The mortality represents about half the total biomass at the sites in the Jervis Inlet.
Grieg said it continuously works to improve biosecurity and all of its sites perform algal monitoring by taking daily samples which are analysed using advanced image analysis techniques. This allows for the identification of the species, prevalence and depth distribution of any algae present.
The company said that the HAB in Jervis consisted of Heterosigma, a species of microscopic algae that cause acute mortality in fish, adding: “Due to extraordinarily high concentration and spread throughout the entire water column, use of aeration treatments or other protective measures could not prevent the incident.”
Grieg Seafood said it was insured against such incidents, and that estimated costs including individual share of insurance were limited to NOK 25 million (£2.3m). The costs will be charged the Q2 2018 results.
The fish affected were scheduled to be harvested in the second half of 2018.