Survival of Young American Alligators on a Florida Lake

Abstract

A capture-recapture study was conducted on Orange Lake, Florida, from 1979 through 1984 to estimate survival rates of young in an American alligator (Alligator mississippiensis) population. Hatchlings remained together in sibling groups (pods) for at least their 1st year and then began to disperse during their 2nd spring and summer. Mortality through mid-November of their 1st year was negligible. Jolly-Seber (JS) survival estimates of hatchlings for 6 and 12 months were 76 and 41%, respectively. The 2-year JS estimate for the 1980 cohort was 8%. Minimum-Known-Alive (MKA) survival values were 72 and 46% of JS estimates for 6 months and 1 year of age. Survival during the 2nd 6 months of life (spring-summer) tended to be lower than survival during the 1st 6 months (fall-winter). J. WILDL. MANAGE. 51(4):931-937 Generally, crocodilians are secretive; they quickly learn to avoid humans and frequently inhabit areas that are difficult to access. These factors render them difficult subjects for survival studies. Therefore, most references to survival rates have been based on casual observations (Neill 1971), small samples (McIlhenny 1935, Modha 1967, Murphy 1977, Webb et al. 1977), or interpretation from population size structures (Chabreck 1966, Graham 1968, Nichols et al. 1976, Webb et al. 1983, Taylor and Neal 1984). Recent mark-recapture studies have attempted to estimate survival rates, using MKA estimates, for American alligators (Deitz 1979) and saltwater crocodiles (Crocodylus porosus) (Messel et al. 1981:390-394) <2 years of age. The MKA survival rate estimator is used extensively by crocodilian researchers because of its simplicity. However, as its name implies, MKA estimates are biased (Nichols and Pollock 1983). Survival rates are an important component of population models (Nichols 1987), especially when models are intended for use in predicting population responses to harvest strategies. Furthermore, growing emphasis on early age-class harvest will make juvenile survival rates increasingly important when evaluating the suitability of various alligator populations for egg or hatchling removal. Our work on Orange Lake, Florida, provided an opportunity to conduct long-term mark-recapture studies on a relatively abundant and accessible population of alligators. Our primary objectives were to estimate survival rates of juvenile alligators using the JS estimator (Jolly 1965, Seber 1965) and to determine the relationship between the JS and MKA estimators. We are grateful to D. C. Deitz, T. M. Goodwin, J. E. Thul, W. C. Maahs, M. L. Jennings, M. F. Delany, and H. F. Percival for field assistance in this study. We also thank H. F. Percival, M. F. Delany, T. G. O'Brien, C. T. Moore, nd M. L. Jennings for providing valuable comments and T. L. Steele for typing this manu-