NWA - Chapter 8.1

## Georges Bank Cod

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

The Atlantic cod, Gadus morhua, is a demersal gadoid species distributed in the Northwest Atlantic from Greenland to North Carolina. In USA Atlantic waters, three major groupings of cod have generally been recognized: (1) Georges Bank; (2) Gulf of Maine; and (3) Southern New England-Middle Atlantic (Wise 1962; Serchuk and Wigley 1986). Tagging studies (Smith 1902; Schroeder 1930; North American Council on Fishery Investigations 1932; 1935; Wise 1962), parasite infestation research (Sherman and Wise 1961), spawning time data (Colton et al. 1979), and growth rate analyses (Penttila and Gifford 1976; Serchuk and Wood 1979) indicate that minimal interchange occurs between the Gulf of Maine and Georges Bank groups, but that extensive mixing prevails between cod on Georges Bank and in the Southern New England-Middle Atlantic region. A seasonal southwesterly movement of cod from the South Channel area of Georges Bank occurs in autumn followed by a northeasterly return in spring. Wise (1958) postulated that the autumn movement was not a migration of Georges Bank fish as concluded by Schroeder (1930)) but rather a return of Southern New England-Middle Atlantic fish to their native grounds for winter spawning. The presence of ripe spawning individuals off the New Jersey coast (Smith 1902; Schroeder 1930; Wise 1958) and the occurrence of cod eggs and larvae as far south as North Carolina (Schroeder 1930; Berrien et al. 1978) suggest the possibility that cod in the Middle Atlantic may comprise a genetically distinct subpopulation, separate from those groupings found further north. However, the origin and fate of Middle Atlantic cod eggs and larvae have yet to be delineated, and hence the existence of the Middle Atlantic sub-population remains to be confirmed. Serchuk and Wood (1979) found strong affinities between Georges Bank and Southern New England-Middle Atlantic cod based on growth rates, research vessel survey abundance patterns and catch composition, recruitment patterns, and commercial catch size/age distributions. The relative absence of juvenile cod in inshore and offshore research vessel surveys in the Southern New England-Middle Atlantic region (Serchuk and Wood 1979) suggests that either the southerly populations are not self-sustaining or that offspring from the southern spawning move north as ichthyoplankton or larval nekton and subsequently return south several year later as adults.

The demographic similarities between Georges Bank and Southern New England Middle Atlantic cod are so pronounced that the two groups are presently considered to comprise a single stock (i.e., Georges Bank and South; commonly referred to as the Georges Bank stock).

The Georges Bank cod stock is the most southerly cod stock in the world. Georges Bank cod are omnivorous feeders and commonly attain lengths up to 130 cm (51 in) and weights up to 25 to 35 kg (55 to 77 lbs). Maximum age is in excess of 15 years, although young fish (ages 2-5) generally comprise the bulk of the catch. Sexual maturity is attained between ages 2 to 6; spawning occurs during winter and early spring. Commercial fisheries for cod on Georges Bank have existed since the 1700s and modern landings statistics are available since the late 1880s. Annual commercial catches since 1960 have ranged between 11,000 and 57,000 metric tons, and have averaged about 33,000 tons per year. The commercial fisheries are conducted year-round with otter-trawls and gill nets as primary gear. Recreational fishing is also important and recreational landings have averaged about 6,000 tons in recent years.

Cod in spawning condition are found on Georges Bank nearly year-round but peak spawning normally occurs between February and early March (Colton et a'. 1979). Spawning occurs over the entire Bank, but is frequently concentrated on the northeastern part (Northeast Peak). The pelagic eggs drift to the southwest and hatch in about 2-3 weeks at typical spring temperatures (Lough and Bolz 1989). A semi-persistent clockwise gyre on Georges Bank acts as a retention mechanism for cod eggs and early-stage larvae (Smith and Morse 1985; Lough and Bolz 1989). Lough and Bolz (1989) found that the cross-shelf distribution of larvae is consistent with estimated dispersion rates and observed and predicted near-bottom, cross-isobath currents. Larval retention is probably also enhanced by the proximity of larvae to the bottom in areas less than 70 m deep (Lough and Bolz 1989).

The transition from pelagic to demersal life normally occurs in late May-early June when larvae are about 4-6 cm in length or about 3 months old. Juveniles are typically associated with pebble-gravel substrates; the gravel beds probably reduce predation [due to the coloration of the cod on the gravel], and furnish an abundance of prey items. (Lough et al. 1989). Year-class strength appears to be set by the time cod become demersal juveniles. Growth of Georges Bank cod is rapid - age 0 fish attain an average size of 26 cm by the end of their first year of life. Sexual maturity commences at age 2 (40-60 cm), and by age 5 (70-80 cm) virtually all cod are sexually mature.

