Decline in Deloraine platypus numbers of concern, says report
A platypus under the Deloraine Bridge over the Meander River photographed by Deloraine wildlife photographer Joy Kachina. Photo supplied.
Sharon Webb
Platypus numbers in the Meander River at Deloraine dropped by 25 per cent in the past year possibly because of water pollution upstream, a new report from the Australian Platypus Conservancy has said.
Director of the conservancy Geoff Williams said the reduction in platypus sightings is significant and of concern.
While Mr Williams said that the Meander River platypus population is not under immediate threat of collapsing, he warned that reasons for the drop needed investigating soon.
‘We’ve seen too many platypus populations on the mainland drop to disastrous levels because official intervention has come too late,’ Mr Williams said.
‘As with all sudden or unexplained fluctuations in animal populations, it would be good to investigate its causes sooner rather than later to decide whether action is needed to halt any decline before it reaches a critical point.’
The conservancy has asked DPIPWE, Tasmania’s environment department, to work with Meander Valley Council to investigate substantial changes to land use or site development in or upstream of Deloraine over the past one to two years.
‘These may have depleted vegetation cover on the banks, increased rates of erosion and sedimentation or increased pollutant levels in the water, including nutrients from livestock or chemical fertiliser.’
A DPIPWE spokesperson said the department will liaise with the conservancy to better understand the implications of the monitoring.
‘Without additional data it is difficult for us to determine whether this is the result of a threat, a methodological issue or natural variation,’ she said.
‘Many factors can influence visual counts of platypus, including survey methods, weather conditions and environmental conditions.
‘However, we encourage further monitoring by the Deloraine community, who are to be applauded for monitoring platypus in their neighbourhood.’
Deloraine resident Anne Gilles, who has recorded over a thousand scans for platypus over the past two years, said many local residents had noticed a drop in numbers.
‘It’s satisfying that data collected systematically by volunteers has confirmed scientifically that there is a threat to Deloraine’s platypus.
‘I certainly hope that DPIPWE and Meander Valley Council will examine the situation seriously,’ she said.
Mr Williams downplayed suggestions that the removal of five willows from the Deloraine riverbank in late 2020 contributed to the loss of platypus habitat.
He said the platypus populations are known to be remarkably resilient to major bank reconstruction projects. ‘It is unlikely that the scale of the willow removal activities was solely, or even primarily responsible for the significant fall.
‘Furthermore, the decline in sightings was already apparent in the months before willows were removed, indicating that other factors were involved.’
Platypus monitoring at Deloraine from June 2019 to May 2020, compared with June 2020 to May 2021, showed 25 per cent fewer platypus sightings.
Mr Williams said this was statistically significant, with less than four per cent likelihood that the difference was due to chance alone.
At the Tasmanian Arboretum at Eugenana, the nearest site to Deloraine with valid platypus monitoring, there is no change in numbers.
