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.
Left - Female scallop with mature roe, February 2006, Belfast, Maine
Right - juvenile scallops, shown with the end of a paper clip (dark, rectangular shape at bottom), for scale
Sea scallops prefer areas with relatively high flow, and will most commonly be found on hard-bottom areas, with sand, gravel or cobble as the preferred substrates; rarely will they be found on mud, which is typical of low-flow, depositional conditions. They will prefer temperature ranges more aligned with European oysters and blue mussels, rather than those typical of Eastern oysters and hard clams: the upper range for reliable scallop survival is between 65 and 70 deg F. Scallops also require higher-salinity areas, rather than estuarine conditions.
Predators of scallops are numerous, particularly on the smaller sizes. Lobsters and several crab species will readily prey on scallops, with starfish as perhaps the most devastating species. A scallops' shell will thicken as it grows, providing some defense, although the main defense is the scallops' well-developed ability to swim. This ability diminishes as the scallop grows however, and studies indicate that scallops above roughly 3.5" in shell height are unlikely to swim any great distance. On the other hand, even large scallops can swim some distance, to evade predators or to seek better conditions for feeding.
The principal market for sea scallops in the United States is for the adductor muscle, or meat. Scallop meats generally range from $7 to $13 dollars per pound, although recently-decreased Japanese production is increasing the prices paid to scallopers in the inshore Maine fishery. In 2010, landings of scallop meats in Maine waters amounted to approximately 185,000 lb., worth just under $1.5 million USD. By contrast, total scallop landings in the US were 58 million pounds, worth over $380 million USD.
Products and markets for scallop meats are well developed, and 'diver' or 'dayboat' scallops are enjoying particular favor in recent years, by virtue of their top quality, and good associations between fisherman and consumer.
The market for other products - such as roe-on, whole or live scallops - is presently limited, but it appears that this is due to lack of production, given that many buyers, chefs and restaurant owners have indicated a strong interest in obtaining such products. Unless grown under careful monitoring, landing of whole scallops is disallowed, because of the scallops' ability to accumulate deadly levels of biotoxins. That said, producers in the US have had several tries at growing scallops through aquaculture, and Canadian producers appear to be having a measure of production and economic success.
As a practical matter, generating cash flow at several points in the production process is a standard approach for Japanese and others who are currently producing scallops of other species, eg: Patinopecten yezzoensis. Producers might plan on selling scallop seed, selling individuals in the 50-75mm range into the 'Princess Scallop' market, and larger animals sold as roe-on, whole, or live; and the largest might be used to produce adductor muscles into the high-end market for big scallop meats. Other opportunities exist for other food and non-food markets: scallop seed as garnish in salads and other dishes (small scallops often have distinctive and very attractive shells), small shells and scallop pearls to be used as jewelry, and larger shells for the culinary trade.
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.
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. Fishermen typically use about 2 meters of spat bag stuffing such as Netron® per collector. 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.
Scallops after 14 months in the spat bag
The sister-state relationship with Aomori Prefecture has continued through the years, and in 2011, a delegation from Japan visited several sites in Maine. While at the Darling Marine Center, they provided an interested audience with a nice overview of several aspects of the scallop and seafood industries in Japan. The items below are courtesy of that delegation, and Hugh Cowperthwaite of Coastal Enterprises, Inc.
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.
Scallops in a bottom tray, Darling Marine Center
Depending on how large the seed is to start, and what size scallop your market is looking for, growout can take 1 to 3 years. Spat bags that spend 12 months in the water can expect to yield scallops in excess of 25mm, although problems may be encountered with fouling, smothering or mortality due to starfish, which will also settle inside the spat bag.
Harvesting and Final Products
While there are many considerations that the potential scallop producer must think about (efficiency, rapid delivery to cool storage, etc), the most critical issue is being sure the product is safe to consume. For all tissues other than the adductor muscle, biotoxin accumulation is an issue that every grower must address. Working with the appropriate agencies at the state level and elsewhere is necessary to ensure public health, and it is currently illegal to land anything other than the adductor muscle in Maine, unless specially permitted by Maine DMR to do so.
Diversification is already happening in the market for products made from scallops. While a premium will always be paid for top-quality meats, products are appearing that feature half-shell whole scallops with garnishes and sauce, roe-on scallops, and live scallops. Advances in freezing and packaging technology have improved the ability of the producer to get a high quality product to the consumer in an appealing, flavorful and affordable fashion.
►Disease & Health
For prospective sea scallop producers, two of the most common problems to keep an eye out for include burrowing polychaete worms (such as Polydora websteri), and the orange boring sponge (Cliona sp.). While not immediately a danger to the scallop itself, these pests can deform and weaken the shell, causing the scallop to expend energy to make repairs, and lowering the overall condition of the scallop.
Blisters caused by shell-boring polychaete worms (T. Harper photo)
Off-color meats can be caused by a variety of factors, but is generally an indication of a weakened state for the individual scallop. The adductor muscle is where much of the scallops' energy is stored, and when much of the energy is used up, the meat can appear gray, or even bluish. The energy loss can be from a post-spawning phase, poor feed or water quality conditions, pollution, or many other reasons.
Orange and reddish meats have been observed from both offshore and inshore scallops periodically as well. Bourne and Bligh reported in 1965 (Journal of the Fisheries Research Board, Canada) that this phenomenon only occurs in female scallops, and that the orange color comes from the carotenoid compound zeaxanthin. Zeaxanthin is apparently responsible for the red coloration of scallop roe, and an excess of this compound in the feed results in deposition into the adductor muscle, and sometimes other tissues such as the mantle.
►Relevant Documents Specific to Species (click to download)
Proceedings of the 2004 Scallop Aquaculture Workshop, Halifax, Canada. (1.8 MB, pdf format)
NOAA Technical Memorandum - Life History of the Sea Scallop (2004) 300 kb, PDF format
1996 Report on Scallop Polyculture in Maine (1.6MB, PDF)
A definitive reference is available: Scallops: Biology, Ecology and Aquaculture, Second Edition. 2006. S. E. Shumway and G. J. Parson, Editors. Elsevier, Inc., San Diego, CA., USA. 1460 p.