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

► We studied the genetic population structure of an ecological indicator (the eelpout, Zoarces viviparus). ► We found a strong isolation-by-distance pattern and a decrease in genetic diversity from the North Sea to the north-eastern Baltic Sea. ► These results suggest that migration occurs only among adjacent populations and that no long distance migration occurs. ► The findings are important information for environmental monitoring programmes, which use the eelpout as a biomarker.