Genetic differentiation between inshore and offshore Atlantic cod (Gadus morhua) off Newfoundland: a test and evidence of temporal stability

The genetic difference between inshore overwintering Atlantic cod (Gadus morhua) from Trinity Bay, Newfoundland, and offshore overwintering cod from the Grand Bank region (Ruzzante et al. 1996; Can. J. Fish. Aquat. Sci. 53: 634-645) has remained stable during 1992-1995. Cod collected inshore during 1995 in Trinity Bay (N = 150) were again genetically distinguishable (using (delta mu)(2) and D-SW) from offshore cod (N = 140) collected between 1992 and 1994 and were genetically indistinguishable from inshore cod (N= 123) also collected between 1992 and 1994. Farm-held cod (N = 30), captured inshore in 1992 and pen reared until 1995 were most likely to have been drawn from an inshore overwintering population; they were genetically different from offshore cod and were indistinguishable from wild inshore cod showing high antifreeze activity and from cod collected in relatively cold (similar to 0.2 degrees C) water in December 1995. The farm-held cod were genetically different from cod collected in relatively warm (similar to 3.3 degrees C) water but were indistinguishable from cod with low antifreeze activity. Despite evidence of weak genetic heterogeneity within the pool of wild cod collected inshore in 1995, which was not associated with antifreeze activity nor with water temperature, the magnitude of the genetic differences between inshore and offshore cod has remained unchanged during the period 1992-1995.