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NZ King Salmon chief executive Grant Rosewarne said warm sea temperatures at the company's Waihinau Bay farm, in Pelorus Sound, had contributed to the deaths. Rosewarne would not say for commercial reasons how many salmon had died, or how many fish were at the farm, but said the mortality rate was a "multimillion-dollar problem to solve".
Water temperatures at the Waihinau Bay farm had stayed above 18 degrees Celsius for three months, Rosewarne said.
"I don't think we've ever had it quite as bad as this year."
King salmon cannot regulate their body temperature. They function best when water temperatures are between 12C and 17C.
The increased salmon death rate in the Pelorus Sounds started in mid-February, Rosewarne said.
The water temperature dropped about half a degree last weekend after rain, but was still above 18C.
"We won't really see an improved situation until the temperature starts to drop," Rosewarne said.
"We're completely at the whim of the weather."
The Waihinau Bay farm is the only NZ King Salmon farm affected by higher than normal mortality rates.
No "primary pathogen" was pinpointed during investigations into what was causing the fish deaths, and there was no risk to human health, Rosewarne said.
Staff were "extremely disappointed" about the deaths and had done the best they could for their stock, he said.
"Within the constraints we've got, we have done everything possible to keep the stress low and give these fish the best chance possible of getting through the summer."
Losing fish in what was already a tight market had disappointing consequences for company growth, Rosewarne said.
"The losses at Waihinau Bay will impact our production in the short-term, however we have enough time to put a plan in place to avoid significant effect.
"Our longer-term production is not affected, although we will have to re-assess our site utilisation."
Lower flow water sites, such as the Waihinau Bay farm, were particularly at risk of sustained periods of high water temperatures.
Rosewarne said staff were unable to move the fish to another site because it would put further stress on them, and there were no suitable alternative sites for the fish.
The two-year-old salmon at the Waihinau Bay farm had been at sea for about nine months.
The temperature was not the only factor contributing to the large number of salmon deaths, Rosewarne said.
"It's never the sole reason. It's one of the more extreme things that can stress the fish out but there's other things that can affect it as well."
Giving the fish a nutritious and easily digestible diet helped ease the stress on them, Rosewarne said.
The company had changed to a more expensive feed at its Waihinau Bay farm after a high mortality rate last year, but the feed had not been as successful as hoped.
There would be another review of feed after the high number of salmon deaths this summer, Rosewarne said.
Other strategies to limit the effect of high water temperatures included selecting and breeding fish that had survived high temperatures in the past, and reducing stress on fish by keeping seals at bay with a large gap between the fish pen nets and the outer predator nets.
The dead fish are barged out from the farm and transported to North Island company Kakariki Proteins.
Staff at Kakariki Proteins sterilise and dry out the fish, extracting the oils and protein, before turning the salmon into dried pet and animal food.
Kakariki Proteins does not make products for human consumption.
Source: Marlborough Express