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

Mike Letnic*

Parks and Wildlife Service of the Northern Territory, PO Box 30 Palmerston, NT 0831, Australia *Current address: Institute of Wildlife Research, University of Sydney, Sydney, NSW 2006, Australia. E-mail: [email protected]

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.

Key words: Crocodylus porosus; estuarine; freshwater; human-wildlife conflict; wildlife management.

Introduction In the 19th century C. porosus was reported to have been a common species in northern Australia (Searcy, 1909). From the time of European settlement in the late 19th century until 1971, C. porosus in the Northern Territory were hunted for their hides or for sport and were destroyed as pests (Searcy, 1909; Webb and Manolis, 1989). By the late 1960s C. porosus had become a rare species (Messel et al., 1981),

© Koninklijke Brill NV, Leiden, 2008 APPLIED HERPETOLOGY 5: 243-252 Also available online - www.brill.nl/ah 244 M. Letnic and the population was unable to support a viable hunting industry. In 1971, the species was declared a protected species in the Northern Territory and by 1974 the species was protected throughout Australia (Webb and Manolis, 1989). At the cessation of the hunting era, most of the C. porosus population was located in the brackish waters of tidal estuaries and freshwater-swamps situated on coastal floodplains (Messel et al., 1981; Webb et al., 1984). Population surveys of C. porosus in the Northern Territory were commenced by Messel in the 1970s (Messel et al., 1981) and were conducted in the tidal sections of rivers at night using a boat and spotlight. The population monitoring program established by Messel has continued through to the present day, primarily for the purpose of monitoring C. porosus populations which are now subject to regulated commercial harvesting (Stirrat et al., 2001). Most published studies describing the distribution and population dynamics of C. porosus were conducted in the saline or brackish waters of tidal rivers where they are easily visible and occur at relatively high densities (Messel et al., 1981; Stirrat et al., 2001; Read, 2004). These studies have shown that C. porosus populations have increased in the tidal estuaries of northern Australia since they were declared a protected species. Despite estimates that 20-40% of the Northern Territory population of C. porosus occur in freshwaters (Webb et al., 1984; Messel and Vorliceck, 1986) and observations that crocodiles make extensive use of freshwater swamps and rivers (Webb et al., 1977; Jenkins and Forbes, 1985; Webb et al., 1989), few studies have examined the body-size structure or abundance of the species in these habitats (Webb et al., 1983; Webb et al., 1989; Letnic and Connors, 2006). The likely reasons for this bias against studying C. porosus in freshwater habitats are difficulties associated with accessing and viewing crocodiles in floodplain swamps and billabongs (Webb et al., 1983; Bayliss et al., 1986) and in the case of freshwater rivers, low population densities (Letnic and Connors, 2006). Floodplain billabongs and swamps are known to be important habitats and nesting areas for C. porosus (Webb et al., 1983). However, most floodplain billabongs and swamps in the Top End of the Northern Territory are inaccessible for much of the year because the surrounding soils are water-logged (Webb et al., 1983). Moreover, crocodiles are often difficult to see owing to the presence of dense emergent and fringing vegetation that limit visibility and access (Webb et al., 1983; Bayliss et al., 1986). Crocodiles inhabiting freshwater floodplain habitats present complex issues for wildlife managers in the N.T. The main issues posed by these crocodiles are their depredations on livestock, threat to human safety and, the value of crocodile eggs harvested from these habitats to the crocodile ranching industry. The need to develop management solutions aimed at minimising stock losses, reducing the risk of crocodile attacks on people and at the same time ensuring that harvests of crocodiles and their eggs are sustainable has highlighted the paucity of knowledge available on crocodile populations in floodplain swamps and billabongs. In this study I report upon the distribution of crocodiles in the catchment of the Adelaide River, near Darwin, N.T. during the dry season of 2004. The specific aims