In summer 2001, the St. Lawrence River (Québec, Canada) experienced perhaps the largest massive fish kill of
its history, with more than 25 000 carp (Cyprinus carpio) found dead. This experiment therefore investigated the effects
of heat stress on the gene transcription level of heat shock protein 70, cytochrome c oxidase and mitochondrial superoxide
dismutase expression as well as on phagocytosis of kidney cell suspensions and animal behaviour. Our study suggests that
in summer 2001, elevated water temperatures could have contributed to the fish kill through immunosuppression during
an already stressful spawning period.