Genetic differentiation among Atlantic cod from Browns Bank, Georges Bank, and Nantucket Shoals

Abstract--This study examines genetic variation at five microsatellite loci and at the vesicle membrane protein locus, pantophysin, of Atlantic cod (Gadus morhua) from Browns Bank, Georges Bank, and Nantucket Shoals. The Nantucket Shoals sample represents the first time cod south of Georges Bank have been genetically evaluated. Heterogeneity of allelic distribution was not observed (P>0.05) between two temporally separated Georges Bank samples indicating potential genetic stability of Georges Bank cod. When Bonferroni corrections ([alpha]=0.05, P<0.017) were applied to pairwise measures of population differentiation and estimates of [F.sub.ST], significance was observed between Nantucket Shoals and Georges Bank cod and also between Nantucket Shoals and Browns Bank cod. However, neither significant differentiation nor significant estimates of [F.sub.ST] were observed between Georges Bank and the Browns Bank cod. Our research suggests that the cod spawning oil Nantucket Shoals are genetically differentiated from cod spawning on Browns Bank and Georges Bank. Managers may wish to consider Nantucket Shoals cod a separate stock for assessment and management purposes in the future.

**********

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 compensatory response to overfishing but may also be influenced by shifts in underlying genetic control (Policansky, 1993).

Commercial fisheries are conducted year round, using primarily otter trawls and gill nets. The Canadian fishery on Georges Bank is managed under an individual quota system. United States cod fisheries are managed under the New England Fishery Management Council's Northeast Multispecies Fishery Management Plan (FMP) (1) as implemented by the U.S. Federal Register, 50 CFR Part 648 (U.S. Federal Register, 2003). Under this FMP, cod are included in a complex of 15 groundfish species managed by time and area closures, trip limits, gear restrictions, minimum size limits, days-at-sea restrictions, and a permit moratorium. The FMP's goal is to reduce fishing mortality to levels that will allow stocks within the complex to initially rebuild above minimum biomass thresholds, and, ultimately, to remain at or near target levels.

When ecological and evolutionary processes are responsible for stock structuring, it is necessary to incorporate them into strategies designed to manage exploited species (Avise, 1998). High dispersive capabilities of many marine fish often correlate with low levels of population divergence over vast areas (Ward et al., 1994; Graves, 1998) and may be particularly true for species characterized by high fecundity, large population size, and potentially long-distance egg and larval dispersal. Although marine fish predominantly have high dispersal rates and low levels of population structuring, migratory species with continuous distributions may develop and maintain stock structure if they show fidelity to natal spawning sites or limited egg and larval dispersal. Fidelity to natal grounds has been shown in Greenland-Iceland cod (Frank, 1992) and Georges Bank haddock (Polacheck et al., 1992). Genetic divergence between areas originates when populations are formed or through the restriction of gene flow. Cod in some regions are known to migrate long distances, whereas in other regions they are nearly stationary (Lear and Green, 1984). Tagging studies in the Gulf of Maine show little exchange between the region east of Browns Bank and Georges Bank, and the inner Gulf of Maine (Hunt et al., 1999); however exchange has been reported among Bay of Fundy, southern Nova Scotia, Browns Bank, and Georges Bank populations (Klein-MacPhee, 2002). Such exchange among cod from different management areas may be important for stock assessments and management practices. Determining underlying genetic structure of spawning stocks is paramount to the conservation and management of overexploited species.