Lage et al.: Genetic structuring of Gadus morhua 



293 



(0.0114, P<0.017), and between Nantucket Shoals and 

 Georges Bank at Gmol32 (0.0226, P<0.001) and at all 

 loci combined (0.0045, P<0.0i7). Estimates of F ST values 

 between Browns Bank and Georges Bank samples were 

 all nonsignificant. No significant genetic structuring was 

 observed in any comparison when Gmol32 was excluded 

 from the analysis. 



Discussion 



Georges Bank, a large, shallow offshore bank located along 

 the southern edge of the Gulf of Maine off the U.S. and 

 Canadian coasts (Fig. II, supports a large fish biomass. 

 High primary productivity and tightly bound system 

 energetics on the bank result in relatively stable levels of 

 overall biomass and total fish production, although major 

 shifts in species composition routinely occur (Fogarty and 

 Murawski, 1998). The largest spawning aggregation of 

 cod on Georges Bank is found on the Northeast Peak, a 

 gravel region that is an important habitat for the early 

 demersal phase of cod, and may represent a limiting 

 resource for this stock (Lough and Bolz 1989; Langton et 

 al.. 1996). The bank maintains its own circulation pattern 

 in a slow clockwise gyre which may act as a transportation 

 and retention mechanism for planktonic eggs and larvae 

 (Smith and Morse, 1984; Lough and Bolz, 1989). There 

 may be exchange of biota among regions by episodic fluxes 

 of shelf water carrying eggs and larvae away from the Sco- 

 tian Shelf and Browns Bank onto Georges Bank (Cohen 

 et al., 1991; Townsend and Pettigrew, 1996; Bisagni and 

 Smith, 1998). Once on Georges Bank, planktonic eggs 

 and larvae may, depending on depth, be entrained and 

 transported to gravel settlement sites along the western 

 edge of Georges Bank (Smith and Morse 1984; Lough and 

 Bolz, 1989; Werner et al., 1993). However, wind-driven 

 advection may cause egg and larval loss from the North- 

 east Peak and southern flank of Georges Bank (Lough 

 et al., 1989). Cod spawned in the Gulf of Maine usually 

 drift southeasterly towards Georges Bank because of the 

 counterclockwise Gulf of Maine gyre, but the extent of egg 

 and larval exchange between these regions is unknown 

 (Serchuketal., 1994). 



Cod have been found from the surface to depths greater 

 than 450 meters; however few cod proximate to the Gulf 

 of Maine occur deeper than 180 meters (Klein-MacPhee, 

 2002). Browns Bank and Georges Bank are bathymetri- 

 cally separated by the relatively deep (>260 meters) Fun- 

 dian Channel which may act as a barrier to adult migra- 

 tion, whereas Georges Bank and Nantucket Shoals are 

 separated by the relatively shallow (<100 meters) Great 

 South Channel. Although the latter channel is probably 

 not a significant barrier to adult migration, it is an area of 

 strong recirculation towards Georges Bank and could limit 

 egg and larval dispersal. Tagging studies show little ex- 

 change of adults between the inner Gulf of Maine and the 

 region east of Browns Bank and Georges Bank (Hunt et 

 al., 1999), but limited exchange has been reported among 

 the Bay of Fundy, southern Nova Scotia, Browns Bank, 

 and Georges Bank (Klein-MacPhee, 2002). 



The likelihood of determining correct population struc- 

 ture increases when population differentiation is sta- 

 ble over time (Waples, 1998). Results from this study are 

 concordant with observations of temporal stability of mic- 

 rosatellite variation observed in Atlantic cod ( Ruzzante et 

 al., 1996a, 1997, 2001). Tests of population differentiation 

 and subdivision cannot reject the maintenance of genetic 

 homogeneity among Georges Bank cod from 1994 to 1999 

 and thus may indicate some degree of temporal genetic 

 stability among adult Georges Bank cod. 



Our results indicate that cod from Nantucket Shoals 

 are genetically distinct from those from Browns Bank 

 and Georges Bank, and cod from the two Banks are more 

 genetically similar. The observed lack of heterogeneity be- 

 tween Browns Bank and Georges Bank is consistent with 

 gene flow — perhaps due to episodic larval transport and 

 some level of limited adult exchange. Nantucket Shoals 

 cod may be genetically distinct because of egg and larval 

 isolation by entrainment in the Georges Bank gyre or be- 

 cause of limited movement of adults between regions (or 

 a combination of both). Eggs and larvae spawned on Nan- 

 tucket Shoals most likely do not enter the Georges Bank 

 gyre system; these early life history forms may be retained 

 on the shoals or transported to the southwest by prevailing 

 circulation (Fogarty and Murawski, 1998). Some North 

 Atlantic cod stocks have shown substantial differences in 



