Oyster farmers growing Crassostrea virginica are increasingly utilizing off-bottom culture practices as there is a potential to improve growth, survival, shell shape, meat quality, and product consistency relative to traditional methods. Despite this potential, sudden spring/summer mortality events impacting both on and off bottom farms, occurring without apparent connection to disease or harmful algal blooms, have been observed along the East and Gulf Coasts, with mortality reaching 85% in 2014 at some Virginia farms. These mortality events could not be explained by changes in ambient water parameters (e.g., a sudden drop in salinity) and typically, the mortalities do not affect every farm in a given waterbody. This pattern suggests that the seed stock and/or the farming practices may be key factors in these mortality events. For the latter, farming practices can lead to substantially different grow-out conditions for oysters.
Conditions inside an oyster bag (dissolved oxygen, pH, turbidity, and chlorophyll-a) may vary from ambient conditions due to a spatio-temporal lag induced by reduced water exchange rates which may be exacerbated by biofouling and oyster stocking density. This ongoing project addresses three questions: 1) Do biofouling control (air-dried vs. not air-dried) and oyster stocking density (high, normal, and empty) decisions affect the microclimate (water quality parameters) inside grow-out bags; 2) Do any observed differences in the water parameters correlate with C. virginica performance, including oyster health and disease prevalence; and 3) What combination of farm practices can growers employ to maximize oyster performance while minimizing costs?
Significant effects of both stocking density (typical grow-out density and highly stocked at 150% of typical) and biofouling control (air dried periodically or not) on water parameters within floating bags have been observed in work to date. In a sampling on September 19th, 2022, which occurred two days before a split (decreasing the density of oysters within each bag), stocking density significantly affected dissolved oxygen (p < 0.01), where dissolved oxygen was significantly lower in stocked bags, regardless of stocking density, relative to empty bags. The interaction of stocking density and biofouling control significantly impacted pH (p < 0.01), chlorophyll-a (p < 0.01), and turbidity (p < 0.02). pH was lower in stocked bags relative to empty bags, while turbidity and chlorophyll-a were also lower in stocked bags, with the exception of highly stocked, non-air-dried bags where the values were not significantly different from bags in empty treatments.
Results from this ongoing study suggest that farmers can influence the water parameters within their floating bags through various husbandry decisions, which could correlate to oyster performance. We hope to provide producers with better data so farmers can make more informed husbandry decisions while also identifying factors that could be driving these spring/summer mortality events.