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 (FST and RST) 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 (DA) 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 (∼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.

Genetic mixed-stock analysis of Atlantic herring populations in a mixed feeding area

“…Such effects are, however, expected to contribute little to allele frequency variation in marine organisms with extensive overlap between generations and the presumed very large effective population sizes. Our analysis comprised populations exhibiting lower differentiation than MSA studies in most other marine species (Wirgin et al 1997, Ruzzante et al 2000, Beacham et al 2008, Wennevik et al 2008) and approximately an order of magnitude lower than MSA studies in salmonids (Ruzzante et al 2004, Beacham et al 2005, Koljonen et al 2005, Smith et al 2005, Gauthier-Ouellet et al 2009). Using BAYES with data for 8 microsatellite loci typed in 26 Atlantic salmon populations, Koljonen et al (2005) reported that deviation between true and estimated single population contributions averaged 3% in a composite self-assignment test (i.e.…”

“…However, when detailed sampling of the main populations contributing to mixed aggregations is attainable, approaches can be designed to overcome problems with low genetic resolution among populations (see e.g. Ruzzante et al 2000Ruzzante et al , 2006.Atlantic herring Clupea harengus L. is an abundant and widely distributed marine pelagic fish that spawns on substrate in coastal areas throughout most of the north Atlantic (Iles & Sinclair 1982). Most herring populations are migratory and often congregate on common feeding and wintering grounds where aggregations may consist of mixtures of individuals from several populations.…”

Biocomplexity and fisheries sustainability

“…There is growing recognition that many marine fish stocks consist of amalgamations of several geographic components (35,36). It would seem prudent to try to prevent loss of such stock components, including those that appear, at present, to be unproductive.…”

Marked genetic structuring in localised spawning populations of cod Gadus morhua in the North Sea and adjoining waters, as revealed by microsatellites

“…There would therefore be a need to identify the relative contributions from putative stocks, such that they can be monitored independently. Indeed, Ruzzante et al (2000b), working on stock complexes of cod in the north-western Atlantic have demonstrated the efficacy of a mixed-stock-analysis approach based on 6 polymorphic microsatellite loci. It was possible not only to estimate the proportion of cod contributions from various stock components, but also to demonstrate temporal stability of structuring over a 2 yr period.…”