Regional genetic population structure and fine scale genetic cohesion in the Southern blue whiting Micromesistius australis

Southern blue whiting Micromesistus australis support a substantial commercial fishery in South America. There is growing evidence of a high level of demographic independence between stocks associated with the two main spawning grounds, one in the SW Atlantic (SWA) and one in the SE Pacific (SEP), but the potential genetic structuring of these stocks is unknown. In this study adults collected from sites throughout SWA and SEP waters were genotyped at hypervariable microsatellite markers to investigate genetic structuring between and within regions. Allele frequency-based analyses reported highly significant genetic differentiation between regions, indicating low levels of allo-recruitment. Ancillary data on migratory behaviours support natal homing as a prominent stock isolating mechanism. Genetic differentiation was also detected among samples from around the Falkland Islands: kinship analyses indicated that this was due to non-random genetic relatedness within samples. Despite a general pattern of genetic homogeneity among SEP samples, the northernmost sample exhibited significantly high mean relatedness. The data indicate the occurrence of a further level of structuring within both regions that prevents complete mixing, specifically that schools may be hierarchically structured (from putative subpopulations down to kin-containing groups) and exhibit some degree of ontogenetic cohesion, which may also be a component of homing. The importance of homing and group cohesion as factors influencing resilience to, and recovery from, overexploitation is discussed. This study represents an important baseline for future genetic monitoring and assessment of M. australis. The need for such studies is emphasised by the observation of significantly lower levels of genetic variation among SWA samples, which may reflect genetic erosion, and the subsequent collapse of the SWA stock.

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