Mixed-stock analysis of Atlantic cod near the Gulf of St. Lawrence based on microsatellite DNA

The collapse of various stock complexes of cod (Gadus morhua) in the northw est Atlantic has prompted a clarification of relationships among stock comp onents. Here we examine the genetic composition of >2300 cod collected duri ng 1994-1997 in the Gulf of St. Lawrence and its approaches to determine wh ether: (1) stock components can be genetically identified; (2) population s tructure is temporally stable; (3) components are always separated and, if not, where and when are they mixed; and (4) component contributions to mixt ures can be estimated. We use polymorphism at six microsatellite DNA loci f rom cod collected on or near their spring and summer spawning grounds to ex amine structure and then employ maximum likelihood analyses to estimate con tributions of each component to mixtures overwintering near the entrance to the Gulf. Estimates of genetic structure (F-ST and R-ST) reveal significan t differences among cod populations during stock-separated periods, and the structure appears to be temporally stable. Multidimensional scaling analys is of estimates of genetic distance (D-A) suggest that the structure result s from differences among cod collected within the Gulf of St. Lawrence and those collected near the entrance to the Gulf on either side of the Laurent ian Channel in the Cabot Strait, as well as among cod collected south of Ne wfoundland along the north side of the Channel. Weak genetic heterogeneity among seven regional mixed-stock collections during the overwintering perio d suggests that cod aggregations characteristically found in the overwinter ing region represent population mixtures that differ in the proportion of c od contributed to them by the various stock components, Maximum likelihood estimates indicate no significant temporal changes in component contributio ns to the mixed-stock samples between 1996 and 1997 when all of the winter mixed-stock samples were pooled. The combined contribution of cod from the southern and northern Gulf of St. Lawrence to the mixed-stock samples range d between 46% and 71% (expected 64%), More precise estimates of contributio ns from these two regions are precluded by the weak genetic differentiation detected in our samples. The contribution by cod from the Cape Breton Isla nd region was small and estimated at 3%. Contributions by cod from the east ern Scotian Shelf, southwest Newfoundland and south-central Newfoundland we re in the range of 13-14%, 4%, and 8%, respectively. Contributions by insho re cod from Placentia and Fortune Bays in south Newfoundland were small to negligible (similar to 3% each). The results indicate that future managemen t could be designed around the spatial and temporal scale of the stock stru cture identified during the stock-separated period and around the spatially varying contributions to the overwintering mixed-stock fishery.