First record of a Heterosigma (Raphidophyceae) bloom with associated mortality of cage‐reared salmon in Big Glory Bay, New Zealand

In early January 1989, a dense phyto‐plankton bloom occurring in Big Glory Bay, Stewart Island, New Zealand, was associated with chinook salmon kills. The total loss was estimated to be NZ$17 million. Water samples collected between 5 and 6 January showed Heterosigma cf. akashiwo as the dominant species. This bloom is the first record of Heterosigma in New Zealand, and is the first New Zealand record linking this species to salmon kills. During the period of salmon mortality, Heterosigma attained densities of nearly 2 × 10 6 cells 1 −1 , and was found in greatest concentration on the north‐west side of Big Glory Bay where lowest Secchi disk depths were recorded. Histopathological investigations of salmon which were affected by the bloom revealed gill and intestine pathology characterised by degenerative changes of the branchial epithelium and vasculature. The pathology observed was similar to that reported for Gyrodiniwn aweolum in Atlantic salmon. Impairment of respiratory and osmoregulatory function of gills was implicated as the cause of death in fish exposed to Heterosigma bloom.