An endangered fish found only in Tasmania survives in just one lake, scientists have confirmed, ruling out the possibility of insurance populations being used to save the species.
The Maugean skate lives in Macquarie Harbour on the state's west coast, a body of water heavily impacted by fish farming operations.
The species was thought to also exist at Bathurst Harbour in the southwest, but a study by the Institute for Marine and Antarctic Studies (IMAS) has ruled that out.
The skate was first discovered at Bathurst Harbour but only four individuals have been seen there and none since 1992.
IMAS researchers tested water in the harbour in November 2021 and February 2022 for the presence of skate DNA.
"Despite very extensive sampling, we only detected minute traces of Maugean skate DNA," IMAS researcher and study leader Professor Jayson Semmens said.
"This was most likely only remnant traces of biological material such as egg cases that remained in the sediment, or a few live individuals at the most.
"It is now clear that the only known population is in Macquarie Harbour, where conditions are impacted by historical and ongoing human activities."
Conditions in Macquarie Harbour's water column have deteriorated due to decreases in dissolved oxygen levels, coinciding with the introduction of salmon farming.
Tasmania's Environment Protection Authority (EPA) last week implemented a new system to limit operations at Macquarie Harbour, putting a cap on the farms' total dissolved nitrogen output.
Farming in the harbour had previously been regulated by a biomass limit, most recently set at 9500 tonnes.
The EPA said companies had complied with the figure but annual feed inputs had increased since 2018.
Farming in the harbour peaked in 2014/15 when there was more than 20,000 tonnes of fish present.
Prof Semmens said previous research had shown the skate was impacted by deteriorated oxygen levels at all stages of life.
It is also at risk from heavy metal pollution in the harbour linked to historic mining runoff.
"We can no longer regard Bathurst Harbour as an 'insurance population' against any declines in the Macquarie Harbour population," IMAS researcher and lead report author Dr David Moreno said.
"Conservation of the Macquarie Harbour skates is our only option to ensure the survival of this unique species."
Maugean skates, which have adapted to Tasmania's western estuaries, have one of the most limited distributions of any known species of shark, ray or skate in the world.