Farmed salmon left parts of this harbour almost 'devoid of life' — but there are signs of recovery

As the environmental condition of Macquarie Harbour on Tasmania's isolated West Coast deteriorated in 2016, a fight amongst salmon giants and industry regulators boiled over into the public eye.

Patches of the harbour floor were found to be virtually "devoid of life" and were labelled dead zones — a phrase that took hold. 

Dissolved oxygen levels in the bottom waters fell dramatically towards zero, and bacterial matting was discovered well inside the bordering World Heritage Area. 

Huon Aquaculture took their concerns to court, challenging the state government and Environment Protection Authority over the handling of salmon farming in the harbour.

Ultimately, Huon lost its case. 

But the company succeeded in drawing attention to the issues and, after years of rapid expansion, the amount of salmon that could be farmed in Macquarie Harbour was slashed. 

Years later, the divisions between the major industry players have eased.

Together, they organise an annual cruise to take locals around the harbour, show them the farms and hear from scientists.

This year, Jeff Ross from the Institute of Marine and Antarctic Science told those on board that the sediments on the harbour floor had rapidly recovered and life has returned. 

"There are not persistent or recurrent dead zones in the harbour at the moment," Professor Ross said. 

IMAS researcher Dr David Moreno and associate professor Jeff Ross have been monitoring conditions at Macquarie Harbour.(ABC News: Alexandra Humphries)

Oxygen levels have also improved, but they remain low. 

The problem is being compounded by climate change, with scientists recently confirming a rise in water temperature of between 1.5 and 2 degrees over the past 30 years.

"When you look back at the temperature records in the bottom waters, they actually reflect the warming that we've seen out on the open coast," Professor Ross said. 

"I guess that makes sense; the bottom water is oceanic water that comes in during recharge events."

Life is returning to Macquarie Harbour's floor, but is still facing environmental challenges.(ABC News: Alexandra Humphries)

'It has bounced back fairly well'

Locals want the recovery of the sediments to bring an end to talk of dead zones.

Dianne Coon is satisfied with how Macquarie Harbour is now being managed.(ABC News: Alexandra Humphries)

Dianne Coon from the Strahan Aquaculture Community Forum is frustrated by the phrase. 

"It's a slogan, 'the catastrophe at Macquarie Harbour', and I want to grab those people and say — can you explain to me what it is? Can you show me where the dead zones are?" she said.

Ms Coon said the issues that erupted in 2016 were frightening for locals, but they're satisfied with the way the harbour's now being managed. 

"It seems to me that we can sustain a salmon and trout industry in the harbour with a reasonable number of fish, we can't sustain high numbers," Ms Coon said. 

"It's still at risk, it's got a low oxygen base which makes it difficult, there are some issues about maugean skates that are concerning, but by and large it has bounced back fairly well."

Concerns remain about what low oxygen levels mean for the maugean skate population.(Supplied: Neville Barrett)

The elusive and endangered maugean skate has only ever been found in Macquarie and Bathurst harbours but hasn't been spotted in the latter since the 1980s. 

IMAS researcher David Moreno said scientists were worried about what low oxygen levels may mean for the population, which is roughly estimated to be about 3,000. 

"The population seems to be getting older, and we seem to be catching a smaller proportion of juveniles of smaller individuals," he said. 

"We cannot tell definitively why that's happening … if it were to be a decline in recruitment, that could have big implications for the future of the population."

Dr Moreno said answers were urgently needed, to determine whether intervention would be required to help the species survive. 

Challenges ahead for future sustainability

With signs of recovery emerging, Tasmania's environmental watchdog is due to reconsider how much salmon should be farmed in the harbour. 

The permitted biomass peaked at 21,000 tonnes in 2016, but it's since been more than halved to 9,500 tonnes. 

A new biomass decision will come into effect in May.

Huon, Petuna and Tassal have not said whether they will seek an increase, saying in a joint response that they would farm to whatever limit is set. 

But EPA director Wes Ford said they would need to make the case for a higher limit if they want change. 

"At this stage, there's no evidence that I have that suggests it needs to be reduced," Mr Ford said. 

"Any proposal for an increase to the biomass is really a matter for the companies to put to me."

Only 9,500 tonnes of salmon is currently permitted to be farmed in the harbour.(ABC News: Alexandra Humphries)

Mr Ford took control of regulating the industry just months before Huon's concerns about the harbour were aired nationally on ABC's Four Corners program in October 2016. 

Early the following year, he reduced the biomass cap to 14,000 tonnes and ordered the lease closest to the World Heritage Area — operated by Tassal — be destocked. 

That lease, known as Franklin, remains Tasmania's most heavily regulated salmon farming site.

The EPA is due to reconsider how much salmon should be farmed in Macquarie Harbour.(ABC News: Sophie Zoellner)

Mr Ford said on reflection, he was comfortable with how he had handled the situation.

"The message that I would give to people about Macquarie Harbour is that salmon farming can be managed sustainably. It's about getting the right level of biomass," he said.

"At 9,500 tonnes, Macquarie Harbour is showing definite signs of recovery and that could be a sustainable biomass for the industry for decades to come."

According to Professor Ross, the harbour's long-term sustainability relies on finding the right balance. 

"I've always said that at the right biomass, history would suggest that you can grow salmon in Macquarie Harbour," he said. 

"The challenge now is finding out what that biomass is."

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