Sea scallops – also known as the Atlantic Sea Scallop, or the Giant Sea Scallop – are filter-feeding bivalves. They occur in coastal and offshore waters from Labrador to North Carolina, along the US and Canadian coasts. Best estimates conclude that scallops can live over 20 years, and grow to be over 9″ in diameter. Sexes are separate (not hermaphroditic), with sexually maturity at age 2. In Maine waters, spawning generally occurs in July and August, but there may be wintertime spawning as well, since ripe male and female gonads have been observed in January, February and March. Fertilization happens externally, and larvae develop in the water column. The larval period is generally 40-45 days depending principally on temperature. Competent larvae – those that are ready to settle – have limited ability to determine their settlement site, and to delay metamorphosis for a short time, if conditions are not ideal. Post-set juveniles will attach to the substrate with a byssal thread, and can retain that ability until they are over 1″ in diameter. A settling sea scallop larva is about 450 microns, or about a half-millimeter in size.
Aquaculture of P. magellanicus is still in a fairly early stage of development, so there is still a good deal of experimentation under way. Thus far, traditional equipment such as lantern nets, bottom cages and pearl/lantern nets are in use, with some ear hanging as well. P. magellanicus has a different body type and behavior that other commonly-cultured pectinid species – such as the King Scallop (Pecten maximus) and the Yesso, or Japanese Scallop (Patinopecten yessoensis) – so it will be a process of investigation, trial and error to find the best husbandry and equipment for the sea scallop.
1）Hatcheries vs. Wild Spat Collection
Hatchery production of sea scallops has proven to be very difficult; many efforts over the years in the US and Canada have all encountered problems, making the process very unpredictable. With a larval period of roughly 40 days, cultures of larvae are very susceptible to losses. Ciliates, bacterial contamination and proper feed all contribute to problems in larval cultures, despite a fair amount of investigation in the US and in Canada.
On the other hand, collection of wild spat has proven to be very productive in certain areas along the Maine and Massachusetts coasts, with numbers per collector often reaching 2,000-3,000. Following a trip to Aomori Prefecture, Japan, in 1999, by a delegation of fishermen, scientists and managers from Maine, local fishermen started experimenting with Japanese spat collectors, and rapidly found success in waters just a little offshore, but still within state jurisdiction.
Spat collectors are bags of plastic mesh, with a mesh size of about 1.5mm. The bags are filled with plastic mesh of a different type, usually semi-rigid polyethylene (although gill netting and other materials have been used), to make a pillow shape. Collectors are strung to single, vertical lines, or deployed in an array, and suspended to float in the lower third of the water column. Collectors too close to the surface will foul, and if set within a fathom or two of the bottom will tend to gather sediment, or may abrade on the seabed. In Maine waters, collectors are generally set in the month of September, usually following the drops in water temperature the accompany storms or other weather fronts. Collectors usually stay in the water through the next several months, and need to be checked frequently to prevent tangles, and to keep them out of towing areas by shrimpers, scallopers and draggers. Scallops will not be readily visible until January or February, and by May of the following year should be in the 3-10 mm range.
Nursery culture of juvenile scallops is usually done in pearl nets or fine-meshed shellfish bags set in bottom racks. P. magellanicus may prefer a stable platform, rather than one that moves, so bottom cages may avoid the stress that a suspended cage might induce. Any container having fine mesh will foul quickly, so a proper maintenance program must be planned for, and proper site selection can help to minimize this. In general, greater depths will help to reduce fouling rates, but benefits from reduced fouling must be balanced against any difficulties in handling, equipment and access caused by operating in a deeper environment.
As sea scallops grow, they get increasingly prone to damaging one another: one scallop will clasp onto another’s shell, causing mantle damage to both – often referred to as ‘knifing.’ This is usually a response to two linked issues: improper stocking density, and feed/temperature/salinity conditions. Scallops growing in ideal conditions will probably be able to handle higher stocking densities, whereas animals in sub-optimal conditions will move around in the cage more, seeking a better environment, and resulting in higher knifing rates. Scallops that bite one another will be misshapen (oblong, rather than rounded), and will suffer lower growth and higher mortality. However, stunting may reduce shell length but increase shell height, and therefore produce ‘taller’ meats – all considerations that must be evaluated by the grower.
Stocking density is usually measured as a percentage of the bottom that is covered by scallops, and typically, 30% to 60% bottom coverage is appropriate for a given type of gear and location, but each grower will have to determine the proper density for his/her site.
Common growout equipment include pearl nets, lantern nets, ear hanging, and a variety of bottom cages: shellfish bags in racks, flat plastic trays, soft mesh trays, etc. Talking with your local extension agent, gear supplier or aquaculture association can help in determining what the best options are for you, and where to purchase equipment.