Petuna Aquaculture's chief executive, Ruben Alvarez, said Petuna was currently dealing with elevated mortality at its marine farm in Rowella, in West Tamar.

"The farm experienced a temperature spike in an unusually short time frame, followed by a prolonged period of long hot days," he said.

"For now, our immediate concern is to safeguard those fish that have not been affected by removing those that have perished, as well as supporting our farming team who are working to manage the situation."

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EPA director Wes Ford said he was first notified of the early signs of the incident about 10 days ago, but said it didn't really begin at a mass scale until last Tuesday.

"As understood by the vets, the incident was a temperature-related issue," he said.

"The warm summer we've had to date, and the lack of any significant rainfall flowing down the river, has caused the temperature in the Tamar River to increase."

"If you look at the history of the Salmon industry, you'll see this sort of thing has happened during various hot summers ... this is one of the challenges of farming in the marine environment."

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Mr Ford said a long-term solution to the issue of water temperature could be to implement selective breeding of fish, by selecting genetic lines of fish that are more tolerant to warmer water.

He also said it was unknown how many fish were effected by the incident at Petuna's farm at the moment, but revealed the number was definitely in the thousands.

"The company has a standard process to recover mortalities, in which the fish are extracted using a fish pump, put into a truck, and taken to an EPA approved facility for disposal,' he said.

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After becoming aware of the circumstances, Foundation Fish Farm Campaigner at the Bob Brown Foundation, Rebecca Howarth, condemned the State's fishing industry and called for urgent change.

"Salmon are sentient beings and experience stress and pain ... they die from suffocation en masse," she said.

Tasmanian Alliance for Marine Protection (TAMP) Co-Chairman, Peter George, said the incident came as no surprise to the group which has relentlessly urged the industry to transition to land-based production.

"Tasmania's waters are warming far faster than anywhere else in Australia and the entire industry is already struggling with temperatures rising towards levels salmon cannot survive in," he said.

"TAMP calls on the EPA to be absolutely honest in revealing the details and cause of the fish kill, particularly in light of recent reports of other major salmon kills in the south of the state."

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