A comparison of saltwater crocodile (Crocodylus porosus) populations in freshwater-floodplain and tidal river habitats of the Adelaide River catchment, Northern Territory, Australia

Abstract

Because they are subject to commercial harvesting, Crocodylus porosus populations in Australia are monitored to detect trends in their population growth. Monitoring programs are restricted primarily to estuarine waters with high crocodile densities, subsequently little information is available on populations in freshwater habitats. In this study I compared the body-size structure and minimum number of C. porosus known to be alive in tidal-estuarine and freshwater-floodplain habitats of the lower Adelaide River catchment, Northern Territory (N.T.) during the 2004 dry season. A total of 669 C. porosus was sighted. Of these individuals, 546 (81.6%) were sighted in tidal-estuarine waters and 123 (18.4%) were sighted in freshwater, floodplain billabongs and creeks. Crocodiles were observed up to 17.3 km from tidal waters. Crocodiles less than 1.2 m total length (TL) were the most frequently sighted size category in both the freshwater-floodplain (68%), and tidal river (31%) populations. Crocodiles 1.2-3.4 m TL comprised 62% of those seen in the tidal river but only 22% of those in freshwater floodplain habitats. Crocodiles >3.4 m TL comprised 10% and 6.5% of the sighted population in freshwater-floodplain and tidal river habitats, respectively. The body-size structure of C. porosus inhabiting the freshwater floodplain and tidal river habitats of the Adelaide River differed markedly from that observed in marine and freshwater river habitats where crocodiles less than 1.2 m are rarely encountered.