MITOCHONDRIAL-DNA SEQUENCE VARIATION AND GENETIC STOCK STRUCTURE OF ATLANTIC COD (GADUS-MORHUA) FROM BAY AND OFFSHORE LOCATIONS ON THE NEWFOUNDLAND CONTINENTAL-SHELF

Bay cod, Atlantic cod (Gadus morhua) that over-winter in the deep-wate r bays of northeastern Newfoundland, have historically been regarded a s distinct in migration and spawning behaviour from offshore (Grand Ba nk) cod stocks. To investigate their genetic relationships, we determi ned the DNA sequence of a 307-base-pair portion of the mitochondrial c ytochrome b gene for 236 adult cod taken from the waters off northeast ern Newfoundland, including fish found over-wintering and spawning in Trinity Bay. Although 17 genotypes were found, a single common genotyp e occurs at a frequency of greater than 80% in all samples, and no alt ernative genotype occurs at a frequency of greater than 3%. Genotype p roportions did not differ significantly among samples. Measures of gen etic subdivision among sampling locations are nil. Cod over-wintering in Trinity Bay are not genetically distinct from offshore cod. In comb ination with tagging and physiological studies, these data suggest tha t there is sufficient movement of cod between bay and offshore locatio ns to prevent the development or maintenance of independent inshore st ocks. Adult cod that over-winter in Trinity Bay appear to represent an assemblage of temporarily nonmigratory fish that have become physiolo gically acclimated to cold-water inshore environments. The pattern of genetic variation in northern cod suggests a recent population structu re characterized by extensive movement of contemporary individuals sup erimposed on an older structure characterized by a bottleneck in the p opulation size of cod in the north-western Atlantic.