Depth-associated genetic structure within redfish (Sebastes mentella) across the North Atlantic: one broad region, two depth strata, two incipient species?

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Genetic and shape analyses of archived otoliths were used for investigating redfish (Sebastes mentella) population structure at depths above and below 500 m, at the scale of the North Atlantic (i.e., Canadian waters, West Greenland shelf, and Irminger Sea). DNA was extracted from 853 archived otoliths sampled in 2009-2011 and was genotyped at 13 microsatellite loci. An individual-based clustering method detected two genetic groups; both groups were observed within each region. Overall, these groups segregated by depth, but (i) juveniles from the genetic 'deep group' were usually found at depth shallower than 500m, in regions close to the continental shelf/slope and (ii) large adult specimens from the genetic 'shallow group' strongly dominated all sampling depths in the southwest Irminger Sea and Flemish Cap. No consistent spatial structure was observed within each group, but introgression with Sebastes fasciatus was observed on the Flemish Cap within the genetic 'shallow-group'. Specimen size disparity between regions resulted in a dominant effect of allometry which confounded otolith shape comparisons between regions. The present study supports the hypothesis of incipient speciation and calls for a better integration of genetic identity, depth, and life-history information into redfish management practices.