Key findings

The Skagerrak harbours distinct populations for the majority of species. For these species, there are clear genetic and/or morphological differences between Skagerrak populations and populations in a least one of the adjacent seas (Figure 1). In some of those species, including herring, lumpfish, cod, plaice, sea trout, harbour porpoise, bladderwrack and toothed wrack, the Skagerrak populations are divergent from populations in both the North Sea and the Kattegat. Several species share a genetic barrier on the south-western tip of Norway, between the Skagerrak and the North Sea, and also in the south, between the Skagerrak and Kattegat. For a few species no population structure was detected in the North Sea-Skagerrak-Kattegat area: three-spined stickleback, European plaice, brown crab, Norway lobster, and green sea urchin.

Figure 1. Population structure between the Skagerrak and the adjacent North Sea, Kattegat, and Baltic Sea. The boxes show the species for which population structure between two areas has been assessed, and the percentage of these species for which population structure was found (in green).

More than half of the assessed species also have multiple distinct populations within the Skagerrak (Figure 2A). Population structure is most common among coastal sites (Figure 2B), and between coastal and offshore populations (Figure 2C). Population structure is rare in offshore areas, and has only been described for lumpfish, Atlantic bluefin tuna, the cold-water coral Lophelia pertusa and the phytoplankton Skeletonema marinoi (Figure 2D).

Figure 2. Population structure within the Skagerrak for species assessed in the scientific literature. Subplots show whether any population structure has been found A) broadly within the Skagerrak, B) among coastal sites, C) between coastal and offshore sites or, D) among offshore sites. The boxes show which species have been assessed, and the percentage of these species for which population structure has been found (in green).

Skagerrak populations generally have the potential to disperse to the adjacent North Sea, Kattegat, and Baltic Sea. Dispersal of organisms, i.e. connectivity, into the Skagerrak from adjacent seas is high in most assessed species (Figure 3A), whereas connectivity out of the Skagerrak is high to the North Sea for all assessed species, but slightly lower southward into the Kattegat and the Baltic Sea (Figure 3B). Dispersal distances within the Skagerrak are highly species-specific and may range from a few to hundreds of kilometres, meaning that management needs to be species-specific.

Figure 3. Connectivity of Skagerrak species with the adjacent North Sea (pink), Kattegat (blue), and Baltic Sea (orange). The figure summarises connectivity A) into, and B) out of the Skagerrak, assessed either by tagging or oceanographic modelling. The boxes show which species have been assessed, and the proportion of these species for which high connectivity has been found (in colour).