Numerous studies have reviewed how different husbandry decisions on oyster farms (e.g. stocking density, biofouling control, and gear type) correlate to oyster performance, but not much is understood about how these decisions affect the water parameters inside oyster grow-out containers. Ambient conditions on leases (monitored directly or through local water monitoring stations) might appear suitable for oyster performance, but different husbandry decisions could potentially affect the environment within the growing containers and cause significant deviation from ambient conditions .
Conditions inside an oyster bag may vary from ambient conditions due to reduced water exchange rates which may be exacerbated by biofouling communities and oyster stocking density. This ongoing project explores if stocking density (measured as percent of bag full with oysters) and total blockage (mesh blockage added to biofouling blockage) affect water parameters (dissolved oxygen, pH, turbidity, and chlorophyll-a) inside oyster grow-out containers at commercial shellfish aquaculture operations . Paired water samples were collected from May through October of 2022 and 2023 from the inside and less than 0.3 meters outside of oyster-growing containers at 22 farms in North Carolina, Florida, and Virginia across various gear types, salinities, and water temperatures. The differential (inside sample – outside sample) was calculated to standardize the results across the different sites.
Preliminary data analysis using a linear model suggests that there is an interaction between stocking density and total blockage on the percent dissolved oxygen (DO) and pH differentials (p < 0.01 for both). The combined effect of increasing stocking density and total blockage results in a magnified decrease of DO and pH . DO decreases 3.9% for each 10% increase in stocking density and total blockage, while pH decreases 1.6 x 10-3 with these 10% increases . These predictors explain 67% of the variance for DO and 56% for pH. Chlorophyll-a is affected by stocking density (p < 0.01) and explains 19% of the variance in the results. For each 10% increase in stocking density , chlorophyll-a decreases by 1.13 mg/L. The lowest DO, pH, and chlorophyll-a values were observed in bags with greater than 50% stocking density and 70% blockage (30% DO decrease; 15 mg/L chlorophyll-a decrease; 0.4 pH decrease). There is no relationship between the stocking density and total blockage for turbidity (p = 0.33 for the overall model; R2 = 0.05).
Results from this study suggest that farmers can influence the water parameters within their bags through various husbandry decisions. Ambient condition absolute values are not necessarily a good metric to describe what is happening inside grow-out containers. Water parameters within containers can be substantially lower than the outside environment. As predicted climate change could lead to further decreased dissolved oxygen and pH levels, understanding how culture practices might affect conditions within grow-out containers is essential .