SREL Reprint #1871

Long term population studies of American alligators inhabiting a reservoir: initial responses to water level drawdown

I. Lehr Brisbin, Jr.1, J. Merlin Benner1, Laura A. Brandt2, Robert A. Kennamer1, and Thomas M. Murphy3

1Savannah River Ecology Laboratory, P.O. Drawer E, Aiken, South Carolina 29802, USA
2Department of Wildlife and Range Sciences, University of Florida, Southwest Florida Research and Education Center,
P.O. Drawer 5127, Immokalee Florida 33934, USA
3South Carolina Department of Wildlife and Marine Resources, Rt.2, Box 167, Green Pond, South Carolina 29446, USA 

Abstract: A population of American alligators (Alligator mississippiensis) inhabiting a 1130 ha nuclear reactor cooling reservoir has been studied for more than 20 years, producing a data base that now can be used to evaluate the responses of these animals to subsequent changes in their habitat. Beginning in June and continuing through September 1991, the water level in this reservoir was lowered 6m to allow for repair work to the dam. The study reported here extended from July 1991 through the summer of 1992, during which time the reservoir remained at the lowered level. The drawdown reduced the water surface area by 50%, exposing and killing the majority of the lake's submerged/emergent aquatic vegetation. Both during and immediately after drawdown activities, the number of alligators counted in the reservoir by night eyeshining techniques increased possibly as a result of increased visibility of smaller animals due to the lack of emergent vegetative cover. High numbers of alligators were also observed during aerial census flights during the spring following drawdown. Fourteen adult alligators outfitted with radio transmitters in September 1991 revealed differences in spatial distributions and movement patterns between the sexes during the fall and following winter in the drawdown reservoir. Males showed more extensive fill movements while most females tended to remain close to the locations where they were originally captured. There was no evidence that the drawdown adversely affected the winter survival of adult alligators in Par Pond. Six of the telemetered alligators spent the winter in moderately deep water along a <300m stretch of exposed reservoir shoreline. An additional female was found wintering with young in an extensive circular subterranean den system that remained dry throughout the winter due to the lowering of the water level. Six alligators (four males/two females) were recovered elsewhere in the area after having been marked or telemetered in Par Pond. Two of these alligators were later killed in smaller nearby impoundments, most likely by larger alligators residing in the habitats to which these emigrants had moved. Three nests initiated before drawdown activities all successfully hatched young. Despite the greater distances of these nests from the receded shoreline, all three females continued to tend these nests and subsequently, moved as much as 100 m with their newly-hatched young into the lowered reservoir. Due to the lack of cover however, it is unlikely that many of these young survived. Unfavourable conditions for nesting, and habitat conditions that have undoubtedly resulted in low survival of juveniles, have probably been the most important impacts of the reservoir drawdown upon its resident alligator population.

SREL Reprint #1871

Brisbin, I.L., Jr., J.M. Benner, L.A. Brandt, R.A. Kennamer, and T.M. Murphy. 1992. Long term population studies of American alligators inhabiting a reservoir: initial responses to water level drawdown. pp. 53-76. In: Crocodiles - Proceedings of the 11th Working Meeting of the Crocodile Specialist Group of the SSC of the IUCN - The World Conservation Union. IUCN, Victoria Falls, Zimbabwe. 

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).