MITOCHONDRIAL-DNA DIFFERENTIATION IN WESTERN NORTH-ATLANTIC POPULATIONS OF HADDOCK (MELANOGRAMMUS-AEGLEFINUS)

Mitochondrial DNA polymorphism was examined in haddock (Melanogrammus aeglefinus) from five offshore banks in the western North Atlantic. A total of 21 genotypes were observed in an initial survey of 54 haddock with 12 restriction endonucleases. Five enzymes that revealed most of the genetic variation were used in an expanded survey from which we observed 22 genotypes among 133 haddock. Both the 12- and 5-enzyme results showed that haddock from each of the five banks comprised a mixture of divergent genotypes that could each be assigned to one of at least four groups of related genotypes differing by mean estimated divergences of 0.6-0.8%. These divergent genotypes may have arisen in past haddock populations that were more isolated than those of the present. Although genotype frequencies did not differ significantly in most pairwise comparisons of banks, two measures of genetic (dis)similarity were significantly correlated with geographic distance as well as with the presence and relative size of deepwater (>180 m) channels separating the banks. One of the genotype groups also exhibited a geographic cline in frequency of occurrence. These features indicate that gene flow among populations is restricted, and hence support the existence of discrete stocks of haddock.