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.
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