Hybridization and Introgression in Black Hakes (Merluccius polli and M. senegalensis): Evolutionary Dynamics and Conservation Implications in the Contact Zone Exploited by Multi-Species Fisheries.

Hybridization is relatively common between closely related species that share part of their distribution. Understanding its dynamics is important both for conservation purposes and to determine its role as an evolutionary mechanism. Here we have studied the case of black hakes (Merluccius polli and Merluccius senegalensis) in its contact zone. The area of study is located in the FAO fishing area 34, in Mauritania and Senegal waters, where both species are exploited jointly in multi-species fisheries involving national and foreign fleets. Using a ddRADSeq approach and based on a set of 5820 SNPs and a total of 240 individuals, we identified one F hybrid and several backcrossed individuals among 90 M. polli samples and none in 90 M. senegalensis samples obtained in 2020, suggesting unidirectional introgression towards M. polli. Hybridization signals were not found in any of the 60 historical samples from 2000. Excluding the hybrids and developing two separate sets of SNPs (5093 SNPs for M. polli and 2794 SNPs for M. senegalensis), our results detected two distinct genetic clusters within M. polli that show different genetic diversity estimates, with one of the clusters showing a higher potential vulnerability to exploitation. This pattern was observed in both contemporary and historical samples, and both groups presented subtle depth segregation. Moreover, 109 outlier loci were identified between the two groups, that could be developed into molecular markers to further study differentiation between both clusters and contribute to improved stock assessment and management of these important demersal resources.