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First record of blooms of Cochlodinium sp. (Gymnodiniales, Dinophyceae) causing mortality to aquacultured salmon on the west coast of Canada

Abstract Blooms of Cochlodinium sp., monitored for the first time on the west coast of Vancouver Island from August to October 1999, caused substantial mortality to farmed salmon, accounting for economic losses of about CAN $2 million. Cells of the alga were 25–40 μm long, 20–30 μm wide and had a torsion of 1.8 to 2 turns. Pairs of cells were common, forming up to 25% of the biomass; apart from the absence of longer chains, the morphology of the species, including the anteriorly placed nucleus and numerous golden chloroplasts, matched the gross morphological description of Cochlodinium polykrikoides. A strong diurnal pattern was observed in blooms at farm sites, with high cell concentrations overnight at depths of up to 25 m and dense surface blooms during the day. Surface concentrations of up to 60,000 cells ml−1 peaked in early September. Fish stopped feeding when cell counts exceeded 500 cells ml−1 in the net-pens, and mortality was observed above 2000 cells ml−1. Bioassays in the field with Atlantic salmon (Salmo salar) smolts demonstrated lethality after 120 min exposure and over 90% mortality after 500 min, when cell concentrations varied from 10,800 to 2700 cells ml−1 as the bloom moved through the test site. Under controlled laboratory conditions, S. salar smolts died within 27 min of exposure to 7200 cells ml−1 and 55 min with 3400 cells ml−1; although fish appeared distressed at 1000 cells ml−1, only 20% died within the 24 h bioassay. Enhanced toxicity of Cochlodinium sp., observed when the dinoflagellate was oxygenated or aerated, may have been a function of agitation, leading to increased release of lethal reactive oxygen species. Mitigation protocols of net-pen enclosures, with 14 m deep tarps and upwelling of deep water by aeration, proved less effective against Cochlodinium sp. than for Heterosigma akashiwo, the major harmful algal bloom killer of farmed salmon on the west coast of Canada.