ResearchCite this article: Colbeck GJ, Duchesne P, Postma LD, Lesage V, Hammill MO, Turgeon J. Social structure involving long-term associations with relatives should facilitate the learning of complex behaviours such as long-distance migration. In and around Hudson Bay (Canada), three stocks of beluga whales form a panmictic unit, but have different migratory behaviours associated with different summering areas. We analysed genetic variation at 13 microsatellite loci among 1524 belugas, to test hypotheses about social structure in belugas. We found significant proportions of mother-offspring pairs throughout the migratory cycle, but average relatedness extended beyond close kinship only during migration. Average relatedness was significantly above random expectations for pairs caught at the same site but on different days or months of a year, suggesting that belugas maintain associations with a network of relatives during migration. Pairs involving a female (female-female or male-female) were on average more related than pairs of males, and males seemed to disperse from their matrilineal group to associate with other mature males. Altogether, our results indicate that relatives other than strictly parents, and especially females, play a role in maintaining a social structure that could facilitate the learning of migration routes. Cultural conservatism may limit contributions from nearby summer stocks to endangered stocks such as the Eastern Hudson Bay beluga.

Migratory connectivity between areas frequented by wide-ranging animals provides crucial information for conservation and management. In and around Hudson Bay (Canada), three stocks of beluga whales (Delphinapterus leucas) are associated with distinct summering areas. We analyzed genetic variation at mtDNA and 13 microsatellite loci among individuals (N [ 1400) harvested by 23 Inuit communities to identify mating units and assess temporal and spatial differences in the way stocks use common migratory pathways. Strong structure at mtDNA and a lack of convincing evidence for nuclear genetic differentiation indicate that both males and females adopt distinct migratory routes towards summering grounds while probably interbreeding on wintering grounds. Spatiotemporal variation in stock composition indicates that subsistence hunting targets all three stocks. While representing ca. 5% of belugas in Hudson Bay, the endangered Eastern Hudson Bay stock accounts for 17% of the overall subsistence harvest by Inuit communities of northern Nunavik (Quebec), and ca. 30% of the spring harvest along northeastern Hudson Bay. Despite interbreeding, cultural conservatism of maternally transmitted migration routes seems to prevent the re-establishment of stocks in previously frequented estuaries. This phenomenon supports the current use of demographic population models based on stock composition for developing behavior-based management strategies.

The capelin (Mallotus villosus) is a widespread marine fish species for which previous work has identified geographically distinct mtDNA clades, the frontiers of which are well within adult and larval dispersal capabilities. Here, we use AFLPs to test for the presence of nuclear gene flow among clades. In addition, we evaluate genetic structuring within one clade, the Northwest Atlantic (NWA). We found that each of the mtDNA clades corresponds with a unique nuclear DNA genetic cluster. Within the NWA clade, we detected individuals with small but significant amounts of genetic ancestry from other clades, likely due to historical introgression. Further support for historical introgression comes from analyses of variance in locus-specific differentiation, which support introgression between some clades and divergence without gene flow between others. Within the NWA, we identified two genetic clusters that correspond to sites in geographically adjacent areas. However, these clusters differ primarily at 'outlier' loci, and a genetic subdivision (K=2) was not supported by genetic clustering programs using neutral loci. Significant neutral F(ST) differentiation was found only between sites that otherwise differed at outlier loci. Thus, these populations may be in the initial stages of 'isolation by adaptation'. These results suggest strong between-clade reproductive isolation despite opportunities for gene flow and support the hypothesis that selection can contribute to divergence in otherwise 'open' systems.