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

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. When microalgal cultures containing more than `log_10` `6.3` \\(culturable\\) \\(bacteria\\) `mL^-1` were fed to larvae, the larval growth rate was reduced within 2-3 days. Small (15-25%), major (25-90%) or total (> 90%) losses occurred within 1-5 days. If no further cultures of microalgae exceeding the bacterial threshold were fed, surviving animals grew satisfactorily to the ready-to-set stage. If further cultures exceeding the bacterial threshold level were fed further small, major or total (> 90%) losses were experienced within 1-7 days. During the course of the study, techniques for the disinfection of microalgal growth medium were altered from pasteurisation to 0.2 ˜í¬¿m membrane-filtration technology. This resulted in the mean levels of culturable bacteria in the growth media decreasing from `log_10 4.1 mL^- 1` to `log_10 1.5` `100 mL^-1` Scanning electron microscopic examination revealed a decrease in the proportion of microalgal cells colonised by bacteria when microalgal growth medium disinfection techniques changed from pasteurisation cooling to membrane-filtration. The proportion of microalgal cultures (at harvest) containing> `log_10 6.3` `bacteria` `mL^-1` decreased from 55.6% (using pasteurised-cooled seawater as microalgal growth medium), to 20.6% and 10.9% in two successive production seasons during which 0.2 ˜í¬¿m membrane-filtered seawater was used as microalgae growth medium. The yield of ready-to set larvae rose during the same seasons from 1.3% to 3.2% to 4.6% of fertilised eggs.