Mitochondrial DNA analyses of the Cape hakes reveal an expanding, panmictic population for Merluccius capensis and population structuring for mature fish in Merluccius paradoxus

The Cape hake species, Merluccius capensis and Merluccius paradoxus are the most important resource of the South African and Namibian demersal fishery, but it is unclear whether there is a single population of each shared by both countries. We analysed the population structure and evolutionary history of these two species using the variable 5′ region of the mtDNA control region for 311 specimens of M. capensis and 333 specimens of M. paradoxus sampled between Lüderitz (southern Namibia) to south of Cape Point (South Africa). 107 haplotypes for M. capensis and eight haplotypes for M. paradoxus were recovered. AMOVA and pairwise Φst analyses revealed no structure in M. capensis, however significant genetic differentiation between Namibian and South African ‘populations’ was detected for M. paradoxus. This was only restricted to mature fish older than 3 and 4 years and not for juvenile fish younger than 3 years. Analyses reveal that M. capensis has undergone population expansion (Fu’s Fs = −26.65, P < 0.001), possibly within the last 4500–23,000 years, whereas M. paradoxus has not. Our study highlights the utility of genetic markers to unravel the evolutionary history of sympatric species, as well as addressing management issues within regions where commercially valuable fish stocks are shared between nations.