Macrogeographical population differentiation in oceanic environments: a case study of European hake (Merluccius merluccius), a commercially importantfish

Polymorphism at six microsatellite loci was used to study genetic variabili ty and population structure in six geographically distant natural populatio ns of European hake (Merluccius merluccius L.). Four hundred and eighty-thr ee individuals were sampled from Trondheimsfjord in Norway, the Celtic Sea, the southern Bay of Biscay, Fare off Portugal, the Mediterranean Sea north of the coast of Tunisia and the Adriatic Sea. Population subdivision was f ound between Mediterranean and Atlantic samples, theta = 0.029 (P < 0.001). No substructuring was found between samples within the Mediterranean Sea, theta = 0.003 and R-ST = 0.007 (P > 0.05). The Atlantic population structur e appears to be more complex than previously suggested by the placement of stock boundaries by the International Council for the Exploration of the Se as (ICES). Analyses based on various models of microsatellite evolution all suggest that differentiation exists between Bay of Biscay and Portugese sa mples, theta = 0.013 (P < 0.001), R-ST = 0.036 (P < 0.001) which are curren tly managed as one stock. By contrast, fixation indices indicated no differ entiation between southern Bay of Biscay samples and Celtic Sea samples, th eta = 0.003 (P = 0.02), phi(ST) = 0.007 (P = 0.10) which are managed as sep arate stocks. These results suggest that if the observed trends are stable through time, current management policy of European hake may need revision.