NWA - Chapter 8.3

## Georges Bank Sea Scallops (Placopecten magellanicus)

### by Frederic M. Serchuk and Edward B. Cohen

Sea scallops (Placopecten magellanicus) occur along the continental shelf of North America from the Strait of Belle Isle south to Cape Hatteras (Posgay 1957). North of Cape Cod, scattered concentrations occur in shallow water, often just below the low tide mark (Posgay 1950; Dickie 1953); further south aggregations are restricted to deeper, cooler offshore waters (Merrill 1971). Sea scallops are intolerant of water temperatures above 20-22°C (Posgay 1953; Dickie 1958), and hence the southern extremity of their range, and their distribution in coastal estuaries, are likely circumscribed by temperature (Dickie and Medcof 1963; Merrill 1971). Scallop beds sufficiently dense and extensive enough to support commercial fishing exist from Port au Port Bay, Newfoundland to the Virginia Capes (Posgay 1957), generally at depths between 40 and 100m (Merrill 1962; Posgay 1979). Although individualized movement of scallops within beds is common, tagging experiments (Baird 1954, 1956; Dickie 1953, 1955; Posgay 1963) indicate an absence of directed population movements or seasonal migrations.

Spawning occurs in late summer or early fall, varying slightly between years and areas (Bourne 1964). Spawning commences during July at the southern extremity of the range (i.e., North Carolina and Virginia) and proceeds northeastward as the year advances, ending in the northernmost regions by mid-October (MacKenzie et al. 1978). Sexes are separate, although occasionally hermaphrodites occur (Merrill and Burch 1960; Naidu 1970). Fertilization is external; individuals in the same general area may go from completely ripe to completely spent within a week (Posgay and Norman 1958). The environmental stimuli triggering spawning are unknown, but may be associated with thermal destratification and an increase in bottom temperature (Posgay 1953), tidal cycles (Dickie 1953), or temperature elevation and depression relative to thermal acclimation (Naidu 1970).

Fertilized sea scallops are buoyant, and undergo typical molluscan development as pelagic larvae (Merrill 1961; Culliney 1974). Sea scallop larvae have only a slight capacity for regulating their vertical distribution (Tremblay and Sinclair 1990a; 1990b). Scallop larvae in a stratified area on Georges Bank were strongly associated with the pycnocline while those at a well mixed site were distributed throughout the 40-50 m water column (Tremblay and Sinclair 1990a). There is some evidence of a weak diel migration by scallop larvae in a shallow embayment (Tremblay and Sinclair 1990b). The vertical distribution of scallop larvae on Georges Bank showed no relation to food concentration and was apparently determined by the position of the pycnocline (Tremblay and Sinclair 1990a). Even in the shallow embayment where some vertical migration was observed (Tremblay and Sinclair 1990b) the dominant factor in the vertical distribution was the pycnocline depth.

Duration of the planktonic phase in nature is unknown; laboratory culture experiments at 15°C indicated that spatfall occurs 35 days after fertilization (Culliney 1974). The distribution of spatfall is presumably related to prevailing surface current patterns during the pelagic period. Generalized sea surface circulation patterns indicate a prevailing southwesterly flow from Georges Bank (Bumpus 1976; Beardsley et al. 1976; Butman et al. 1982) suggesting that progeny of a given sea scallop aggregation are unlikely to settle out in the vicinity of the parental beds (Merrill 1965; Posgay 1979). Larvae spawned on Georges Bank, however, may frequently be retained there due to a semi-persistent gyre facilitating completion of metamorphosis in this region (Posgay 1979).

The interaction between larval scallops and the currents on Georges Bank needs to be more clearly defined. A tentative hypothesis is that the beds on the northeast peak, the northern edge and the great south channel areas of Georges Bank are thought to be self sustaining. This is likely due to larvae being retained on the bank by gyral circulation, long enough for the larvae to metamorphose and settle to the bottom. Scallop beds on Georges Bank may also receive larvae from the Gulf of Maine and Scotian Shelf and may supply larvae to the Mid-Atlantic region. The question of the source of supply of larvae to the various beds and the genetic makeup of the beds (i.e., are there several stocks or just one) need increased attention.

Sexual maturity may be attained as early as age 1, with the initial spawning occurring after deposition of the first growth ring (age 1.5 or 2) (Naidu 1970). Size at sexual maturity may vary from 23 to 75mm (Naidu 1970; Posgay 1979) but fecundity of the younger age groups contributes little to total egg production. By age 5 or 6, however, female scallops may each produce about 2 million eggs (Posgay 1979).

During the first several years of life, growth in both shell size and meat weight is rapid. Between ages 3 and 5, sea scallops commonly increase 50-80% in shell height and quadruple in meat weight, while about 10% die, per year, from natural causes (Merrill and Posgay 1964). In this interval, the number of meats per pound is reduced from about 100 to 23. Between ages 8 and 9, annual growth falls to less than 10% per year, so that at age 8 (ca. 133mm shell height, 11 meats/lb) the increase in weight due to growth roughly balances the loss in weight due to natural mortality.

Scallop abundance on Georges Bank has recovered from the nadir reached in 1983-1984 and is now at a relatively high level (CUD 1989). Current US fishing effort is excessive and landings are above long term sustainable catch. However, US landings and catch per unit effort (CPUE) may remain high during 1989 and 1990 because of recent strong year classes (1982-1984) in the fishery. US landings were 6100mt and Canadian landings 4300mt in 1988. On the Canadian side landings are slightly below the long term historical average and fishing effort has recently decreased through reduction of the fishing fleet (M. J. Tremblay, pers. comm.).

