Genetic assessment of seasonal alongshore migration in Merluccius capensis in the Benguela region

Shallow-water hake (Merluccius capensis) is a valuable demersal resource distributed throughout the Benguela Current region, from southern Angola to eastern South Africa. Previous molecular work revealed distinct populations confined to the northern and southern Benguela sub-systems, with asymmetrical migratory patterns. Here, we re-evaluated the geographic position of the genetic break in shallow-water hake for two seasonal sampling events: summer (February - March) and winter (June – August) with the intention of exploring the hypothesis of seasonal migration patterns across the geo-political boundary between Namibia and South Africa. A total of 503 samples were obtained between northern Namibia and the Cape Peninsula, South Africa, and amplified for nine microsatellite markers. Results suggest the presence of small seasonal differences in population composition, with “northern” individuals occurring in the southern Benguela samples in both summer and winter (9% and 3.3% overall, respectively), but “southern” individuals only being found in samples from north of the Orange River collected during the winter (~5% overall). Seascape genetic analyses suggest that the two populations are isolated by environment and that depth and sea surface temperatures might be important drivers in shaping the observed patterns. These results refined the asymmetrical migration patterns previously reported and further suggest that shallow-water hake migration patterns appear to fluctuate moderately with the seasons. We recommend that future stock assessments should be used to evaluate the likely consequences of these migration patterns under various fishery management scenarios.