The Distribution and Abundance of Platypuses in The Thredbo River-Lake Jindabyne System.

Abstract

The Thredbo River is an upland alpine stream, with a steep gradient over its 50 km length contained within a 251 km2 catchment in the Australian Alps. The altitude gradient from its source to Lake Jindabyne is 850 metres. The distribution and abundance of platypuses were determined over a two year period using volunteer sightings, disciplined transect walking and pool observation, gill-netting in selected pools and fyke-netting in riffle areas. Under low flow conditions, 93 pools were identified downstream of Dead Horse Gap, 85% of which were judged to be in the medium to small category. Under snow melt conditions, as river flow increases and smaller pool 'disappear' in the torrent, the carrying capacity of the river is significantly reduced. Some base-line data were available from the 1980s. Platypuses were uncommon or absent in the upland and common, but not abundant, in the mid and lowland segments of the river. Small numbers of platypuses were located in two larger tributaries, but none in the remainder, which had very steep gradients and narrow channels. Low recruitment rates, an overall decline in numbers, as judged by catch effort compared to base-line data, a probable increase in transience suggesting a non-stable population, are all signs that the population is under considerable stress. Episodic flood events, habitat degradation and the dam wall barrier preventing reinvasion from downstream segments of the population after flood-induced population losses, when considered together with population data, suggest that the platypus population is vulnerable.