The Department of Fisheries, Forestry and Agriculture says more than 200,000 salmon died as the result of an unusual incident at the Marine Harvest aquaculture site on the south coast known as the Gorge.

The department says the company reported on September 11 that just over 92,000 of the total number of 900,000 fish on site were affected by a sudden drop in low dissolved oxygen levels.

As of September 21, the number of fish killed was 212,000, or 24 per cent of the fish on site.

The department says the incident is site-specific and no abnormal mortalities have been identified at other sites. The affected fish were at marketable size and removal and cleanup operations are ongoing.

Mitigation measures include the installation of deeper nets and aeration equipment on site.