Mitochondrial DNA sequence variation and genetic stock structure of Atlantic cod (Gadus morhua) from bay and offshore locations on the Newfoundland continental shelf


  • This paper is part of an ongoing collaborative effort by-Canadian geneticists, physiologists, and oceanographers to understand the biology of Atlantic cod with a view towards recovery and better management of there source. Steve Carris interested in differences among the population genetic structures of a variety of marineand terrestrial vertebrate species as revealed by molecular markers, an interest first developed in the laboratory of the late Allan Wilson. Joe Wroblewski holds the NSERC /Fishery Products Intema tional/ National Sea Products Research Chair in Fisheries Oceanography at Memorial University.

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Bay cod, Atlantic cod (Gadus morhua) that over-winter in the deep-water bays of northeastern Newfoundland, have historically been regarded as distinct in migration and spawning behaviour from offshore (Grand Bank) cod stocks. To investigate their genetic relationships, we determined the DNA sequence of a 307-base-pair portion of the mitochondrial cytochrome b gene for 236 adult cod taken from the waters off northeastern Newfoundland, including fish found over-wintering and spawning in Trinity Bay. Although 17 genotypes were found, a single common genotype occurs at a frequency of greater than 80% in all samples, and no alternative genotype occurs at a frequency of greater than 3%. Genotype proportions did not differ significantly among samples. Measures of genetic subdivision among sampling locations are nil. Cod over-wintering in Trinity Bay are not genetically distinct from offshore cod. In combination with tagging and physiological studies, these data suggest that there is sufficient movement of cod between bay and offshore locations to prevent the development or maintenance of independent inshore stocks. Adult cod that over-winter in Trinity Bay appear to represent an assemblage of temporarily nonmigratory fish that have become physiologically acclimated to cold-water inshore environments. The pattern of genetic variation in northern cod suggests a recent population structure characterized by extensive movement of contemporary individuals superimposed on an older structure characterized by a bottleneck in the population size of cod in the north-western Atlantic.