Strong isolation-by-distance in the absence of genetic population structure in the eelpout (Zoarces viviparus, Linnaeus 1758)

The eelpout (Zoarces viviparus) is a benthic marine fish, which has long been assumed to be philopatric. This made it particularly suitable for environmental monitoring programs as it allows matching its content of pollutants to the area of catch. However, a recent small scale genetic study in the Baltic Proper suggested that gene-flow might be stronger than generally believed and may occur frequently up to a distance of at least 90 km. As challenging the assumption of philopatry might have strong implications for environmental monitoring studies, we tested the hypothesis of philopatry at a larger geographical scale using ten microsatellite loci. A total of 220 individuals sampled from eight locations covering almost the entire geographic range of the species was genotyped. Our results show that genetic diversity decreases from the North Sea to the northeastern Baltic Sea. No strong population structuring was found, but a highly significant isolation-by-distance pattern was detected, suggesting a stepwise migration pattern among populations. Thus, the hypothesis of long-distance migration can be falsified. It is more likely that only limited gene flow exists among adjacent populations without any barriers between them. Our results suggest that dispersal in the eelpout is weak enough to retain this species as an important bioindicator. However, we suggest that reference stations should be placed in an appropriate distance to avoid misleading results from migrating individuals. We conclude that a more precise knowledge on migration rates of the eelpout is required in order to get more reliable insights into the potential area over which the concentration of environmental pollutants is integrated.