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

Abstract

The collapse of various stock complexes of cod (Gadus morhua) in the northwest Atlantic has prompted a clarification of relationships among stock components. Here we examine the genetic composition of >2300 cod collected during 1994-1997 in the Gulf of St. Lawrence and its approaches to determine whether: (1) stock components can be genetically identified; (2) population structure is temporally stable; (3) components are always separated and, if not, where and when are they mixed; and (4) component contributions to mixtures can be estimated. We use polymorphism at six microsatellite DNA loci from cod collected on or near their spring and summer spawning grounds to examine structure and then employ maximum likelihood analyses to estimate contributions of each component to mixtures overwintering near the entrance to the Gulf. Estimates of genetic structure (F(ST) and R(ST)) reveal significant differences among cod populations during stock-separated periods, and the structure appears to be temporally stable. Multidimensional scaling analysis of estimates of genetic distance (D(A)) suggest that the structure results 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 Laurentian Channel in the Cabot Strait, as well as among cod collected south of Newfoundland along the north side of the Channel. Weak genetic heterogeneity among seven regional mixed-stock collections during the overwintering period suggests that cod aggregations characteristically found in the overwintering region represent population mixtures that differ in the proportion of cod contributed to them by the various stock components, Maximum likelihood estimates indicate no significant temporal changes in component contributions 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 ranged between 46% and 71% (expected 64%), More precise estimates of contributions from these two regions are precluded by the weak genetic differentiation detected in our samples. The contribution by cod from the Cape Breton Island region was small and estimated at 3%. Contributions by cod from the eastern Scotian Shelf, southwest Newfoundland and south-central Newfoundland were in the range of 13-14%, 4%, and 8%, respectively. Contributions by inshore cod from Placentia and Fortune Bays in south Newfoundland were small to negligible (similar to 3% each). The results indicate that future management could be designed around the spatial and temporal scale of the stock structure identified during the stock-separated period and around the spatially varying contributions to the overwintering mixed-stock fishery.