Changes in the distribution and abundance of saltwater crocodiles (Crocodylus porosus) in the upstream, freshwater reaches of rivers in the Northern Territory, Australia

Abstract

Since they were declared a protected species in 1971, populations of saltwater crocodiles (Crocodylus porosus) have increased in the tidal rivers, freshwater swamps and marine waters of the Northern Territory. The recovery of the C. porosus population has been accompanied by an increase in the incidence of ‘problem crocodiles’ that represent a threat to people in freshwater and marine habitats. Despite the implications for human safety, little is known about C. porosus populations in the freshwater reaches of rivers, well upstream of tidal influence. In this study, we examined the density and body-size structure of C. porosus populations in three freshwater rivers using a combination of data from spotlight and helicopter surveys conducted between the 1980s and 2005, and the inland extent of C. porosus using distribution records in the Northern Territory. Since the 1980s, the density of C. porosus in upstream, freshwater reaches of the Daly and Roper rivers has increased, as has the inland extent of C. porosus on the Daly River. Although C. porosus was not detected in spotlight surveys of the Victoria River, helicopter survey and anecdotal records indicate that C. porosus was present after 1989. In all, 52.1% of the crocodiles sighted in spotlight surveys were 2.1–3.4 m long. Distribution records show that C. porosus occurs up to 235 km inland and at elevations of up to 126 m above sea level. The potential distribution of C. porosus is likely to be similar to that of barramundi (Lates calcarifer), a readily identifiable diadromous fish that must spawn in estuarine waters and occurs at elevations of up to 178 m above sea level. Because the density and inland extent of C. porosus in freshwater rivers is likely to increase, it is recommended that: (1) programs communicating crocodile awareness use the linkage between the presence of barramundi and the potential presence of C. porosus; (2) that crocodile warning signs be erected in upstream areas within the potential range of C. porosus; (3) that research be conducted on developing techniques to detect and exclude C. porosus from swimming areas; and (4) that widespread systematic surveys be undertaken to document the inland extent of C. porosus.