"We put an amazing effort in preparing for the summer because we always know it's going to be a difficult time for our fish and there's a range of things we can do to try and improve the situation," Rosewarne said.
"We were thought we were on top of it and indeed we were, but unfortunately it was a particularly hot summer that started quite early and has continued on."
Records show 243 tonnes of fish waste was dumped in December, followed by 394.7 tonnes in January.
In February alone 632 tonnes of fish were dumped.
New Zealand King Salmon's farms in the Pelorus Sound, where the water is warmest, have been the most affected.
Rosewarne said several factors led to fish dying in high numbers, but the strongest correlation with fish mortality was high temperatures.
"Salmon are a cold water species and the ideal temperature is around 15 or 16 degrees Celsius, but when it gets to 18 degrees, and it's sustained there for weeks, that's when the problems really start."
The salmon producer now expected its underlying profit for the current financial year to be between $6.5 million and $7.5m, down from $10.5m and $12.5m.
Rosewarne said it was one of the reasons the company had applied to establish a farm in the cooler, deeper and faster current conditions of the Cook Strait.
The resource consent application for the Blue Endeavour project, off Cape Lambert was lodged in October 2019, with a decision expected by mid-year.
Conditions in the Marlborough Sounds were perfect for salmon nine months of the year but during the summer months, the sea temperature - particularly in Pelorus Sound, was too warm, he said.
Clare Pinder of environmental watchdog group Guardians of the Sounds said the amount of dead fish being dumped was appalling.
"Something is seriously going wrong at King Salmon's farms, it might be yet another unusual heatwave, it could be poor husbandry, it could be overstocking, it could be disease, but it's a huge proportion of their harvest that have died this year."
Monitoring data showed sea temperatures near the proposed Blue Endeavour site showed it's too warm to successfully farm salmon but the company was pinning its hopes on the expectation it would be successful in the cooler waters of the Cook Strait, Pinder said.
"The problem is it's not cooler, there have been temperatures in the range of 18 to 19 degrees this summer, well above the optimum range of 12 to 16 degrees Celsius."
According to Marlborough District Council landfill data going back five years, there had never been as many dead salmon dumped in such a short time, she said.
"If we were driving through the countryside and we saw dead sheep or dead pigs or dead horses in the paddock, they'd be an outcry. So why is there not an outcry on king salmon trying to grow fish when the environmental conditions are not suitable for them?"
MPI said it was aware of the higher than usual death rates at King Salmon's farms this summer.
Biosecurity New Zealand aquatic and environment health team manager Dr Michael Taylor said it was investigating the deaths, but warmer water temperatures are likely to be a contributing factor.
It has been meeting weekly with the company and says there have been no signs of exotic bacterial or viral disease infection but that Biosecurity NZ is awaiting further test results.
- RNZ
