Connectivity and Population Structure in a Marginal Sea—A Review

ABSTRACT Aim The current biodiversity crisis calls for conservation measures that limit or reduce the negative human impact on key habitats and vulnerable wild populations. To effectively protect biodiversity at all levels, including intra‐specific diversity, conservation measures should, ideally, be aligned with the connectivity and genetic structure of wild populations. In this review, we synthesise the scientific literature on connectivity and population structure of marine species in a marginal sea. Location The study focuses on the Skagerrak—a marginal sea in the northeast Atlantic Ocean. Methods We reviewed a total of 172 scientific publications assessing connectivity or population structure in 48 species. From this material, we summarised the main patterns of connectivity and population structure across species, as well as the taxonomic and geographic representation of the scientific literature within this field. Results Our review shows that contemporary connectivity with adjacent seas is high, but asymmetric, for most species. Simultaneously, most species have multiple distinct populations in the Skagerrak, separate from those in adjacent seas. Within the Skagerrak, population structure is common both among coastal populations and between coastal and offshore populations, but less frequent among offshore populations. In many mobile species, multiple populations temporarily overlap in certain areas, but retain their genetic divergence through homing or other barriers to gene flow. Main Conclusions Even in one of the most intensively studied marine regions within the field of connectivity and population structure, there are still large knowledge gaps limiting both our understanding of connectivity and its application in management decisions. Nevertheless, it is evident that the presence of population structure despite high connectivity, and temporal variability in population assemblages, poses a challenge for area‐based protection measures. This underscores the need for adaptive management that monitors and manages intra‐specific diversity on multiple temporal and spatial scales.