Microevolution of eelpout, Zoarces viviparus, in the Baltic Sea

Abstract

Zoarces viviparus was sampled in various sub-basins of the Baltic Sea (Gulf of Finland, Gulf of Riga, and Baltic Proper) by different biotopes (eurytherm, cold stenotherm, and their transition areas). The somatic growth rate, absolute fecundity, and number of vertebrae differed significantly between the biotopes suggesting the existence of different eelpout populations in the northeastern Baltic. Adaptation of the originally cold stenotherm eelpout enabled the fish to occupy the more productive eurytherm biotope and thus facilitated an increase in the abundance of the species. An important precondition for this was the existence of transitional abiotic conditions that promoted the break of the species through the temperature barrier, but also simultaneous or earlier adaptation of the cold stenotherm invertebrate prey complex to the eurytherm conditions. As a result of adaptations, the phenotype of the species has changed compared to the original (stenotherm) stock. Adaptation of the fish to new conditions has not caused changes in its intraovarian growth.