The decline in platypus numbers has likely been underestimated, according to research from the University of NSW.
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The study, which examined data on the species from the past 258 years and compared with recent data, was published in Global Ecology and Conservation.
It found changes to the species' distribution, with no recorded sightings of the mammal in more than 41 per cent of sub-catchments across its range in the past 10 years.
It also found considerable historical declines, with past accounts of platypus abundance that far exceed those of the present day.
This includes the capture of 22 platypuses in a single day on the Yarra River in Melbourne in 1908.
UNSW researcher Dr Gilad Bino, one of the authors of the study, believes the long-term declines were likely due to the impact of the historical fur trade, from which the platypus never fully recovered.
"Subsequent impacts of river regulation, habitat destruction, pollution, predation and drowning in enclosed fish and crustacean nets further increased their decline," Dr Bino said in a statement.
Another author of the study, Professor Richard Kingsford, said the decline of the species was of increasing concern.
"Improved and ongoing monitoring of platypus is essential to increase understanding and inform effective management of this iconic mammal," he said in a statement.
Australian Associated Press