In a narrow-necked harbour on Tasmania’s west coast, an ancient bottom-dweller is losing its grip on existence as rings of farmed salmon fatten up.
Some environmental campaigners suggest the maugean skate’s slide towards extinction – in the one place it exists – is a relatively clear-cut case of cause and effect.
First and foremost, they blame fish farming for driving dissolved oxygen levels in Macquarie Harbour down to dangerous levels.
They point to a dire report that says the skate’s population has almost halved in just seven years as proof the salmon rings must go.
But does all or even most of the blame for the skate crisis really rest with the large salmon companies that operate there?
The scientists behind the report say there’s no doubt aquaculture is a big part of the problem.
However, Jayson Semmens and David Moreno from the University of Tasmania warn there’s danger in focusing on just one reason the fish is teetering on the brink.
The species, which dates back to the time of the dinosaurs, is also feeling the effects of hydro-electric dams that have altered the delivery of oxygenated fresh water into the harbour.
The harbour’s bottom waters have also warmed significantly due to climate change, with knock-on effects for oxygen availability.
And there are other pressures, including gill-net fishing and heavy metal pollution from old mines.
All of it in a harbour that’s never flushed well thanks to its deep central basin and the narrow, shallow neck that connects it to the recharging forces of the ocean.
The situation is complex but that complexity must inform what happens next, or the skate might not be saved.
“The modern conditions cannot be unentangled from one another,” Dr Moreno said.
“This is a system that is very heavily modified by human activity and the accumulation of those activities have to be managed holistically if we want to keep the skate alive.”
The long-lived war over fish farming in Tasmania has been reignited by the researchers’ warning that the skate might be one extreme weather event away from extinction.
The Bob Brown Foundation says one of the first steps to help the skate survive is to remove fish farms from Macquarie Harbour.
But Prof Semmens and Dr Moreno say that might not change anything.
“There might not be any change because there are such complex interactions,” Prof Semmens said.
“There’s also the possibility that we’ve changed the ecosystem – that the ecosystem has come to new equilibriums and we may not be able to completely reverse it.
“It’s not as simple as let’s do this and this changes.”
Jeff Ross has spent years studying Macquarie Harbour’s health.
The University of Tasmania associate professor said there was no doubt fish farming was linked to a deterioration in the harbour’s oxygen levels a decade ago, when the industry was radically expanding.
As the number of salmon ratcheted up, the environmental challenges became abundantly clear.
The increase in salmon biomass was halted and destocking followed.
“They’ve more than halved the biomass of salmon,” Dr Ross said.
“And we’ve seen some real improvements in terms of the benthos – the animals that live in the sediments.”
But Dr Ross said there had only been slight improvements in dissolved oxygen levels.
“So are we guaranteed of seeing a positive response to decreasing salmon aquaculture? I think it’s very unclear,” he said.
As the state and federal governments meet with threatened species experts to decide how to improve the skate’s survival prospects, all three researchers want the focus to be on everything, not one thing.
“If we don’t fix all parts of the problem there’s no point,” Prof Semmens said, warning an argument over salmon farming took the focus off the skate.
“We need to be thinking about all the things we need to do to help it.”
The scientists refuse to venture outside their lanes and suggest what levers authorities should pull.
But they say authorities in charge of the harbour’s health will need to pull as many levers as they can, all at the same time.
Greens senator Peter Whish-Wilson has spent years questioning the Commonwealth about what it’s doing to ward off the loss of the skate.
There was a sense of frustrated relief when the threatened species commissioner told a parliamentary hearing the federal environment minister planned to write to her Tasmanian counterpart, urging “extreme intervention” to save the skate.
The minister’s office has not released the letter and Senator Whish-Wilson says Australians deserve to know more.
He said some of the threats could be addressed quickly and time was running out for the skate.
Researchers are not finding evidence of young skates and the existing adults are ageing, meaning they’ll soon be unable to reproduce.
“We can permanently ban gill netting and we can take out fish farms and completely destock and give the skate the best chance possible,” Senator Whish-Wilson said.
“They’re actions we can control.”
Salmon Tasmania said the skate needed a team effort and fish farmers were ready to work with the government on a conservation action plan.
Hydro Tasmania said it would consider any evidence-based proposal to alter its water flows.
Federal environment minister Tanya Plibersek has promised Australians no more species will vanish on her watch.
The Threatened Species Scientific Committee, which advises the federal government, is due to meet shortly to consider upgrading the skate from endangered to critically endangered.
Australian Associated Press