Occurrence of the Toxic Phytoflagellate Prymnesium parvum and Associated Fish Mortality in a Norwegian Fjord System

At the end of July 1989, toxin-producing Prymnesium parvum was spread through a fjord system in the surge of freshwater released from a hydroelectric power plant. In total, 750 tonnes of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) died in fish farms. Prymnesium parvum germinated in the brackish surface layer of a fjord branch which, during July, was characterized by longer residence time, higher temperatures, and lower nitrogen and silicate concentrations than the rest of the fjord system. Nutrient loading (especially phosphate) from fish farms may, however, have stimulated local growth of the alga. At the time of the first observed fish mortality, the salinity was 5‰ and the temperature 18 °C. Pelagic concentrations of P. parvum were generally low, with a maximum of 2.2 × 106 cells∙L−1 found close to a fish farm. Denser concentrations of P. parvum were, however, found in association with benthic substrates. Phosphorus limitation was probably important for the production of toxin by P. parvum.