Crocodylus johnstoni in the McKinlay Area, N.T. VIII. A Population Simulation Model

Abstract

A population simulation model was constructed for the McKinlay River population of the freshwater crocodile Crocodylus johnstoni. C, johnstoni are long-lived reptiles (50 y +) which take 9-16 years to reach maturity. As a consequence, the effects of legal hunting before 1963 are still reflected in an unstable population age structure. No quantitative data on the original population densities exist, nor are there data on the densities at which equilibrium can be expected in the future. The model examines the dynamics of a population which is still expanding and does not take into account density-dependent factors which may ultimately limit the population. If the population was undisturbed for 10 years, the model predicted the mean natural rate of population increase would be 1 .5% per annum. However, the population has been disturbed during research activities, and when these disturbances and manipulations were simulated the model predicted a 4% decrease in the population between 1979 and 1983. Independent surveys in both 1979 and 1983 indicated a 5% decrease in the population. This consistency has been interpreted as indicating that the model's predictions are not grossly erroneous. Sensitivity tests were carried out in which most parameters in the model were independently varied by plus or minus their estimated error, while other parameters were held constant. The resultant changes in the estimated population size after 10 years indicated the model was most sensitive to the age-specific mortality estimates. C. johnstoni management has the conservation requirement of maintaining or even enhancing the density of wild populations. Sustained-yield harvesting can theoretically be achieved without compromising this requirement by harvesting eggs and/or hatchlings, and later returning a proportion of the harvest to the population when they are larger and have a greater probability of surviving. When released into the wild, captive-raised C. johnstoni survive as well as wild ones of equivalent sizes. The model was used to simulate egg and hatchling harvests with different collection and return rates, and different ages of returned animals. It was also used to simulate harvests of post-hatchling crocodiles, without a return of captiveraised animals.