Distribution and Demographic Attributes of Platypus Populations Near Melbourne.

Abstract

Information on platypus populations occurring along streams in the Melbourne region was collected by mark- release studies in the period I989-I996. In all, 256 records of animals were obtained along waterways in the Yarra River, Maribyrnong River and Dandenong Creek catchments. Ornilhorhynchus anatinus were captured at approximately one-quarter of the sites sampled by fyke nets, with animals encountered to within about 15 km of inner Melbourne. Evidence of reproduction was obtained along seven waterways in the Yarra River catchment (Badger Creek, Watts River/Graceburn Creek, Olinda Creek, Mullum Mullum Creek, Diamond Creek, Plenty River/Jacks Creek and the Yarra River itself) as well as Monbulk Creek in the Oandenong Creek catchment. The earliest date on which a juvenile was captured at any site was 24 February. Across populations, sex ratio (expressed as the number of adult or subadult females per adult male) tended to be positively correlated with population density. In all seasons, nearly all animals were found to be in moderate to good physical condition (i.e. tail fat index = 2 or 3). The three thinnest individuals encountered in this study were adult females captured in March, at least two of which were lactating. Along the Watts River and Mullum Mullum Creek, six individuals (comprising about 10% of animals captured) had one or more pieces of plastic or rubber litter caught around their neck or thorax.