Genetic inference of demographic connectivity in the Atlantic European hake metapopulation (Merluccius merluccius) over a spatio-temporal framework

Once the metapopulation structure of the European hake (Merluccius merluccius) has been outlined, temporal stability of its stock units and their connectivity pattern are key factors to understand its population dynamics and design appropriate management policies. Using five microsatellite markers from the European hake and an extensive sampling coverage within stocks, we have tested the connectivity pattern between the two managed fish stocks of this species, the Northern stock and the Southern stock, over the last decade. On the spatial domain, the non-significant genetic variance between stocks (FCT = 0.001, k = 1 gene pools) as opposed to the low but significant divergence within stocks (FSC = 0.008) suggests that demographic exchange occurs between stocks. On the temporal domain, the non-significant variation among years (FCT < 0.001) suggests that the genetic background within the Atlantic hake metapopulation did not change significantly over the last decade. The pattern of connectivity observed fits a classic Wright continuum model and the negative correlation between restriction to gene flow (FST) and migration rate (m) responds closely to theoretical expectations. No evidence of stable differences in gene diversity was observed between stocks, what is consistent with an intermittent and variable intensity of bi-directionally exchange between them. The current pattern of connectivity indicates that uncoupling exists between actual management areas and the genetic commonality within this metapopulation. Nevertheless, the geographic segregation and the relative demographic independence of both stocks advocate for considering two sub-management units. A subsidiary stock assessment capable of modelling fishing effort upon stock interdependence would add up for their sustainability. Indeed, actual EU subsidiary management of both stocks matches social, economic and cultural needs as well as fishing capacity and fishing tradition of local fleets.