Human activities and climate change-related impacts exert multiple pressures on the marine environment, resulting in changes to biodiversity and ecosystems, including decreased ocean productivity, shifts in food-web dynamics and species’ geographical distributions. Assessments show that the Lusitanian to Boreal species ratio has significantly increased in the Greater North Sea region and Celtic Seas and is correlated with sea surface temperature. While this northward expansion is attributed to a combination of ocean warming and increasing impacts from multiple human activities, their cumulative effects are still poorly understood.

Marine ecosystems perform key environmental functions: they regulate climate, prevent erosion, accumulate and distribute solar energy, absorb carbon dioxide, and maintain biological control. Most of the global warming of the past 50 years has occurred in the oceans. Mitigation of and adaptation to climate change are key EU policy objectives and central to, for instance, the European Green Deal through the EU Adaptation, Biodiversity 2030 and Farm-to-Fork strategies, as well as the Marine Strategy Framework Directive (MSFD) and Water Framework Directive. One MSFD criterion for good environmental status (GES) is related to the distributional range of species, where distribution patterns should be in line with prevailing physiographical, geographical and climatic conditions. Monitoring species distribution is therefore important for assessing environmental status and the impacts of climate change.

Analyses carried out over 45 years reveal that the number of fish species has increased in the assessment area of the Greater North Sea, Baltic Sea and Celtic Seas. This is mainly related to an increase in the number of warm-favouring (Lusitanian) species, with the number of cool-favouring (Boreal) species increasing by a lesser degree. As a result, significant increases in the ratio of the number of Lusitanian species to the number of Boreal species (L:B ratio) have been observed, notably in the Greater North Sea and Celtic Seas.

Changes in L:B ratios are apparent in the North Sea and the Skagerrak-Kattegat, but not in other areas. While ratios fluctuate from year to year, trends are visible when looking at the entire time series. Furthermore, it seems that Lusitanian species have not spread in all northward directions but have followed two particular routes, through the English Channel and north around Scotland.

In a few areas, such as the Danish Straits (Skagerrak-Kattegat) and the Baltic Sea, the numbers of cool-favouring and warm-favouring species have increased by a similar magnitude, so the L:B ratios have remained stable. However, the increased number of non-classified species in some sub-divisions in more recent years might well be relevant, since these species are likely to be of Lusitanian origin. No changes in the L:B ratios were observed in the Bay of Biscay or the Iberian Coast as dominance of Lusitanian species are expected at these (southern) latitudes.

Significant correlations were found between changes in sea surface temperature (SST) and in the L:B ratio for the Greater North Sea and Celtic Seas. The most statistically significant correlations were found with time lags of 1 and 2 years between the SST and L:B ratio time series. A time lag between changes in SST and in fish distribution is expected, since fish species take time to respond to changes in SST, for example by migrating or through changes in reproduction or survival.

Factors other than temperature, such as a decline in fishing pressure in some areas, could have contributed to the increase in species numbers over time. However, the fact that increased numbers mainly reflect increases in warm-favouring (Lusitanian) species, i.e. increases in L:B ratios, indicates that changing temperature is at least partly responsible for the change in distribution.

Increasing temperatures are expected to further accelerate ocean warming and cause large-scale changes in marine ecosystems, although the full extent of these changes is unknown. The combined effects of warming and ocean deoxygenation, driven by excessive nutrients, may further alter species distributions and vertically compress habitats. This may lead to increased overlap among competitors, shifts in food web dynamics with diverging predator-prey interactions, affecting biomass production, ecosystems, fish stocks and the fisheries that depend on them.