The variation in growth of six strains of the fish-killer raphydophyte Heterosigma akashiwo, from different geographic areas, was studied, as a function of salinity (5, 20 and 35 psu), irradiance (40 and 100 µmol m⁻² s⁻¹) and temperature (17 and 23°C). Two of the strains were isolated from the Nervion River outer estuary (Bilbao, Bay of Biscay, Northern Spain), and the other four were from established cultures in culture collections, isolated from Florida (USA), Narragansett estuary (USA), New Zealand and Norway. The objectives of the study were to determine: (i) whether there were across-strain growth differences under a given set of temperature, irradiance and salinity conditions and (ii) in which locations of the Nervion River estuary would the strains grow optimally on the basis of their response to those conditions. Salinity was the first limiting factor. Little or no growth was observed at 5 psu. Growth of all the strains at salinities of 20 and 35 depended primarily on temperature and secondarily on irradiance. The Florida strain was the most tolerant, showing similar growth under all of the experimental regimes tested. The Narragansett strain was the least tolerant, its growth being impaired under low temperature and low irradiance.

Heterosigma akashiwo is one of the smallest raphidophytes and is known for its ability to form dense, fish-killing blooms. In many parts of the world, H. akashiwo is a permanent resident of the alg...