Eastern Oyster Crassostrea virginica Growth and Mortality in New Hampshire (USA) Using Off‐Bottom Farm Gear

Abstract

Eastern oyster farm production has increased dramatically in the northeasternUnited States in recent decades. Farming methods vary, but different configurations of off-bottom "rack-and-bag" gear are commonly used and there is little published research on how different methods affect oyster growth. Field experiments were conducted during 2016-2018 on two farm sites in New Hampshire to assess the effects on oyster growth and mortality of different gear deployment configurations: (1) bag height above the bottom; (2) different combinations of oyster seed size, bag mesh size, and stocking density; (3) rack-and-bag versus bottom tray; (4) oyster seed size and over-winter mortality; and (5) envelope-versus box-style bags.Bag stocking density consistently had the strongest effect, with oyster growth being up to 3 times faster in bags that were stocked at 0.5 L compared with 2.0 L of wet oysters. Additionally, stocking density had a stronger effect on the oysters that were in 6-mm mesh bags than on those that were in 4-mm mesh bags. Oysters that were in bags in the top level of the rack grew significantly faster in one experiment but not in another. There was no significant difference in oyster growth comparing rack-and-bag and bottom trays. The over-winter mortality of oysters that were raised in rack-and-bag gear averaged 17% in one experiment and 46% in another. Early growth was similar for oysters that were deployed in box-and envelope-style bags, but by the final measurement (at 2.4 months) oysters in the box-style bags were 18% larger. The major findings were interpreted in the context of the notion that food supply and the factors affecting it have strong effects on the growth of farmed oysters, so this dynamic should drive strategies to improve gear deployment methods. Additionally, the typical situation of widely varying differences in uncontrollable factors (e.g., currents or phytoplankton concentrations) among sites and their effects on gear deployment methods may best be addressed by site-specific empirical studies that are conducted by farmers.In many areas, eastern oyster Crassostrea virginica aquaculture has been shifting from wild harvest in leased or otherwise permitted sites to a variety of gear-intensive methods that use hatchery-produced seed oysters (MacKenzie et al. 1997;Brennessel 2008). Seed oysters are initially grown in a nursery system (e.g., upwellers), and