Genetic and morphological stock structure of the pearlside, Maurolicus muelleri (Pisces, Sternoptychidae), among Norwegian fjords and offshore area

Genetic variability between marine fish populations with high dispersal pot ential is usually expected to be low. However, divergence between such popu lations may still occur due to partial isolation. It is hypothesised that f jord habitats with restricted circulation may favour fine-scale geographic isolation of fish populations. In the present study, the population structu re of M. muelleri among five west Norwegian fjords and one offshore locatio n in the North Sea is elucidated by using allozymes and selected morphologi cal measures. Allele frequency differences mainly at EST*, ME*, PGI-1* and PGM-1* revealed that the offshore sample was different from individual fjor d samples. Similarly, genetic differences were found among some fjord sampl es too. However, the overall F-ST estimates indicated a low level of geneti c differentiation. Univariate and multivariate analysis of selected morphol ogical measures of the fish also displayed significant differences between the offshore and fjords, and among some individual fjords. These results su ggest that, despite the potential of considerable dispersal of M. muelleri among adjacent areas, most individual fjord populations represent biologica l units that are genetically differentiable from each other and from the of fshore unit. Partial isolation is probably among the main causes of such ge netic divergence.