Simulation of a commercially harvested alligator population in Louisiana

Abstract

A model was constructed to simulate the dynamics of a commercially harvested alligator (Alligator mississippiensis Daudin) population inhabiting the privately owned coastal marshland of Cameron and Vermilion parishes, Louisiana.In the model, nesting effort, nest flooding, desiccation mortality, and predation on alligator eggs and young were all determined as functions of monthly water depth averages.Cannibalism was considered to be the major density dependent factor operating on the population and was determined as a function of total population density and marsh water depth.The model contained a freeze mortality which was based on minimum winter temperatures.In addition, the model included a harvest option which resulted in alligator hunting mortality.Comparison of simulation results with 1970-1973 nest count results demonstrated reasonably close agreement between simulated and ob- served data.Simulations of a June drought, a severe summer drought, and an August hurricane produced drastic population declines, although rapid recoveries were made in subsequent years.Environmentally stochastic simulations produced extremely irregular population response curves and resultant age structures.Simulations were utilized to examine population response to various differential harvest rates in which age and sex-specific proportions of animals taken were similar to those observed in the 1972 and 1973 Lou- isiana harvest seasons.These simulations demonstrated that under exist- ing habitat conditions a base population of 100,000 animals should be maintained for at least 20 years when subjected to an annual differential harvest rate slightly greater than 5 percent.Simulations were conducted using proportional harvest rates in which animals of various sizes were taken in proportion to their relative abundance in the population.Comparison of proportional and differential harvest strategies indicated that proportional hunting can result in increased yields of alligator hide and resultant income.Simulations with egg collection management programs produced greater population increases than similar simulations with no management.