Vulnerability of marine fish larvae to the toxic dinoflagellate Protogonyaulax tamarensis

Fish larvae and early postlarvae proved highly vulnerable to the toxins of the dinoflagellate Protoyonyaulax tamarensis. Capelin Mallotus villosus and Atlantic herring Clupea harengus harengus larvae from the St. Lawrence Estuary, Canada, were exposed to variable concentrations of a toxic strain (treatment) and a non-toxic strain (control) of P. tamarensis. When exposed directly to the dinoflagellate, mortality due to the toxin (treatment minus control) was strongly correlated with the percentage of larvae that ingested cells. At cell concentrations (1500 cells ml-') that compared with local bloom densities, mortality reached 92 and 77 % d-' in capelin and herring, respectively. The mortality of herring postlarvae fed toxic microzooplankton ranged from 17 to 36 % d-' Age-and dosedependent variations in vulnerability were linked to early ontogenetic changes in the feeding efficiency and food selectivity of both species. We conclude that the present proliferation of toxic dinoflagellates in coastal waters could jeopardize the early survival of fish and recruitment to fisher~es by narrowing the spatiotemporal window within which spawning leads to successful reproduction.