Definitions and Methods

Population structure is the tendency of species to separate into more or less distinct spawning groups or populations. Such populations will independently evolve and maintain genetic differences and adaptations, unless there is genetic connectivity between them.

Connectivity is the passive or active dispersal of individuals – eggs, larvae, spores, seeds, swimming adults, etc. – from one location or population to another. If some individuals reproduce in the new location, this leads to genetic connectivity (gene flow), important for evolution and local adaptation. If the dispersal affects numbers and biomass in the receiving population there is demographic connectivity, important for ecological interactions and fisheries management.

This policy brief is based on a systematic review of 172 scientific studies published between 1990 and 2023, assessing population structure and connectivity in 48 marine species, both within the Skagerrak and in relation to the adjacent North Sea, Kattegat and Baltic Sea. Population structure was assessed primarily using molecular genetic tools, but also with morphometry and chemical isotope analyses. Connectivity was assessed either by studies of tagged individuals or by oceanographic modelling of propagule dispersal. The scientific literature was strongly dominated by fish species, particularly cod and herring.