“Platypus and fish are all shedding DNA into the water – it can be skin cells, hair cells, scales,” says ecologist Josh Griffiths. “A lot of it is actually urine and faeces – which, next time you’re swimming in the river, is probably not a great picture.”

There are a few chuckles from the dozen or so members of the Moorabool Catchment Landcare Group, who have joined Griffiths at a park in Ballan, a town in Victoria, Australia, located 78 kilometres north-west of Melbourne.

Griffiths works for EnviroDNA, a company that detects DNA from environmental samples in order to monitor populations of animals in particular locations. In partnership with Odonata, a biodiversity non-profit, the team is hoping to map the whereabouts of platypuses in Victorian waterways. Group members have volunteered their Saturday morning to join the search.

Expecting dreary weather, everyone is dressed accordingly – rain jackets and wellies abound – but it turns out to be a brilliant autumn morning. In the trees surrounding us, crows caw as Griffiths explains the programme for the day and gives a short safety briefing. “Look out for snakes and stings— so typical outdoor issues,” he says, and because we are all Australian, nobody bats an eyelid.

Our job today is to take samples from the Moorabool river, which is flowing at around 10 per cent of its usual volume. There are 18 sampling sites in total. People are assigned three or four sites each and given printed satellite maps with marked locations as well as testing kits in bright blue fabric lunchboxes.

Though most of us are wearing waterproof shoes, we are told not to get into the water if possible. This is partly to avoid disturbing sediment in the water, which can clog up the fine sampling filter, and partly to avoid contaminating the water with anything that may be on our boots, like dog faeces.

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Environmental DNA (eDNA) moves with the flow of water, travelling anywhere between 100 metres and 1 kilometre, says Griffiths. If any sample tests positive, it means there are platypuses in the upstream vicinity.

Logging their locations is increasingly important. Platypuses are found on the eastern mainland of Australia and in Tasmania. The animal is classified by the International Union for Conservation of Nature as a near-threatened species, but populations have declined in recent decades.

Research from the University of New South Wales concluded in November that platypus habitat has shrunk by 200,000 square kilometres in the past 30 years – an area roughly the size of England and Scotland combined.

In January, the government of the state of Victoria listed the animal as a threatened species for the first time. The platypus is also officially endangered in South Australia.

Major threats include vegetation clearing and urbanisation. “Because they’re dependent on aquatic ecosystems, essentially every time we modify our rivers, that’s going to have an impact,” says Griffiths. “Probably the number one threat is changes to flow regimes – typically that’s a reduction in water availability.”

Farley Connelly, who also works at EnviroDNA, demonstrates how to take samples, drawing up creek water through a large syringe and pushing it through a fine filter in a thin spurt. Gradually, the filter browns with collected matter.

The group scatters, driving off to take two samples at each of their assigned sites. I stay to chat and sample with Griffiths and Connelly. Our samples will eventually be tested in the lab with a platypus-specific probe, which binds to any platypus mitochondrial DNA that is present. Two samples of eDNA can detect the presence of a platypus with an accuracy of 97 per cent, says Griffiths.

The testing process seems simple – essentially just water collection – but it has revolutionised the way in which platypus populations are monitored, he says.

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Platypuses have home ranges of 1 to 2 kilometres and tend to be solitary animals, meaning that population surveys have historically been hard to carry out.

Because of their low density, Griffiths believes that mapping platypus whereabouts is a more practical measure of population health than quantifying numbers of animals at individual locations. In the past, he has done overnight trapping surveys, which involves a laborious process of setting and checking nets every few hours for the nocturnal creatures. “They’re not easy to spot,” says Griffiths, which somewhat alleviates my disappointment at not managing to see one today.

The team is planning to launch a Victoria-wide citizen science mapping project later in the year, sampling during platypus breeding season between August and October.

The hope is that by identifying waterways that platypuses historically inhabited but are now scarce in, the team may be able to develop programmes to increase populations, such as boosting water flows during dry summer months.

The team will also be testing for eDNA of other vertebrates, including fish, frogs and birds. “Platypus is the hook, but really it’s a waterway biodiversity survey that we’re going to be doing,” says Griffiths.