Temporal and spatial genetic variation in spawning grounds of European hake (Merluccius merluccius) in the Bay of Biscay

Polymorphism at five microsatellite loci were screened to determine the gen etic variability and the temporal stability of population structure in natu ral populations of European hake (Merluccius merluccius, L.) within the Bay of Biscay. In addition, the control region (900 bp) and two protein coding genes (ATPase, subunits 6 and 8, 842 bp and a partial sequence of the ND1, 800 bp) of the mitochondrial DNA (mtDNA) were sequenced from geographicall y distant populations from the extremes of the species range. One hundred i ndividuals from either side of a supposed stock boundary within the bay wer e collected in autumn 1997. This sampling strategy was repeated during hake spawning seasons in late spring of 1998 and 1999. Low levels of population subdivision were found between putative populations within years. Similarl y, low levels of differentiation were found between autumn 1997 northern sa mples and spring 1998 southern samples which were collected 7 months later on spawning grounds. These results are discussed in relation to ecological, behavioural and oceanographic information. Sampling effects, which may inf luence these results, are also discussed. Theta (theta) estimates were sign ificantly different from zero in every other pairwise comparison between ge ographical areas (north and south of the Bay of Biscay) and between years w ithin the same area (P < 0.05). Hierarchical analysis of molecular variance (AMOVA) does not confirm the temporal persistence of population structure. These results are discussed in relation to variance in reproductive succes s, and temporal spawning patterns, which. may exist within the bay. mtDNA v ariability was very low between geographically distant samples from Norway and the Mediterranean Sea with only 10 variable sites found in a total of 2 542 bp of mtDNA, these differences being exclusively in the D-loop.