Human activities and climate change exert increasing pressure on marine environments, leading to changes in biodiversity, decreased ocean productivity, shifts in food-web dynamics and species’ geographical distributions. The number of warm-favouring species in the Greater North and Celtic Seas have increased and are positively correlated with recent past sea surface temperatures. Responses to increasing water temperatures take time and include changes in migration, reproduction, survival, and potentially productivity. Ocean warming, acidification and eutrophication are expected to further drive changes in species distribution and abundance, impacting ecosystems and access to fishing. 

Figure 1. Temporal development of the ratio of the number of Lusitanian species to the number of Boreal species
Temporal development of the ratio of the number of Lusitanian species to the number of Boreal species

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 in 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, the Fit for 55 package, 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 on marine life.

Analyses carried out over 47 years reveal that the number of fish species has increased in the assessment area of the North-East Atlantic Ocean. 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 and Celtic Seas.

While L:B ratios fluctuate from year to year, some trends are visible when looking at the entire time series. Changes in ratios are most apparent in the North Sea, Irish Sea and West of Scotland. 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, with L:B ratios remaining stable. However, the increased number of non-classified species (unknown) in some sub-divisions in more recent years might well be relevant as these species are likely to be of Lusitanian origin, warranting further examination. No changes in the L:B ratios were observed in the Bay of Biscay or the Iberian Coast as dominance of Lusitanian species are to be expected at these (southern) latitudes.

Figure 2. Temporal development of the number of species of each biogeographical affinity group (top) and of the ratio between Lusitanian and Boreal species with sea surface temperature (bottom) by marine region
Temporal development of the number of species of each biogeographical affinity group (top) and of the ratio between Lusitanian and Boreal species with sea surface temperature (bottom) by marine region

Significant correlations were found between changes in sea surface temperature (SST) and in the L:B ratio for the Greater North 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. For the Baltic Sea, correlation was significant with a time lag of 2 years. A time lag between changes in fish distribution and SST is expected. 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 could have contributed to the increase in species numbers over time. However, the fact that increased numbers mainly reflect increases in warm-favouring 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 ocean warming, acidification and deoxygenation 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.