Alaska Department of Fish and Game
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Did You Know?
The beauty of scallop shells has long been internationally recognized. The image of scallop shells is used to adorn everything from postcards, stamps, family crests, oil company logos, and trail signs along the Camino de Santiago walking pilgrimage in Spain.
The Alaska scallop fishery is based upon the weathervane scallop, also called the giant Pacific scallop, scientific name Patinopecten caurinus. Unlike most bivalves (clams and mussels), scallops cannot burrow to escape predation, but instead they detect predators with primitive 'eyes' and have limited ability to swim away by rapid opening and closing of their shells. Scallops require a very large adductor, ‘hinge’ muscle, to maintain this ability. This muscle is what we think of as a "scallop," although it is only one part of the animal.
Size and sex determination
The biological measurement for scallops is shell height. This is measured from the base of the hinge to the edge of the scallop shell. Weathervane scallops sexually mature around age 3 or 4. They spawn annually, usually in early summer and are about 100 millimeters in shell height when they are sexually mature. The sexes can be distinguished by the color of the gonads; female gonads are orange-red to red in color, and male gonads are creamy white. Otherwise, male and female gonads are similar in size and shape.
There are other scallop species in Alaskan waters, but none as large as the weathervane scallop. These include: Pacific pink scallop, Chlamys rubida; spiny pink scallop, C. hastata; white scallop, C. albida; arctic pink scallop, C. behringiana; C. islandica; purple-hinged rock scallop, Crassadoma gigantea; Vancouver scallop, Delectopecten vancouverensis; and Alaska glass-scallop, Parvamussium alaskense.
Growth and Reproduction
Scallops are dioecious (have two sexes), are broadcast spawners, and reproduce by assembling and releasing clouds of gametes (eggs and sperm) which are fertilized in the water column. The signal for scallops to assemble and spawn is thought to be increasing water temperature, and in Alaska spawning occurs in May and June. Fertilized eggs settle to the bottom where they develop after a few days into a tiny transparent-shelled veliger larvae. The veliger larvae swims into the water column where it feeds on microplankton (small free-floating plants) for a period of about three weeks before settling to the bottom to begin its life as a benthic (bottom-dwelling) filter feeder.
Weathervane scallops begin to reproduce at three or four years of age at a shell height of about three inches, and are of a commercially harvestable size at about four inches shell height and six to eight years of age. Scallops reach a maximum age of approximately 28 years in Alaska. Like other bivalve mollusks, scallops are aged by counting the rings on their shell, which are formed by alternating periods of slow and fast growth associated with seasonal changes in temperature and food availability. The two valves of scallop shells are not identical, and the scallop body lies on the bottom valve, which is more rounded in shape, while the top valve is flatter.
Range and Habitat
Weathervane scallops are distributed from California to the Bering Sea and as far west as the Aleutian Islands. Scallop beds in Alaska are located on mud, clay, sand or gravel substrate in depths of 120 to 390 feet. Adult scallops assemble in dense "beds"; these have a characteristic oblong shape, with the longer axis parallel with the direction of the prevailing current.
Status, Trends, and Threats
Commercial fisheries for weathervane scallops in Alaska are currently prosecuted in offshore waters from Cape Spencer in Southeast Alaska to north of Unimak Island in the Bering Sea.
Historical harvest of weathervane scallops in Alaskan waters peaked at 1.85 million pounds of shucked scallop meats in the 1969/70 season. A second peak of 1.79 million pounds occurred in the 1992/93 season. The long-term average harvest for 1967–2010/11 seasons is 703,590 pounds, and the 40, 30, 20, and 10-year averages are 703,590; 669,024; 697,854; 714,080; and 476,117 pounds respectively. An optimum yield of 0 to 1.8 million pounds of shucked scallop meats was established in 1996. In 1998, optimum yield was redefined as 0 to 1.24 million pounds. The 2012 Scallop Stock Assessment and Fishery Evaluation Report found that statewide scallop harvests have not exceeded the upper limit of optimum yield and thus are not overfished.
The primary threats to this species include habitat damage, ocean acidification, overfishing, and predation by crabs, seas stars and octopus.