con-
centrations to fresh water animals. Maz-
zotti and Dunson (1984) showed in labo-
ratory experiments that hatchling
crocodiles could gain mass in 100% sea
water (35 ppt) if exposed to brackish water
(4 ppt) once a week. It rains frequently
during late summer in southern Florida,
and fresh water seeps forming brackish
water lens 5 cm thick (5 ppt over 35 ppt)
lasting for 3-4 days duration have subse-
quently been observed ( : -: -., unpub-
lished) in areas ..... .. :- ?. by I:. ..
during 1978-1981. Thus, osmoregulation
by drinking fresh water made available
by frequent rains remains the most likely
explanation for the growth of C. acutus
June .' '-. i
in saline environments in southern Flori-
da. The exceptionally fast rate of growth
for hatchlings in the cooling system may
have been the result of food supply and/
or the elevated temperatures compared
with the adjacent canals.
In addition to the behavioral and phys-
iological specializations of hatchling
American crocodiles, the placement of
nests in the southwestern portion of the
cooling system may also be important to
the survival of the hatchlings. Not only is
this area the coolest in the cooling system,
but it is also the only area where brackish
water has been observed. The southwest-
ern portion of the cooling canal system is
also closest to the canals that the hatch-
lings eventually disperse to (Gaby et al.,
1985). Apparently, hatchling C. acutus
have all of the requirements needed for
growth and survival at the Turkey Point
power plant site in southern Florida.
Acknowledgments.-This study was supported by
contracts from Florida Power and Light Co. to Con-
nell Associates. Cooperation from the Florida Game
and Fresh Water Fish Commission, and the U.S. Fish
and Wildlife Service Permit office is greatly appre-
ciated. N. Gillies, J. Gleman, S. Carney, M. Ahearn,
P. Krauss and M. D'Orazio assisted in collection of
field and laboratory data. This paper is from a por-
tion of a thesis done in : fulfillment of the de-
gree of Doctor of Fhri.,. ..:.i The Pennsylvania State
University. R. Gaby, J. Bohnsack, W. Dunson, D.
Pearson, T. lldhams and G. Kelly critically re-
viewed drafts of this manuscript. L. Weatherholtz
and L. Brandt prepared the manuscript.
COLBERT, E. H., R. B. COWLES, AND C. M. BOGERT.
1946. Temperature tolerances in the American
alligator, and their bearing on the habits, evolu-
tion, and extinction of the dinosaurs. Bull. Am.
Nat. Hist. 86:333-373.
DUNSON, W. A. 1970. Some aspects of electrolyte
and water balance in three estuarine reptiles, the
diamondback terrapin, American and "salt water"
crocodiles. Comp. Biochem. Physiol. 32:161-174.
-- 1982. Osmoregulation of crocodiles: salin-
ity as a :. .. limiting factor to Crocodylus acu-
tus in Florida Bay. Copeia 1982:374-385.
ELLIS, T. M. 1981. Tolerance of sea water by the
American crocodile, Crocodylus acutus. J. Her-
EVANS, D. H., AND T. M. ELLIS. 1977. Sodium
balance in the hatchling American crocodile Croc-
odylus aculus. Comp. Biochem. Physiol. 58:159-
GABY, R., M. P. MCMAHON, F. J. MAZZOTTI, W. N.
GILLIES, AND J. R. WILCOX. 1985. Ecology of a
population of Crocodylus acutus at a power plant
site in Florida. J. Herpetol. 19:189-198.
GIBBONS, J. W., R. R. SHAHITZ, AND 1. L. BRISBIN.
1980. Thermal ecology research at the Savannah
River plant: review. Nuclear Safety 21:367-379.
GRIGG, G. C., L. E. TAPLIN, P. HARLOW, AND J.
WRIGHT. 1980. Survival and growth of hatchling
Crocodylus porosus in salt water without access to
fresh drinking water. Oecologia (Berl.) 47:264-266.
HOLLAND, W. E. 1974. Thermal tolerances of fish
from a reservoir receiving heated effluent from a
nuclear reactor. Physiol. Zool. 42:110-118.
HUTCHINSON, V. H. 1961. Critical thermal maxi-
ma in salamanders. Physiol. Zool. 34:92-125.
JOHNSON, C. R., G. J. W. WBB, AND C. TANNER.
1976. Thermoregulation in crocodilians-II, A
telemetric study of body temperature in the Aus-
tralian crocodiles, Crocodylus johnstoni and Croc-
odylus porosus. Comp. Biochem. Physiol. 53A:143-
JOHNSON, C. R., W. G. VOHT, AND E. N. SMITH.
1978. Thermoregulation in crocodilians--[1.
Thermal preferenda, voluntary maxima, and heat-
ing and cooling rates in the American alligator,
Alligator mississipptensis. Zool. J. Linn. Soc. 62:
KLSHLAN, J. A. 1980. The Status of Crocodilians in
South Florida. IUCN Crocodile Specialists Group,
LANG, J. W. 1979. Thermophilic response of the
American ::,.. .r and the American crocodile to
feeding. Copeia 1979:18-59.
MAGNUSSON, W. E. 1978. Nesting Ecology of
Crocodylus porosus, Schneider, in Arnhem Land,
Australia. Ph.D. Dissertation, University of Syd-
ney, Sydney, Australia.
MAZZOTTI, F. J. 1983. The Ecology of Crocodylus
acutus in Florida. Ph.D. Dissertation, The Penn-
sylvania State University, University Park.
MAZZOTTI, F. J., AND W. A. DUNSON. 1984. Ad-
aptations of Crocodylus acutus and Alligator for
life in saline water. Comp. Biochem. .1 791:
NEILL, W. T. 1971. The Last of the Ruling Rep-
tiles. Columbia University Press, New York.
SPOTILA, J. R., 0. H. SOULE, AND D. M. GATES. 1972.
The biophysical ecology of the alligator: heat en-
ergy budget and climate space. Ecology 53:1094-
Accepted: 21 June 1985
Associate Editor: James Spotila
[Vol. 42. No. 2