Smolt are being introduced to New Zealand King Salmon's Waihinau farm in Pelorus Sound after tests ruled out disease as the cause of fish deaths there.

King Salmon aquaculture general manager Mark Preece said overseas testing showed there were no infectious salmon pathogens on the farm.

The Primary Industry Ministry, which was also investigating the cause of fish deaths, had confirmed in an email that they were not diseased, Mr Preece said. He expected this to be formally confirmed in a written report, expected within weeks.

Unusually high numbers of salmon have been dying at the 500,000-fish farm since late summer. Affected fish were covered with small lesions that looked like swollen mosquito bites.

The ministry did not require controls to be placed on the movement of smolt or the farm itself, Mr Preece said. However, King Salmon would continue to run its Waihinau farm under a voluntary "code red" status until the ministry confirmed its disease-free status. Restrictions included not moving equipment on or off the farm, not harvesting fish and appropriate disposal of dead fish.

King Salmon chief executive, Grant Rosewarne, said the fish deaths delayed the transfer of smolt to the Waihinau farm by about a month. The young fish were introduced last week.

Affected salmon on the farm were still dying because they were off their feed but no new fish were showing symptoms, he said. The company would start harvesting the new smolt salmon, grown since 2010, next month.

Mr Rosewarne said there were lots of theories on what caused the fish to die but he suspected the problem was stress caused by several factors. Two seals that lived in the cages for six months had only just been removed, temperatures exceeded a critical 17 degrees Celsius in summer and some feed had gone mouldy during wet weather.

Mr Rosewarne said staff had not counted the salmon to find out how many were lost because this would cause further stress. One clue was the reduced amount of feed eaten but he did not know how much the volume fed out had dropped.

The Pacific king salmon species was not susceptible to any pathogenic diseases, including infectious salmon anaemia, which was a problem in some countries, such as Norway and Chile, he said.

- The Marlborough Express

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