Skagerrak serves as a crucial maritime gateway linking the North Sea with the Baltic Sea. The region is characterized by a unique seascape with a deep channel known as the Norwegian Trench, which extends from the Norwegian Sea to the inner reaches of Skagerrak. The Norwegian and Swedish coasts of Skagerrak are characterized by a narrow rocky plateau that descends via steep slopes into the trench, whereas the southern Danish side features shallow sandy slopes with sporadic rocky habitat. Strong currents transport water to Skagerrak from both the northern and southern North Sea, where upwelling brings deep, cold, and nutrient-rich water to the surface, whereas less saline surface water flows from the Baltic Sea. This mixture of diverse habitats and water bodies creates a unique and exceptional ecosystem with a high biodiversity, where the access to deeper and colder water may represent a climate refuge from marine heat waves in coastal and shallow areas. While rich in habitats and species, their status is in many cases poor. Being one of the most intensively trawled areas in the world, there is scope for improvement in the management of Skagerrak’s biodiversity and fisheries resources, to restore the services provided and the ecological resilience of this precious ecosystem.
By reviewing the available scientific literature (Chapter 1), we show that a majority of species have populations inhabiting the Skagerrak that are genetically and/or morphologically distinct from surrounding populations in the North Sea, Kattegat, and Baltic Sea. Additionally, many species also have several distinct populations within the Skagerrak. Despite this, functional connectivity across Skagerrak is high in most species, meaning individuals from several populations may coexist in certain areas during parts of the year, especially in highly mobile taxa such as many fish species. Management needs to consider that different populations may coexist at certain times in a given area. This is especially relevant in fisheries management, when different stocks coexist, and where genetic mixed-stock analysis should be implemented to disentangle and estimate the proportions of the different stocks.
A policy brief on population connectivity in Skagerrak is available here:
We identified a range of commercially valuable fish species that currently lack advice or full analytical stock assessments, or where the stock units are poorly defined, and the current advice therefore applies to an area far beyond the reach of Skagerrak (Chapter 2). Not all of these are of commercial interest, and fisheries advice is thus not relevant for all. However, monitoring trends in abundance can also support red list status assessment and biodiversity monitoring. The analyses highlight the remarkable diversity of fish in the Skagerrak, documenting 236 unique species or species groups across two surveys over almost 40 years. This extensive inventory underscores the ecological richness of the region and establishes a foundational dataset for understanding the area’s biological complexity. Declines in certain species emphasise the impacts of high fishing pressure and/or environmental changes, signalling the need for targeted management actions. However, summarizing complex temporal patterns into single metrics poses additional challenges. While this study estimates relative abundance indices, reducing these temporal patterns to a single value may obscure year-to-year variability or mask historical baselines critical for understanding long-term trends. Given that many species in this study are deep-water and long-lived, alternative metrics, such as ratios of recent to historical averages or time series analyses over longer periods, could provide more robust insights into population changes.
A policy brief on fish species with management shortcomings in Skagerrak is available here:
Marine protected areas (MPAs) are tools for biodiversity conservation in a wide sense including protection of ecosystem functions and services. Preliminary analyses suggest that marine protection in Skagerrak is weak overall, especially considering regulation of mobile bottom-contacting fishing gear (Chapter 3). Nature protection in the Skagerrak consists of MPAs ratified under the OSPAR agreement (EU and non-EU members), the Natura 2000 network of MPAs (EU member states) and national parks. Partially protected areas (PPAs) in the form of small-scale spatial protection of European lobster through gear restrictions are implemented widely in Norway and Sweden. Protected areas in Skagerrak are largely restricted to shallow coastal habitats (0-200 m), meaning that important deep-water habitat types are poorly protected. One notable exception is the Bratten MPA with fully protected no-take zones (Sweden). The deepest parts of the Skagerrak are situated in the Norwegian Trench, which is home to vulnerable marine ecosystems, and constitute the most important carbon sink in the greater North Sea region. Current protection of this area is weak and in urgent need of attention. Establishment of a coherent network of MPAs in the Skagerrak will require international coordination, where marine spatial planning should consider the benefits of protection to biodiversity, fisheries and climate change adaptation and mitigation.