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

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.