However, Danny Boulton, of Sustain Our Sounds, which is fighting an application to develop nine new salmon farms in the Marlborough Sounds, says the risk of disease will increase if salmon farming expands, reports TheMarlboroughExpress.
Niwa aquaculture and biotechnology chief scientist Mr Forsythe said pathogens and parasites that caused significant disease in captive and wild salmon in other countries were not found in New Zealand.
The Marlborough Express has been following the death of fish in unusually large numbers at New Zealand King Salmon's Waihinau farm in Pelorus Sound since last summer.
King Salmon chief executive Grant Rosewarne has said the fish had so far tested negative for a range of diseases.
Fish farmed by King Salmon are chinook, also known as king or quinnat salmon.
Mr Forsythe said chinook were not susceptible to normal diseases of New Zealand native fish. The one exception was the discovery of Aeromonas salmonicida bacteria in dead kanakana (lamprey) in September, and later in trout at a hatchery in Otago.
"The success of New Zealand salmon farming is in part due to the fortunate history that several species of [salmon] were introduced, but their diseases, in the main, were not," he said.
This meant New Zealand salmon farmers did not need to vaccinate their stock.
Chinook were resistant to potentially devastating infectious salmon anaemia (ISA), which affected atlantic salmon, Mr Forsythe said. Reports that chinook in Canada had tested positive for ISA proved incorrect, he said.
Mr Rosewarne says disease problems on overseas salmon farms are irrelevant in New Zealand because of strict biosecurity regulations.
The company had sent samples from fish that died at Waihinau to the Primary Industries Ministry to test for diseases including ISA and the vibrio species of bacteria, he said.
The tests came back negative.
The ministry had since sent samples to laboratories in Canada and Norway with more experience in salmon disease, but these results were not yet available.
Salmon farming had advanced since toxic algae killed many fish at Big Glory Bay in Stewart Island in 1989, Mr Rosewarne said.
"That makes the Waihinau experience most unusual."
Mr Boulton said it was realistic to expect disease in New Zealand farmed salmon.
Stepping up production levels increased the risk of disease, which was why King Salmon wanted more sites to spread this risk, he said. His group claims concentrated feeding at more fish farms would add nitrogen and phosphorus to the sea equivalent to the sewage from almost half a million people, which made disease outbreaks more likely.
In his evidence, King Salmon aquaculture manager Mark Preece has signalled the possibility of antibiotics being used on the farms to treat disease and said the company trialled the use of 52kg of antibiotic in 2000.
Primary Industries Ministry senior communications adviser James Sygrove confirmed no cause had been found for unexplained fish deaths at the Waihinau farm. He would not comment further until the investigation was completed, but expected the test results would be made public.
