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The effect of harmful algae on the summer mortality of juvenile pacific oysters (Crassostrea gigas) Cassis, David


During the summer of 2001, a mass mortality of early juvenile Pacific oysters, Crassostrea gigas Gmelin, was observed at a farm site in Jervis Inlet, British Columbia. During this episode, several toxin producing and potentially harmful algae were detected within the phytoplankton community, with a bloom of Protoceratium reticulatum (Claparede et Lachmann) Buetschli preceding and including the mortalities. Searching for a cause we examined experimentally the rapid response behaviour of juvenile oysters to various species of microalgae. The behavioural response was a strong rejection, complete closure and feeding cessation when exposed to cultures of P. reticulatum and Alexandrium tamarense (Lebour) Balech. While exclusion in pseudo-feces (Amphidinium carterae Hulburt) and mixed reactions were observed with other species (Heterosigma akashiwo (Hada) Hada ex Sournia, Karenia mikimotoi (Miyake et Kominami ex Oda) Hansen and Moestrup, Pseudo-nitzschia pseudodelicatissima (Hasle) Hasle, and Gonyaulax spinifera Diesing). The juvenile oysters feed well on the controls provided (Dunaliella tertiolecta Butcher, Isochrysis galbana Parke, Phaeodactylum tricornutum Bohlin, and Chaetoceros calcitrans Paulsen). Our study suggests that several of these phytoplanktonic species could have contributed to the oyster mortality in 2001 by causing starvation. The bloom of P. reticulatum was identified as the main probable cause for the die-off. In order to avoid juvenile losses, the phytoplankton composition and the size of the oysters should be considered at the time of introduction to farm sites. This study is one of the first to focus on the qualitative and quantitative responses of early juvenile oysters (+/- 5mm shell length) to various potentially harmful phytoplanktonic species.

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