With depleted oxygen levels in Tasmania’s Macquarie Harbour pushing the Maugean skate towards extinction, the salmon farming industry and federal government have combined to fund a “nano bubbles” trial.
Starting next month, a diesel-powered generator on board a barge will be used to draw up low-oxygen water, fill it with highly concentrated bubbles of oxygen, and pump it back into the harbour at 30 to 40 metres depth.
The $7 million two-year trial has been jointly funded by industry body Salmon Tasmania and the Commonwealth, via the Fisheries Research and Development Corporation (FRDC).
The announcement of the trial came the same week that Australia’s Threatened Species Commissioner Fiona Fraser stated that Macquarie Harbour had been degraded “largely due to salmonid aquaculture”.
Commonwealth conservation advice highlighted that the “fastest and simplest” way to improve oxygen levels was for a significant destocking of salmon farms by this summer.
That did not occur, and instead the oxygenation trial is set to begin.
Tasmanian Labor senator Anne Urquhart said it would help to determine whether oxygenation was feasible in the harbour.
“We all want to see the Maugean skate protected,” she said.
“We also want a sustainable salmon industry on the harbour which supports good, well-paid jobs, and makes an important contribution to the local and state economy.
“And to do that we need to increase the levels of dissolved oxygen in Macquarie Harbour and examine the environmental responses to the oxygenation. We also need to know how feasible and scalable the system is.”
The Institute of Marine and Antarctic Studies (IMAS) at the University of Tasmania will design the system and oversee the trial, which will be run by Salmon Tasmania.
IMAS will report the results to the Commonwealth and Tasmania’s Environment Protection Authority (EPA) to determine if it continues.
All three major salmon companies in Tasmania – Tassal, Petuna and Huon Aquaculture, owned by JBS – operate in the harbour.
Fears for skate future in El Niño year
It will not be the first time that oxygenation has been used to improve water quality in Australia.
It is used in the Swan and Canning rivers in Western Australia to reduce the presence of phytoplankton blooms following degradation caused by human activity.
Tasmanian Greens senator Peter Whish-Wilson said oxygenation should have been investigated more than a decade ago when salmon farming expanded in the harbour, but it was too late now.
“This is at least a two-year trial to obtain data, to inform modelling on whether this works and whether this will make a difference to oxygen levels around salmon farms,” he said.
“Given this is an El Niño year, and we’re already seeing significant changes in ocean temperature in December, then this is not a solution.
“This is not the right lever to be pulling to save the skate.”
The salmon biomass in the harbour peaked at 20,000 tonnes in 2014-15, following an expansion in 2012 that will enable it to reach 29,000 tonnes.
Commonwealth conservation advice found there was “significant correlation” between a reduction in dissolved oxygen levels and increases in the salmon farms.
Maugean skate numbers decreased by up to 47 per cent between 2014 and 2021.
As water quality reduced in response to the farm expansions, biomass limits were set at 9,500 tonnes in 2020.
This then resulted in increased feed being used by the farms, causing nitrogen levels to rise. The EPA then introduced nitrogen level restrictions in 2022.
Salmon Tasmania chief executive officer Luke Martin said the industry was being “responsible” in helping to fund the oxygenation trial.
“There are absolutely no guarantees that taking the easy route and blaming salmon will help to save the skate,” he said.
“But what is 100 per cent certain is that reducing aquaculture in Macquarie Harbour will cost jobs and devastate communities.”
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