Fish are harvested at Clay Point Farm, in Tory Channel, by New Zealand King Salmon. (File photo)

Salmon in the Marlborough Sounds are dying in their farms as an "extraordinarily hot" summer continues to take its toll on marine life.

While shellfish were cooking in the sand north of Wellington earlier this week, veterinarians and welfare officers are monitoring salmon in the Sounds as water temperatures keep rising.

King salmon prefer temperatures between 12 and 17 degrees Celsius, but the farms have been warmer than usual since early December, and one farm has been consistently over 19C at a depth of 5 metres for the last three weeks.

New Zealand King Salmon chief executive Grant Rosewarne said the farms had to manage warmer water every summer, but this summer was "extraordinarily hot", forcing the company to review its harvest.

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Salmon could not self-regulate through perspiration like humans, and some had ultimately died, Rosewarne said.

SCOTT HAMMOND/STUFF Salmon are harvested with a 'continuous flow' pumping system at the Clay Point farm in Tory Channel, Queen Charlotte Sound. (File photo)

"Like humans in a heat wave, the salmon's appetite is lower, and they need more room to manage the heat. We are actively managing the biomass on our farms to help the salmon," Rosewarne said.

"In addition, we have adjusted feed times to the beginning and end of each day when the water surface temperatures are cooler, and are putting extra effort into maintaining the farm environment in optimal condition for as low a stress environment as possible."

When salmon died before harvest, they were removed and disposed of, Rosewarne said.

SUPPLIED New Zealand King Salmon chief executive Grant Rosewarne says the company has to manage warmer water every summer. (File photo)

"A small amount of mortalities within farmed livestock of any type is a normal occurrence in any population, and best biosecurity practice is to remove these immediately."

Rosewarne would not say how many had died from the heat so far this summer. They were being dumped at a landfill, Rosewarne said.

"We would never use mortalities for human or pet consumption," he said.

SUPPLIED New Zealand King Salmon sends its harvested fish to be processed at a factory in Nelson. (File photo)

"However we have commenced an ensilage programme with new technology that will shortly allow us to develop other products, like compost, with them."

Some salmon farms were warmer than others. Kopaua Farm, in the Pelorus Sound, was reaching the highest temperatures, he said.

"Fortunately, Kopaua is a high-flow site and is better suited to salmon production. If it were not for this, stress levels would be higher," Rosewarne said.

STUFF Some farms are in warmer water than others. (File photo)

"We actively manage our lower flow sites during the summer months to minimise the biomass we hold on them, ensuring that the majority of our salmon are held on the higher-flow sites.​"

New Zealand King Salmon had applied to move some farms from low-flow sites to high-flow sites in the Marlborough Sounds, arguing it would bring both economic and environmental benefits.

Former Minister for Primary Industries Nathan Guy was supposed to make a decision on the proposal, but new Minister of Fisheries Stuart Nash had since taken over the reins following the change in Government, and had not yet made a decision, his office confirmed on Thursday.

JOHN COWIE/STUFF New Zealand King Salmon is waiting for a decision on relocation of salmon farms to high-flow sites.

Rosewarne said New Zealand King Salmon aquaculture staff were re-assigned to help harvest salmon at Kopaua and Clay Point, in Tory Channel, this week.

They were also using "feeding strategies, net cleaning, density management and oxygen checks" to keep the salmon "comfortable", Rosewarne said.

"Historically, sea temperatures in the Marlborough Sounds don't normally drop until the end of March, and our planning for the summer period takes this into account.

JOHN COWIE/STUFF New Zealand King Salmon wants to move four salmon farms in the Pelorus Sound.

"Our vets and trained welfare teams are closely monitoring the salmon's welfare and ensuring the highest standards of care are applied over this time to help our salmon cope with this extreme situation."

Rosewarne did not expect the amount of product entering the market to be severely affected by deaths related to heat, he said.

"Demand for our branded premium salmon has consistently exceeded available supply, and with the hot weather recent New Zealand sales have been particularly strong," Rosewarne said.

"Our most recent forecast for the current financial year is for sales of around 8000 metric tonnes, significantly up on [financial year 2017's] actual sales of approximately 7200 metric tonnes."

Juvenile fish, or smolt, were not affected as temperatures were less variable in their spring-fed freshwater hatchery in Canterbury, Rosewarne said.