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The degree of differentiation between capelin (Mallotus villosus) populations in the Estuary and Gulf of St. Lawrence was evaluated using conventional and truss morphometric analyses together with electrophoretic analyses of protein-coding loci. Samples of spawning fish were collected in the same year at eight sites across NAFO divisions 4R, 4S, and 4T. While the results of conventional morphometric analyses were not conclusive, truss and electrophoretic analyses differentiated the sampled sites along an east–west axis. Two samples (one from the Southern Gulf and the other from the Northeastern Gulf) did not follow this trend. Processes are suggested to explain this pattern. Variations in allelic frequencies among age-classes of fish have been observed in some samples and may be attributed to selective environmental pressures. This could explain the apparent population differentiation observed, although the distribution of the alleles revealed a high gene flow.

A restriction fragment length polymorphism (RFLP) analysis was carried out on nuclear ribosomal DNA (rDNA) of the redfishes Sebastes fasciatus and Sebastes mentella from the Gulf of St. Lawrence in an attempt to describe new molecular markers that would discriminate these two sibling species. The RFLP analysis revealed heterogeneity in the size of the repeat unit within and among individuals that is most likely the result of variation in the length of the intergenic spacer. Double digestion of rDNA with the restriction enzymes EcoRI and ScaI and subsequent hybridization with a 28S probe revealed the presence of three patterns of fragments. Two fragment groups seem to characterize S. mentella and S. fasciatus. Both fragment groups were present in several specimens, suggesting either a restriction-site polymorphism in S. mentella or a hybrid origin for these redfish. Discriminant analysis clearly differentiated all three rDNA patterns. Comparisons of genetic variations at the MDH* locus and of the number of soft rays in the anal fin among the three rDNA-defined groups suggest that if the individuals showing the two groups of fragments are hybrids between S. fasciatus and S. mentella, introgression has also occurred in the Gulf of St. Lawrence.

Branton. 2006. Extensive sampling and concomitant use of meristic characteristics and variation at the MDH-A* locus reveal new information on redfish species distribution and spatial pattern of introgressive hybridization in the Northwest Atlantic. J. Northw. Atl. Fish. Sci., 36: 65-80. doi: 10.2960/J.v36.m573 Abstract Variability at the MDH-A* locus, of anal fin ray number and extrinsic gasbladder muscle pattern is used to describe the distribution of Sebastes fasciatus and S. mentella in the Northwest Atlantic and to define the geographical boundaries of the area of introgressive hybridization between the two species. Sebastes mentella distribution extends from the Gulf of St. Lawrence northward while S. fasciatus is distributed from the southern Grand Banks southward as described in earlier studies. Sebastes fasciatus is also found in the southern Labrador Sea where the species appears to reach its northernmost limit. The distribution of the two species overlaps mainly in the Gulf of St. Lawrence and the Laurentian Channel, around the Grand Banks and on Flemish Cap. This area of sympatry comprises a smaller area where heterozygous individuals at the MDH-A* locus are observed and where introgressive hybridization occurs. This area is mostly restricted to the Gulf of St. Lawrence and the Laurentian Channel. The west-east gradient in the abundance of introgressed individuals suggests that the Gulf of St. Lawrence is the centre of introgressive hybridization and that larval dispersion or migration of juveniles and adults takes place from the Gulf of St. Lawrence towards south Newfoundland and the Grand Banks. In contrast, the absence of heterozygous individuals at the MDH-A* locus outside the Gulf of St. Lawrence and the Laurentian Channel indicates that the dispersion of these individuals is limited for all stages of the life cycle.

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