Aquaculture companies challenged as climate change kills fish

Aquaculture companies are grappling with higher rates of fish dying because of warmer waters.

New Zealand King Salmon had one in five of its salmon die last summer, prompting an increased focus on how to future-proof its business.

Salmon swim in aerated water to keep conditions cool, and are fed a nutritious diet to combat conditions, chief executive Grant Rosewarne said.

"If we do that, it lowers stress and they can cope with the higher temperatures better," he said.

Last summer's mortality rate was more than double the year before, at 8 percent.

"In the wild maybe only 3 [percent] would actually survive," Mr Rosewarne said.

"We're trying to turn that around and in a good year 95 percent will survive, and in a bad year, 80 percent."

According to NIWA, high pressure over the Tasman Sea last summer prevented the mixing of cool deeper ocean water with surface water, meaning the ocean water was abnormally warm.

NZ King Salmon are taking the lead of companies in other countries like Norway and looking to farm in the open ocean.

"The water is deeper, it's colder, the currents are flowing faster so they are better conditions," Mr Rosewarne said.

It would take at least 10 years for that to happen, he said.

In the short term the company are adapting to changes at its hatchery.

"We've always bred our fish to be what chefs' want - what they want to eat and what they want to serve their customers," Mr Rosewarne said.

"Now because of climate change we're altering that, saying we need to have more temperature-resistant fish."

Mr Rosewarne hoped the changes will tide them over with fewer salmon casualties this summer.