#### 8.3.1 References

Baird, F. T., Jr. 1954. Migration of the deep sea scallop (Pecten magellanicus). Maine Dept. Sea Shore Fish., Fish. Circ. 14, 8 pp.

Baird, F. T., Jr. 1956. The sea scallop (Pecten magellanicus). Maine Dept. Sea Shore Fish., Fish. Education Series, Unit No.2, 11 pp.

Beardsley, R. C., W. C. Boicourt and D. V. Hansen. 1976. Physical oceanography of the New York Bight. ASLO Spec. Symp. 2, 20-34.

Bourne, N. 1964. scallops and the offshore fishery of the Maritimes. Fish. Res. Bd. Can. Bull. 145, 60 pp.

Bumpus, D. F. 1976. review of the physical oceanography of Georges Bank. ICNAF Res. Bull. 12, 119-134

Butman, B., R. C. Beardsley, B. Magnell, D. Frye, J. A. Vermersch, R. Schlitz, R. Limeburner, W. R. Wright and M. A. Noble. 1982. Recent observations of the mean circulation on Georges Bank. J. Phys. Oceanogr. 12, 569-591.

Conservation and Utilization Division (CUD). 1989. Status of the fishery resources off the northeastern United States. NOAA, NMFS, NEFC, Woods Hole Laboratory. 110 pp.

Culliney, J. L. 1974. Larval development of the giant scallop, Placopecten magellanicus (Gmelin). Biol. Bull. 147, 321-332.

Dickie, L. M. 1953. Fluctuations in abundance of the giant scallop, Placopecten magellanicus (Gmelin), in the Digby area of the Bay of Fundy. Fish. Res. Rd. Can. MSS Rept. Biol. Sta. No. 526.

Dickie, L. M. 1955. Fluctuations in abundance of the giant scallop, Placopecten magellanicus (Gmelin), in the Digby area of the Bay of Fundy. J. Fish. Res. Bd. Can. 12, 797-857.

Dickie, L. M. 1958. Effects of high temperatures on survival of the giant scallop. J. Fish. Res. Bd. Can. 15, 1189-1211.

Dickie, L. M. and J. C. Medcof. 1963. Causes of mass mortalities of scallops (Placopecten magellanicus) in the southwestern Gulf of St. Lawrence. J. Fish. Res. Bd. Can. 20, 451-482.

Mackenzie, C. L., A. S. Merrill and F. M. Serchuk. 1978. Sea scallop resources off the northeastern U.S. coast. Marine Fish. Rev. 40, 19-23.

Merrill, A. S. 1961. The sea scallop fishery. Bull. Amer. Malacol. Union 28, 14.

Merrill, A. S. 1962. Abundance and distribution of sea scallops off the Middle Atlantic coast. Proc. Nat. Shelf. Assoc. 51, 74-80.

Merrill, A. S. 1965. The benefits of systematic biological collecting from navigation buoys. ASB Bull., 12, 3-8.

Merrill, A. S. 1971. The sea scallop. Ann Rept. (1970). Amer. Malacol Union. 24-27.

Merrill, A. S. and J. B. Burch. 1960. Hermaphroditism in the sea scallop, Placopecten magellanicus (Gmelin). Biol. Bull., 119, 197-201.

Merrill, A. S. and J. A. Posgay. 1964. Estimating the natural mortality rate of the sea scallop (Placopecten magellanicus). ICNAF Res. Bull. 1, 88-106.

Naidu, K. S. 1970. Reproduction and breeding cycle of the giant scallop Placopecten magellanicus (Gmelin) in Port au Port Bay, Newfoundland. Can. J. Zool., 48, 1003-1012.

Posgay, J. S. 1950. Investigations of the sea scallop, Pecten grandis. In Third Report on investigations of methods of improving the shellfish resources of Massachusetts. Comm. of MA, Dept. Nat. Res. Div. Mar. Fish., 24-30.

Posgay, J. 5 1953. The sea scallop fishery. In Sixth Report on investigations of methods of improving the shellfish resources of Massachusetts. Comm. of MA, Dept. Nat. Res. Div. Mar. Fish., 9-24.

Posgay, J. A. 1957. The range of the sea scallop. The Nautilus. 71, 55-57.

Posgay, J. A. 1963. Tagging as a technique in population studies of the sea scallop. ICNAF Spec. Pub. 4, 268-271.

Posgay, J. A. 1979. Sea scallop Placopecten magellanicus (Gmelin). In: Fish distribution. MESA N.Y. Bight Monogr. No.15. N.Y. Sea Grant Institute, N.Y.

Posgay, J. A. and K. D. Norman. 1958. An observation on the spawning of the sea scallop, Placopecten magellanicus (Gmelin), on Georges Bank. Limnol. Oceanoyr. 3, 142.

Tremblay, M. J. and M. Sinclair, 1990a. Sea scallop larvae Placopecten magellanicus on Georges Bank: vertical distribution in relation to water column stratification and food. Mar. Ecol. Prog. Ser. 61, 1-15.

Tremblay, M. J. and M. Sinclair, 1990b. Diel vertical migration of sea scallop larvae Placopecten magellanicus in a shallow embayment. Mar. Ecol. Prog. Ser., 67, 19-25.