SREL Reprint #2192





Population Studies of American Alligators (Alligator mississippiensis) Inhabiting a Reservoir: Responses to Long-Term Drawdown and Subsequent Refill

I. Lehr Brisbin, Jr., Karen F. Gaines,
Charles H. Jagoe, and Peter A. Consolie

Savannah River Ecology Laboratory
P.O. Drawer E
Aiken, South Carolina 29802, U.S.A.

ABSTRACT: A 6 m drawdown of an I 100 ha reservoir in South Carolina, USA in 1991 continued to have an effect on the reproduction of resident alligators through the summer of 1994 when reservoir refill began. Clutch sizes in 1994 were reduced by 10.9 %. Body condition of hatchlings, as evaluated by weight/length relationships, was also lower during the period of drawdown than before. Nest depredation may have been reduced during drawdown however, providing some compensation for negative impacts upon clutch size and hatchling condition. Changes in numbers of alligators, as counted by night eyeshine censuses, could be explained by changes in visibility due to variation in emergent macrophyte cover along the reservoir shoreline. Within years, eyeshine counts varied between regions of the reservoir in proportion to the amount of available shoreline but not in proportion to the area of open water. Radiotelemetry studies showed movements of alligators varied between-sexes and between drawdown vs. post-refill conditions. Only males moved long distances between regions of the reservoir, and only during the drawdown period. Alligator movements were generally reduced following refill. Wintering behavior during periods of exceptionally cold weather following refill followed expected patterns. Par Pond formerly served as a nuclear reactor cooling reservoir, and its sediments contain elevated concentrations of mercury and some radionuclides. Although no changes in mercury or radiocesium concentrations in alligator tissues could be related to the drawdown or refill, reservoir filling has been shown to enhance mercury bioavailability at other locations. This suggests the potential for increased mercury uptake by alligators in future years following refill. In general, this reservoir alligator population seemed highly resilient to any major negative impacts associated with reservoir drawdown and refill, with the exception of the flooding of nests and drowning of 14.6 % of the eggs in six nests studied. Such losses could be avoided by altering the timing of reservoir refill to avoid periods when eggs are incubating. An understanding of the basic ecology and natural history of any resident population of crocodilians is essential to minimizing detrimental impacts on these animals, as a result of reservoir management programs.

SREL Reprint #2192

Brisbin, I.L., K.F. Gaines, C.H. Jagoe, and P.A. Consolie. 1997. Population studies of American alligators (Alligator mississippiensis) inhabiting a reservoir: responses to long-term drawdown and subsequent refill. p. 446-477. In Proceedings of the 13th Working Meeting of the IUCN/SSC Crocodile Specialist Group, IUCN-The World Conservation Union. Gland, Switzerland.

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