The effect of aquaculture gear on the growth and shape of the oyster Crassostrea virginica during a "finishing period" in Chesapeake Bay, USA

Growth of the oyster aquaculture industry on the US east coast has accelerated in recent decades due to advances in breeding technologies and grow-out methods. As the industry grows, competition in the market place is likely to increase attention to product quality, for example, shell conformation and meat content. This study investigated how gear type and tidal zone location influenced shell shape and product quality of Crassostrea virginica during a four month "finishing" period at an oyster farm on Fishing Creek MD on the eastern shore of Chesapeake Bay. For these experiments, a mix of sub-market sized, triploid oysters from two local farms were deployed in four gear treatments that varied in water column location and exposure to wave action: OysterGro (TM) floats, a rack and bag system, a bottom cage placed in the intertidal, and a bottom cage placed subtidally, which represents the 'control' treatment (no change of gear type for finishing). Shell height, length, width, and weight (total and wet meat) were measured each month from August to December 2015 and an index of shell shape relative to an idealized 3-2-1 ratio of height to length to width was calculated. Overall, oysters grew well in all gear types (3.0-4.8 mm increase in shell height per month) and experienced relatively low mortality (4-8% across treatments) - mean height of oysters from each gear treatment exceeded market size by the end of the experiment. Gear treatment had a significant effect on oyster height and weight at the end of the experiment (P < 2.0e-05) and the OysterGro (TM) treatment produced the largest gains in growth compared to all other treatments (2.3-2.5 fold- increase in wet or total weight). Gear type also affected shell shape (P = 0.023), with significantly lower mean 3-2-1 ratio deviations (more ideal, rounded shells) in the OysterGro (TM) compared to subtidal bottom cage or rack and bag gear. Shell shape improved the most between initial and final time points for the OysterGro (TM) treatment, while the rack and bag and bottom cage subtidal treatments showed no improvement. This study provides some of the first data on growth performance and shell shape using the OysterGro (TM) system in Chesapeake Bay and overall, results indicate that floating gear such as the OysterGro (TM) system may be a valuable method to finish oysters, facilitating good growth, and improving shape over a relatively short period of time.