As shown below, populations of rakali or Australian water-rats occur across much of the Australian mainland and Tasmania and on many offshore islands.
Map courtesy of Strahan R (ed) (1995) The Mammals of Australia, 2nd edition. Reed Books: Chatswood NSW.
Across their distribution, rakali occupy many different freshwater habitats, including swamps, ponds, lakes, rivers, creeks and irrigation channels (Watts and Aslin 1981; Van Dyck et al. 2013; Williams and Serena 2018). They also inhabit coastal mangrove habitats, brackish estuaries and sheltered ocean beaches (Van Dyck et al. 2013; Williams and Serena 2018), and ephemeral rivers and lakes when these fill with water (Watts and Aslin 1981). In Western Australia, rakali have been found to prefer feeding in relatively shallow water near the shoreline (Harris 1978; Speldewinde et al. 2013) and places where reed beds or other low-growing vegetation provide plenty of cover on or near the banks (Smart et al. 2011; Speldewinde et al. 2013). At Barrenbox Swamp in New South Wales, they often foraged near features such as reed clumps or fence posts and logs located in the water (Woollard et al. 1978).
An analysis of live-trapping results in Victoria concluded that rakali are generally most abundant in the state’s west, followed by the Melbourne urban area, the state’s northeast, and lastly Gippsland in the state’s southeast (Williams and Serena 2018). It was suggested that low rakali numbers in Gippsland might partly reflect predation by long-finned eels Anguilla reinhardtii, voracious carnivores that can weigh up to nearly 17 kilograms and are restricted in Victoria to eastern coastal river systems (Cadwallader and Backhouse 1983). Low numbers of rakali captures in Gippsland have also been reported by Smales (1984).
In South Australia, live-trapping studies revealed that a very dense rakali population (62 individuals recorded in 1.2 kilometres of channel) occupied the River Torrens in Adelaide. However, numbers dropped sharply after the gates to a weir were opened, reducing surface flow for 6 weeks to a ribbon of water that was only 1 to 2 metres wide x about 0.5 metres deep. It took more than a year for rakali numbers to recover after the weir gates were closed and the channel refilled (Leigh and Breed 2020). Similarly, rakali were routinely observed at Lake Wendouree in the city of Ballarat in Victoria before the lake dried out in 2007 during a prolonged drought. The species was not seen again until 2013, more than three years after the lake refilled in 2010 (Williams and Serena 2018).
Rakali monitoring at Ballarat
The APC has initiated a citizen science project at Ballarat to monitor the rakali population at Lake Wendouree. If you are interested in participating, please contact the APC to register your interest and obtain further details of the survey methods.
For more information on this project, see the article on the first year of monitoring in Platypus News & Views no. 94, page 3 (February 2024).
LITERATURE CITED
Smales LR (1984) A survey of Hydromys chrysogaster, the Australian Water rat in central Gippsland. The Victorian Naturalist 101, 115-118.
Smart C, Speldewinde PC and Mills HR (2011) Influence of habitat characteristics on the distribution of the water-rat (Hydromys chrysogaster) in the greater Perth region, Western Australia. Journal of the Royal Society of Western Australia 94, 533-9.
Speldewinde PC, Close P, Weybury M and Comer S (2013) Habitat preference of the Australian water rat (Hydromys chrysogaster) in a coastal wetland and stream, Two Peoples Bay, south-western Australia. Australian Mammalogy 35, 188-94.
Van Dyck S, Gynther I and Baker A (2013) Field Companion to The Mammals of Australia. New Holland Publishers: Sydney.
Watts CHS and Aslin HJ (1981) The Rodents of Australia. Angus & Robertson: Sydney.
Williams G and Serena M (2018) Distribution of the Australian water-rat Hydromys chrysogaster in Victoria: findings from community-based sightings and live-trapping surveys. The Victorian Naturalist 135, 71-83.