Microbiological aspects of an oyster (Crassostrea gigas ) hatchery, with special reference to larval mortalities

Abstract

Investigations of the cause/s of larval mortalities at a commercial Pacific oyster '(Crassostrea gigas ) hatchery in Tasmania, Australia, were carried out during successive production seasons. Epidemiological evidence suggested that cultures of microalgae, harvested semicontinuously as food for larval oysters (Crassostrea gigas ) were the major vectors for transmission of bacterial disease. There was no evidence that bacteria associated with intake seawater, fertilised eggs or hatchery fomites (biofilmed surfaces) caused larval losses, or that non-bacteriological factors (eg. nutrition, gene pool, heavy metal contamination) were responsible for larval mortalities. All microalgal clones (Isochrysis sp. clone T. Iso., Chroomonas salina Butcher 3C, Thalassiosira pseudonana Hasle et Heimdal 3H, Chaetoceros calcitrans Paulsen, Dunaliella salina Butcher, Pavlova (Monochrysis ) lutheri Droop, Tetraselmis suecica Butcher)used as food for larvae, were implicated in the transmission of disease. It was apparent that mixed, rather than single, bacterial biotypes were responsible for disease. Bacteria cultured from moribund larvae rarely included presumptive Vibrio or Flavobacterium /Cytophaga spp., and never red-pigmented Pseudomonas spp .