A red tide of gyrodinium aureolum in sea lochs of the firth of clyde and associated mortality of pond-reared salmon

Red tides of the naked dinoflagellate Gyrodinium aureolum Hulburt occurred in sea lochs in the north of the Firth of Clyde, Scotland, during late September 1980. Greatest concentrations of the organism were found in the top 1 m layer of the water column, which was stabilized, and probably also enriched with nutrients, by freshwater input from land drainage. In addition vertical and horizontal concentration must be postulated to explain Gyrodinium cell densities of 2 x 10⁷cells l^-1and chlorophyll concentrations of 2228 mg m^-3near the shore at Otter Ferry, Loch Fyne. On 28 September 1980, water containing the red tide at Otter Ferry was unintentionally pumped into fish ponds at a shore-based salmon farm and resulted in the death, in one pond, of 3000 salmon each weighing about i kg and of 200-300 smolts in another when water was transferred to it from the affected pond. Pathological investigation of affected salmon showed that death was likely to have resulted from asphyxiation and osmotic shock as a result of extensive cellular damage to gills and guts. Results of mouse bioassays, using acidic and ether extracts of flesh and guts from affected salmon, suggest that necrotizing toxin(s) was associated with the cells of Gyrodinium aureolum during the bloom. The clinical signs exhibited by mice injected with toxin extracts were, however, unlike those caused by paralytic shellfish poison or toxins of the Gymnodinium breve type.