New Zealand's biggest salmon farmer is blaming a toxic algal bloom as the most likely cause of a large number of fish deaths at one of its five sites in the Marlborough Sounds.
Ministry for Primary Industries staff have been alerted by the New Zealand King Salmon company to the heavier-than-normal losses at its Waihinau farm in Pelorus Sound.
The company first noticed something was wrong in February, and began comprehensive testing.
But New Zealand King Salmon chief executive Grant Rosewarne says the tests identified no evidence of diseases behind the deaths and the company concluded there is some other natural cause.
Mr Rosewarne says the firm lost about $1 million worth of fish in an algal bloom outbreak at another of its farms in Queen Charlotte Sound about two years ago.
He estimates the latest losses at the Waihinau site may be even higher.