Platypus sightings in the Meander River at Deloraine have reduced by at least 25 per cent in the past 12 months according to a citizen science program as volunteers raise concerns about apparent water quality issues and the removal of trees.
The Australian Platypus Conservancy ran visual surveys using four sites at Deloraine - the caravan park, pontoon, road bridge and weir - for 2019-20 and 2020-21, comparing data month-by-month and noticing a significant decrease in sightings.
In the report, the conservancy stressed that platypus remained "abundant" in the river, but a drop in sightings by a quarter in just one year was worrying given the usual resilience of the Tasmanian variety.
Volunteer Anne Gilles carried out the surveys, which included regular monitoring of each of the sites for five minutes at a time and counting the number of platypus sightings, which were averaged over each month.
During her time on the banks of the Meander, she noticed some changes.
"The river has been very dirty in appearance. The turbidity really changed some months ago, around late last year," Ms Gilles said.
"I was going through some videos of the river from the past, and I couldn't believe how clear the water was back then. Something has been happening upstream for some months, and over the summer months the river smelt bad."
It seemed to coincide with reduced platypus sightings.
"When I first started doing the surveys, at one of my scan sites I'd see a platypus 70 per cent of the time. Now it's dropped to 50 per cent and is continuing to decline," Ms Gilles said.
She plans to take the findings to TasWater in the hope it would carry out a thorough assessment of water quality and nutrient levels near the town's pump station. Ms Gilles is worried that a possible change of use further up the river could be causing more nutrients to enter the water way.
Empire Hotel owner Mark Flanagan ran nightly platypus tours for visitors since taking over the pub three years ago, but had to stop after it became more difficult to spot them.
"We used to be able to see up to eight or nine in the river most nights. It was almost guaranteed at least one sighting," he said.
"This year just gone, the numbers have crashed - I'd say by about 50 per cent. It made it hard to take people down and see them, now we only do the tours by request."
The platypus tours had become an important feature of Deloraine's tourism market, and Mr Flanagan said it could have wider ramifications.
"Providing the platypus experience in the evening was giving people a reason to stay in the town overnight," he said. "It's a lovely tourism town but people don't usually stay for more than a day or two, so we were able to encourage people to stay two or three days instead."
Mr Flanagan pointed to the Meander Valley Council's removal of six willow trees from the bank in December as a possible reason for reduced sightings, along with a reduction in food sources like insect larvae and worms due to changes in the catchment.
Deloraine resident and regular photographer of platypus, Joy Kachina, agreed that the willow tree removal was a factor.
"You used to have platypus feeding under the branches later in the morning, now there's nothing at all, no shelter for them," she said.
The Australian Platypus Conservancy is run by Geoff Williams, based in Victoria.
Platypus are considered extremely elusive on the mainland, but are a common sight in various rivers across Tasmania.
Mr Williams has seen their continued decline in Victoria, and believed any reduction in sightings needed to be investigated before it became a larger issue.
"Deloraine is still a good place for platypus sightings. Nevertheless, the fact is that this decline was picked up, and previous experience shows that it's much better to get on top of the problems before they continue to slide downhill," he said.
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"That has been the case with populations on the mainland and the trends we see there.
"Both the quality of the water in the Meander in regards to sediment, and quality of water in terms of smell, has been off at various times in the past year so there's obviously something going on that needs to be looked at, but it would be premature to speculate until we've identified the source."
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