Monitoring Crocodylus porosus populations in the Northern Territory of Australia: a retrospective power analysis

Abstract

In the Northern Territory of Australia, populations of the estuarine crocodile (Crocodylus porosus) have been subject to an annual egg harvest since the early 1980s. Since 1997, adult and juvenile crocodiles have also been harvested in some catchments. Annual surveys of crocodile populations are conducted in order to ensure that the harvest is sustainable. Boat surveys commenced in 1975 and helicopter surveys commenced in 1989. Retrospective power analysis was used to determine whether the sampling program meets the objectives of the Crocodile Management Program for the Northern Territory. Data collected during boat surveys vary in quality between river systems. The analysis of pooled data from 7 river systems with a residual standard deviation of 0.11 indicates that the power of the current spotlight survey method to detect a decline of 10% per annum in around 4 years is about 0.9. In this time the population would decline by around 33% and would fully recover in 8 years following the removal of the factor causing the decline. This allows detection of a decline within one-third, and recovery within two-thirds, of the estimated generation time of the saltwater crocodile and will allow management actions to be implemented before the impacts on populations are serious. The data from helicopter and boat surveys from a 10-year period were compared. Helicopter surveys did not provide useful management information.