Farms in Tasmania's south have for weeks been in the grips of "elevated mortality" due to an endemic bacterium and warmer waters.
Tasmania's environmental regulator said the figure - about six per cent of the state's annual salmon production - was for February alone, and came from data from southern waste facilities.
The regulator said the scale of the deaths was unprecedented.
"The 2024/25 summer season saw a significant increase in mortality notifications in the southern region compared to previous years," Environment Protection Authority director Cindy Ong said.
"(We had) hundreds of pen notifications in February in the D'Entrecasteaux Channel region alone."
Tasmania's salmon farms must tell the regulator when they experience elevated mortality but won't be required to report the exact amount of dead fish until later in March.
The regulator is investigating whether environmental licences, waste regulations and the state's pollution control act have been breached.
Some waste facilities have been found to be non-compliant with requirements and will likely face enforcement action, Ms Ong said.
The regulator is also continuing an investigation into how congealed fish oil washed up on beaches in southern Tasmania in mid-February.
The industry has been under growing pressure, with approval for farms at Macquarie Harbour on the west coast under federal government review after a challenge from environmentalists who want to safeguard an endangered fish.
Producer Huon Aquaculture, which is experiencing elevated mass mortality at its farms in the south, is being investigating after a video showed workers sealing live salmon in a bin.
The company's RSPCA approval could be under threat, with a spokesman revealing the animal welfare body was seeking further information.
"The inhumane handling of live, sick or injured fish as shown is completely unacceptable," the RSPCA said.
Liberal Premier Jeremy Rockliff, whose party supports the salmon industry, on Thursday said companies were "on notice".
"I expect the highest possible degree of accountability and transparency ... because it is in Tasmania's brand interest and (the) industry's interest as well," he said.
State Greens MP Vica Bayley said he had received reports of salmon matter washing up on the western side of the Huon River, further afield from the February finds.
"What we're seeing raises huge environmental concerns, and is having a clear impact on the community," he said on Friday.
The Environment Protection Authority and industry body Salmon Tasmania have been contacted for further comment.
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