Susceptibility of fish to Chattonella marina is determined by its tolerance to hypoxia

► Fish kill mechanisms by C. marina. ► A significant elevation of blood osmolality is the universal response in moribund fish exposed to C. marina. ► Both respiratory impairment and osmotic distress are important mechanisms leading to fish kill by C. marina. ► The susceptibility of marine fish both interspecies and intraspecies to C. marina inversely related to their tolerance to hypoxia. The harmful alga Chattonella marina has caused massive fish kills and economic losses worldwide. However, the fish kill mechanisms by C. marina have not been identified. The present study has confirmed that a significant elevation of blood osmolality is the universal response in moribund fish exposed to C. marina and the possible reasons leading to contradictory reports were identified. Both osmotic distress and respiratory impairment are important mechanisms leading to fish kill by C. marina. The susceptibility of marine fish to C. marina appears to be inversely related to their tolerance to hypoxia, with the hypoxia intolerant goldlined seabream being the most susceptible, and the hypoxia tolerant green grouper being the most tolerant to C. marina. Further studies in the marine medaka (Oryzias melastigma) showed that fish susceptibility to C. marina is directly related to susceptibility of the fish to hypoxia, but not related to its tolerance to hypersalinity stress.