Genetic differentiation among Atlantic cod (Gadus morhua) from Browns Bank, Georges Bank, and Nantucket Shoals

The Atlantic cod (Gadus morhua) is a migratory gadid found on both sides of the North Atlantic. In the Northwest Atlantic, cod are distributed nearly continuously along the continental shelf from Greenland to North Carolina, spawning in relatively discrete, temporally stable areas, and different regions are regarded as different management units defined primarily by latitude and bathymetry (Ruzzante et al., 1998). Atlantic cod historically supported economically important fisheries in the Northwest Atlantic (Halliday and Pinhorn, 1996). In U.S. waters, cod are assessed and managed as two stocks: 1) Gulf of Maine and 2) Georges Bank and southward (including Nantucket Shoals). Growth rates differ between the two stocks; growth is slower in the Gulf of Maine compared to growth in Georges Bank (Pentilla et al., 1989); each stock is exploited by the same gear type and may show similar biological responses towards such gear selection. Although both stocks support important commercial and recreational fisheries, each is overexploited and remains at a low biomass level (Mayo and O’Brien, 1998; O’Brien and Munroe, 2001; Mayo et al., 2002). Overexploitation may result in significant life-history changes such as a decline in time to reproductive maturity which has been observed in Georges Bank cod (O’Brien, 1998); such changes may be a Genetic differentiation among Atlantic cod (Gadus morhua) from Browns Bank, Georges Bank, and Nantucket Shoals