The death of more than 1000 tonnes of salmon over seven months at farms in a remote harbour "devastates" claims of a sustainable industry, environmentalists say.
But an aquaculture head says the figure is a consequence of the size and scale of operations.
The federal government is reviewing the future of fish farming in Tasmania's Macquarie Harbour, the only home of the endangered Maugean skate.
Scientists say urgently destocking salmon farms is the best way to safeguard the skate, which has been impacted by poor oxygen levels linked to aquaculture.
Between September 2023 and March, 1,149,795kg of salmon died in the harbour, with the highest monthly losses in January (313,895kg).
The data was released by the Tasmanian government after a right-to-information request by anti-salmon campaigners Neighbours of Fish Farming.
"(These figures) devastate the salmon industry's claims of high standards of animal welfare," campaign spokesperson Peter George said.
Salmon Tasmania, which represents Tassal, Huon Aquaculture and Petuna which all farm in the harbour, said the deaths occurred for a "variety" of reasons.
"These incidents happen as part of normal production," Salmon Tasmania CEO Luke Martin told ABC radio.
"There wouldn't be a single farmer … who wouldn't be dealing with livestock losses on a regular basis.
"It's just the reality of the scale of this industry. It is large which gives you large numbers.
"It is something the industry spends a lot of time, a lot of resources (and) a lot of research trying to minimise."
The maximum fish stock level in the harbour is 9500 tonnes, meaning the losses account for about 11 per cent.
Tassal, which produced 37,425 tonnes of salmon Tasmania-wide in 2022/23, says it has a 91 per cent survival rate for its marine-farmed salmon.
An online state government portal notes there have been no significant salmon mortalities in the harbour since 2018.
Greens MP Vica Bayley said according to information from budget estimates hearings there were 66 mortality events statewide from July 1, 2023 to June 30.
He has called for greater transparency around the release of data.
"We need the exact numbers of mortalities, we need to know what these poor animals are dying from," he said.
"We need to know what effect this is having on our precious waterways and native wildlife."
Mr Martin said the industry operated within environmental permits and reported all mass mortalities to the regulator as required.
"We don't believe there is any correlation between the (September to March) numbers … and the conditions of the harbour," he said.
Federal Environment Minister Tanya Plibersek has given no timeline for her review but has previously said it will consider the best scientific advice.
Dozens of leading scientists on Thursday penned an open letter to Ms Plibersek urging her to revoke 2012 approval for expanded farming in the harbour.
Recent data shows oxygen levels are improving and skate numbers have stabilised at a low level.
The state Liberal government supports the industry in the harbour and has warned of job losses if it is shut down.
