Research Document - 2000/099

Population structure of Atlantic cod (Gadus morhua) in the Newfoundland and Labrador area determined from genetic variation.

By T.D. Beacham, J. Brattey, K.M. Miller, K.D. Le, A.D. Schulze, and R.E. Withler

Abstract

The purpose of this study was to describe population structure and to determine the potential for genetic stock identification of Atlantic cod (Gadus morhua) in Newfoundland and Labrador using microsatellite loci, synaptophysin (SypI) locus, and a major histocompatibility complex (Mhc) locus. Variation at seven microsatellite loci (Gmo3, Gmo8, Gmo19, Gmo34, Gmo35, Gmo36, and Gmo37) and SypI was surveyed in approximately 5,050 cod from 19 putative populations. Variation at a class I Mhc locus was surveyed in 2,000 fish from the 19 populations. Ten populations were sampled over two or more years, and variation among populations was on average about 18 times greater than annual variation within populations. Regional structuring of the populations was apparent with inshore and offshore spawning populations forming distinct groups. The Flemish Cap population was the most distinctive of the offshore group, and the Gilbert’s Bay population in Labrador was the most distinctive of the inshore group. In Divisions 2J3KL, no significant genetic differentiation was observed among inshore cod sampling sites in Notre Dame Bay and Bonavista Bay. Some differentiation was observed between sites in Conception Bay and Trinity Bay, and also with other inshore sites, providing some evidence of distinct "bay" stocks of cod along the northeast coast of Newfoundland. All inshore cod samples were genetically distinct from all offshore samples of northern cod. The offshore samples were more heterogeneous, and there may be at least three distinct offshore spawning populations of northern cod. In Subdiv. 3Ps, genetic differentiation was observed between the inshore Placentia Bay and Fortune Bay samples, and the Placentia Bay sample was distinct from offshore samples of northern cod. Simulated mixed-stock fishery samples of northern cod suggested that variation at the seven microsatellite loci, the synaptophysin locus, and Mhc locus C should provide reasonably accurate estimates of stock composition (inshore vs. offshore) when the inshore component comprises at least 50% of the mixture. In Subdiv. 3Ps, bias of estimated stock compositions was marginal when offshore populations (Burgeo Bank, Halibut Channel) comprised the majority of the sample. However, bias in the estimated stock compositions increased when inshore populations comprised the majority of the sample. Increased baseline population sample sizes or additional discriminating markers are likely required to decrease the degree of bias in the estimated stock composition in this application.

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