Crocodylus johnsoni

Common names: Australian freshwater crocodile, freshie, Johnson’s, Johnstone’s or Johnston’s crocodile Range: Northern Australia Revised by Anton Tucker

CITES: Appendix II
CSG Action Plan:
Availability of Survey Data – Good
Need for Wild Population Recovery – Low
Potential for Sustainable Management – High
1996 IUCN Red List: Not Listed (LRlc Lower Risk, least concern.)
Principal threats: Habitat destruction.

Ecology and natural history

The Australian freshwater crocodile is a small to medium-sized freshwater crocodile restricted to tropical northern Australia. It is similar to other fresh water species in the Asian region (e.g. Crocodylus mindorensis, Crocodylus novaeguineae) but morphologically distinct because of its unusually narrow snout. The species is commonly referred to in the literature as Crocodylus johnstoni [see King and Burke 1989 for discussion. Ed.]. Maximum size of males approaches 3m, and the species is generally restricted to freshwater habitats upstream of tidal areas in Western Australia, Northern Territory and Queensland (Webb et al. 1987). This includes almost any type of permanent freshwater habitat including rivers, creeks, swamps and floodplain lakes and lagoons (“billabongs”). In some rivers freshies extend into tidal, saline waters, although they may be restricted from colonizing such areas by saltwater crocodiles (Messel et al. 1981).

Although it has a narrow snout, the diet of this species is very catholic and includes a wide variety of invertebrates and small vertebrates (Webb et al. 1983). Females lay eggs into holes which they dig in sand bars exposed in the dry season. Clutch size averages 13, and predation by monitor lizards (Varanus sp.) and feral pigs is high. Incubation normally lasts 75–85 days (Webb et al. 1983). Recent biological studies include reports of population dynamics (Cooper-Preston and Jenkins 1993, Tucker et al. 1993), aestivation (Kennet and Christian 1993), osmoregulation (Taplin et al. 1993), and the energetics of incubation and development (Whitehead et al. 1992).

Conservation and status

Crocodylus johnsoni is currently at low risk of extinction. The population is large and widely distributed, harvest rates are low and habitats are largely intact. A detailed assessment of conservation status is given by Cogger (Cogger et al. 1993). The human influences on the population which have been identified and require monitoring are loss of riparian habitat, erosion of nesting areas and water diversion for irrigation. Entanglement in fishing nets is reported but is mainly restricted to areas of high crocodile density. Feral pig predation of nests is reported but it is unclear whether this significantly affects populations (Webb et al. 1983b). The frequency and severity of early nest season flooding has increased significantly since the 1800s (Tucker, pers. comm.), but the effect of such natural stochastic events on recruitment of long lived crocodilians is debatable.

Many populations of C. johnsoni have recovered since legal protection was enacted in the 1960s–1970s. Recent population estimates in the Northern Territory range from 30,000–60,000 (Webb et al. 1987). In Western Australia a total population estimate is not available, although combined estimates from separate populations in Fitzroy and Ord rivers and Lake Argyle total at least 47,000 (McNamara and Wyre 1994). The status of the species in Queensland is unquantified but presumed to be abundant (Miller 1993). A large scale research and management program was undertaken in the late 1970s. The species appears to be highly suitable for adaptive management (Webb and Manolis 1993) although their skins are of relatively low value. Sustainable use began in 1983 with the collection of hatchlings for a ranching program. Management programs vary among the states. Western Australia and Northern Territory permit a mixture of ranching, harvesting and farming while Queensland allows only farming. The effects of population biology on sustainability of management actions is addressed by Tucker (1995).

Priority projects

Moderate priority

Investigation of population dynamics: Little conservation action is needed for this species, but C. johnsoni offers superb potential for conducting research on crocodilian population dynamics. An intensive research effort has been conducted by Conservation Commission of the Northern Territory since the late 1970s. A similar program is underway in Queensland. The continuance of these long-term research programs will yield much needed data for management programs for this and other species of crocodilians.

Population status assessment: Continued monitoring of the population status of the species is a necessary component of the ongoing sustainable use and management of the species. The CSG Regional Meeting in Darwin March 1993 recommended regular monitoring of a series of index rivers to assess population changes.

Australian freshwater crocodile, Crocodylus johnsoni. Photo by G. J. W. Webb.