Aquaculture Production of Bay Scallops, Argopectens irradians
Each Spring, adult bay scallops are retrieved from ESL’s seaside grow-out plot and brought to the Castagna Shellfish Research Hatchery. There, ripe scallops are exposed to mimicked environmental cues in attempt to trigger a spawning event in a closed hatchery system. Once spawning has been initiated, egg fertilization is monitored, and soon after the fertilized embryos can be removed from the spawning system and placed into a larval culturing tank.
Bay scallops typically remain in the planktonic larval stage for ten days post fertilization (dpf). During this period, these larval cultures are maintained at a constant temperature and fed calculated rations of cultured microalgae each day. Every other day, the larval tanks are drained over a sieve according to size of the developing larvae. This allows for observations and quantification of the larvae that are used to dictate further culturing practices based on the health and total number. These drains on alternating days also allow for any accumulated waste products, contaminants, or unconsumed food or excess nutrients to be removed from a culture as the larvae are retained on the sieve, and anything smaller passes through the sieve as waste. After quantifications and observations are recorded, the larvae are returned to a clean, pre-heated larval tank according to healthy larval stocking densities.
Bay scallop larvae develop an eyespot when they are ready to set. This darkly pigmented spot appears between the digestive gland and the shells’ valves and signifies that the scallop is ready to metamorphose into its juvenile state. Often times by 10 dpf, these larvae have also developed a foot in addition to their swimming organ, the velum. At this stage, these are classified as pediveligers and are transferred to ESL’s land-based nursery for setting and post-set development.
In the nursery, water from the Wachapreague Channel is filtered to 50-100 microns to minimize sediment load and to prevent other bivalve larvae and marine organisms from colonizing inside the nursery system. The filtered water is collected in a reservoir tank that supplies the nursery. Rectangular raceway tables are connected in series and are continuously supplied with water from the reservoir tank. Upright structures are used to provide vertical surface area for the scallops inside each table, and sieves of appropriate mesh sizes are placed at the outflow to catch any detached larvae/juveniles. To prevent smothering due to sediment, the tables are rinsed and flow rates are adjusted daily. Juvenile scallops are reared in the land-based system for 4 to 6 weeks.
Upon reaching a shell height of 3 millimeters, these individuals are deployed and retained in bottom cages to grow and spawn. The cages are anchored in South Bay, just East of Oyster, Virginia, where a large portion of the seagrass restoration has occurred. This method secures broodstock and also provides for in-situ spawning, producing millions of larvae that are intended to colonize the grass beds and that could ultimately establish a self-sustaining population. Distinct lines are caged separately to have an accessible source of broodstock for aquaculture development. Throughout the grow-out period, gear is maintained and replaced regularly. Scallop survival and growth are also monitored during these maintenance trips and cage stocking densities are reduced as the scallops grow. Upon reaching 14 months of age, these individuals are released directly into the grassbeds.