Broodstock management for the Bras d'Or Lakes oyster breeding program - resistance to MSX
The American oyster (Crassostrea virginica) is an economically, ecologically and culturally important species in Cape Breton, Nova Scotia, but populations have been in decline due to over-fishing, degradation of habitats and by the appearance of the MSX parasite (Haplosporidium nelsoni) in the Bras d'Or Lakes (Pitupa'q) in 2002. Rejuvenation of depleted private leases and public beds through seeding and cultivation programs has been proposed as part of the solution by DFO, Eskasoni Fish and Wildlife Commission (EFWC) and other stakeholders. Importation of oysters from outside of the Bras d'Or Lakes is not permitted in order to protect the native oysters from exposure to Malpeque Disease. Furthermore, there is recent molecular evidence that the Bras d'Or Lakes oyster is a population genetically discrete from oysters found in the Gulf of St. Lawrence. Therefore, aquaculture and the commercial resource must rely solely on resident populations for future culture and enhancement activities.
Since the onset of the MSX oyster disease, several disease management/studies and oyster enhancement initiatives have been undertaken, including the ACRDP project: "Initiation of a Bras d'Or Lakes oyster breeding program for resistance to MSX". A selective breeding program for disease resistance/tolerance is seen by the different stakeholders (DFO, oyster growers, Mi'kmaq elders) as another long-term strategy for the recovery of the Bras d'Or oyster. This R&D project was performed in 2005/06 by UINR at EFWC's research facility in Eskasoni and initiated a rotational breeding plan with oysters from specific sites within the Bras d'Or Lakes. Crosses were performed in the new oyster hatchery and progenies tested both for pedigree and disease status (Canadian Technical Report in prep.).
During the course of the project, it became evident that the surviving oysters collected from MSX infected sites did not optimally condition, even though water and food quantity and quality standards were met: their gonads were only partially full and they did not naturally spawn during a thermal shock in July 2005. Gametes had to be stripped and then fertilisation was performed according to the experimental protocol. Finally, larvae were raised and their performance was comparable to that of larvae raised in standard North-American oyster hatcheries. The direct and/or indirect effects of the MSX parasite on the gametogenesis and spawning of the oyster are not clear but, overall, MSX infection impedes on the abilities of an adult oyster to properly reproduce.
Temperature and salinity are two factors influencing the activity the MSX parasite H. nelsoni (Haskin and Ford 1982, Ford 1985). Temperatures below 5℃ or above 20℃ have been reported to control infection. Previous research has shown that H. nelsoni is inactive or absent at low salinity (10 ppt or lower) and low salinity immersions of oysters have been used as a control measure in Delaware Bay and Chesapeake Bay. However, gametogenesis is also retarded at salinities below 5 ppt.
The present research project proposes to precisely determine the time-temperature-salinity combinations needed for appropriate gametogenesis and spawning in MSX infected broodstock. This is critical to
- ensure the success on the on-going breeding program for resistance to MSX initiated in Eskasoni for the Bras d'Or Lakes oysters, and
- refine timing and zoning of oyster management activities within the Lakes.
2006 - 2007
Atlantic: Gulf of St. Lawrence, St. Lawrence Estuary
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