Assessments of the Georges Bank cod stock have been conducted since the early 1970s but virtual population estimates [VPA] only go back to 1978 (Serchuk and Wigley 1986; Hunt 1988; NEFC, NMFS 1989). Commercial CPUE indices for the USA otter trawl fleet exist since 1964 and Canadian CPUE data are available from 1967 to the present. Stock abundance and recruitment indices derived from autumn (1963 onward) and spring (1968 onward) USA research vessel surveys have been used to monitor changes and assess trends in population size and recruitment of the Georges Bank cod stock. Abundance indices are also available from Canadian research vessel surveys of Georges Bank (since 1986), and inshore spring and autumn bottom trawl surveys conducted by the State of Massachusetts since 1978.

VPA results indicate that fishing mortality on Georges Bank cod doubled between 1978 and 1985 (F=0.39 to F=0.84) and reached a record-high level in 1987 (F=0.95). Spawning stock biomass at the beginning of 1988 was a record-low, about half of that in 1978. Although strong year classes have been produced with regularity (1975, 1978, 1980,1983, 1985, 1987), significant rebuilding of the spawning stock has been hampered by a strong dependence by the fishery on mostly young fish (ages 2 and 3).

VPA results for the period 1978-1987 indicate that variability in year class strength is rather modest - the smallest and largest year classes differ by a factor of 7. The range in spawning stock biomass is more limited; the highest and lowest SSBs differ by only a factor of 3. Age 1 indices from the autumn USA research vessel surveys appear to accurately reflect relative year class strengths suggesting that year class size is determined during the first year of life. Patterns in recruitment of the Georges Bank stock are generally different from those observed in the Gulf of Maine cod stock.

#### 8.1.1 References

Berrien, P. L., M. P. Fahay, A. S. W. Kendall, Jr., and W. G. Smith. 1978. Ichthyoplankton from the RV Dolphin survey of continental shelf waters between Martha's Vineyard, Massachusetts and Cape Lookout, North Carolina, 1965-66. Tech. Series Rept. 15, Sandy Hook Lab., NEFC, NMFS, Highlands, New Jersey.

Colton, J. B., W. G. Smith, A. W. Kendall, P. L. Berrien, and M. P. Fahay. 1979. Principal spawning areas and times of marine fishes, Cape Sable to Cape Hatteras. Fish. Bull. 76, 911-915.

Hunt, J. J. 1988. Status of the Atlantic cod stock on Georges Bank, NAFO Division 5Z and Subarea 6, in 1987. CAFSAC Res. Doc. 88/73, 50 pp.

Lough, R. G. and G. R. Bolz. 1989. The movement of cod and haddock larvae onto the shoals of Georges Bank. J. Fish Biol. 35(Supp. A), 71-79.

Lough, R. G., P. C. Valentine, D. C. Potter, P. J. Auditore, G. R. Bolz, J. Neilson and R. I. Perry. 1989. Ecology and distribution of juvenile cod and haddock in relation to sediment type and a bottom currents on eastern Georges Bank. Mar. Ecol. Prog. Ser. 56, 1-12.

NEFC, NMFS (Northeast Fisheries Center, National Marine Fisheries Service). 1989. Report of the Seventh NEFC Stock Assessment Workshop (Seventh SAW), Woods Hole, Mass. NMFS, NEFC, Woods Hole Lab. Ref. Doc. No. 89-04, 108 pp.

North American Council on Fishery Investigations. 1932. Proceedings for 1921-1930 No. 1, 56 pp.

North American Council on Fishery Investigations. 1935. Proceedings for 1931-1933 No.2, 40 pp.

Penttila, J.A., and V.M. Gifford. 1976. Growth and mortality rates of cod from the Georges Bank and Gulf of Maine areas. Int. Comm. Northw. Atlant. Fish., Res. Bull. 12, 29-36.

Schroeder, W.C. 1930. Migrations and other phases in the life history of the cod off Southern New England. Bull. U.S. Bur. Fish. 46, 1-136.

Serchuk, F.M., and S.E. Wigley. 1986. Assessment and Status of the Georges Bank and Gulf of Maine Atlantic cod stocks. NMFS, NEFC, Woods Hole Lab. Ref. Doc. No.86-12: 84 pp.

Serchuk, F.M., and P.W. Wood. 1979. Review and status of the Southern New England-Middle Atlantic cod, Gadus morhua, populations. August 1979. NMFS, NEFC, Woods Hole Lab. Ref. Doc. No.79-37: 77 pp.

Sherman, K., and J.P. Wise. 1961. Incidence of the cod parasite, Lernaeocera branchialis L. in the New England area, and its possible use as an indicator of cod populations. Limnol. Oceanogr. 6, 61-67.

Smith, H.M. 1902. Notes on tagging of 4,000 adult cod at Woods Hole, Mass. Rept. U.S. Fish. Comm. 27, 193-208.

Wise, J.P. 1958. The world's southernmost indigenous cod. J. Cons. int. Explor. Mer. 23, 208-212.

Wise, J. P. 1962. Cod groups in the New England area. Fish. Bull. 63, 189-203.

Smith, W.G. and W.W. Morse. 1985. Retention of larval haddock Melanogrammus aeglefinus in the Georges Bank region, a gyre-influenced spawning area. Mar. Ecol. Progr. Ser. 24, 1-13.