Eastern oyster, Crassostrea virginica: Productivity and physiology under varying temperature and salinity conditions

Abstract

In the early 1990s, the introduction of suspended culture methods and the development of a market for cocktail oysters (65–75 mm) propelled the oyster aquaculture industry in New Brunswick forward, now producing over 140,000,000 oysters (337,000 floating bags) on 2,649 hectares of private shellfish leases. Recently, the industry has been threatened by an oyster disease outbreak, known as MSX in the Bras D’Or Lakes region (Nova Scotia). The possibility of the MSX parasite spreading into New Brunswick waters, as well as an increasing number of aquatic invasive species in the area, is a real concern for the industry and has prompted an investigation on possible mitigation strategies. The benefits of incorporating an upriver site into the production cycle of oyster aquaculture operations were evaluated. Seed (~ 26 mm shell height) and adult (~ 66 mm shell height) oysters were taken from downriver (salinity ~ 20–30‰) aquaculture sites and transferred upriver (salinity ~ 5–20‰) and monitored for shell and meat growth (May-October). At the same time, the impact of the relay fishery (harvest of oyster in closed marginally contaminated areas for relay to tenures in open areas) on oyster productivity was evaluated; wild upriver oysters were transferred downriver and monitored. Productivity data revealed that seed transferred upriver grew and survived as well as seed that remained downriver, while the mortality rates of adult oysters were lower at the upriver site. Meat content was unaffected in adult oysters transferred upriver. However, adult oysters transferred upriver had a gain in shell height (least squares means ± standard error) over the oysters that remained downriver (2.7 ± 0.5 mm vs. 1.8 ± 0.5 mm) and, in shell width (Median [95% CI]; 2.8 mm [1.9, 3.6] vs. 1.0 mm [0.3, 1.2]). Final organic meat content were approximately 35% less in adult oysters transferred downriver compared to those that remained upriver, confirming anecdotal reports from fishers of the relay fishery. Stress response of adult oysters was also assessed (80 and 150 days post transfer). Stress response measured revealed that regardless of their origin (upriver, downriver), oysters maintained upriver showed high levels of lysosomal destabilization and tissue atrophy. By contrast, the transfer downriver had the opposite effect, i.e. low lysosomal destabilization and low tubule atrophy. Results suggest that there exists no association between productivity parameters and the level of stress response measured by the biomarkers. The field investigation prompted further exploration on the impact of hemolymph fluid osmolality on neutral red retention assay (NRA) outcome. The prescribed saline solution for the assay assumes animals are isosmotic to surrounding water; however oysters sampled were hyperosmotic. Hemolymph osmolality was manipulated under laboratory conditions by subjecting adult oysters to temperature (0, 5, 10, 15, 20ºC) and salinity (6, 27‰) regimes. At 10–20ºC, oysters remained isosmotic to ambient waters, while hyperosmotic at 0–5ºC. When mimicking spring freshets (salinity 6‰), hyperosmotic condition was observed in all temperature groups, and was inversely proportional to temperature. Monitoring of valve activity confirmed a restricted exchange at 6‰ salinity (e.g. spring freshet). With these laboratory results, use of filtered hemolymph fluid (NRAMOD) is recommended instead of the prescribed standard solution (standard NRA) adjusted to ambient seawater