Changes in fish distribution in Europe's seas

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.

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, 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 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 be relevant as these species are likely to be of Lusitanian origin, warranting further examination. No changes in L:B ratios were observed in the Bay of Biscay or Iberian Coast as dominance of Lusitanian species are to be expected at these (southern) latitudes.

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. Most statistically significant correlations were found with time lags of one and two years between SST and L:B ratio time series. For the Baltic Sea, correlation was significant with a time lag of two years. A time lag between changes in fish distribution and SST is expected. Fish species take time to respond to changes in SST, 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, higher 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 fisheries that depend on them.

Supporting information

Definition

This indicator looks at the temporal development of the ratio of the number of warm-favouring (Lusitanian) to the number of cool-favouring (Boreal) fish species within International Council for the Exploration of the Sea (ICES) statistical rectangles and MSFD marine regions.

It investigates shifts in fish species distribution based on information on species’ biogeographical affinity during the period 1967-2020 (figures may show shorter timeseries for wider and more stable data coverage). Analyses are performed for the marine sub-regions in the North-East Atlantic Ocean (Greater North, Celtic Seas and the Bay of Biscay and the Iberian Coast) and the southern part of the Baltic Sea. The relationship between observed shifts in fish species distribution and trends in SST is analysed to investigate if these changes can be related to climate change. Significance is tested using the Mann-Kendall test (p<0.05). This is in line with the MSFD D1C4 criterion relating to the distributional range of species: ‘Distribution patterns should be in line with prevailing physiographic, geographic and climatic conditions’.

Methodology

The data were collected through a range of trawl surveys carried out in European North-East Atlantic waters (https://www.eea.europa.eu/data-and-maps/figures/overview-of-the-international-conference-1). The purpose of these surveys is to estimate fish stocks and their recruitment. The actual data are stored in and can be accessed from the Database of Trawl Surveys (Datras) provided by ICES. Similar trawl surveys are carried out in parts of the Mediterranean Sea; however, these data reside in national databases and are not easily accessible. The spatial and temporal scales vary but, in general, sampling has been relatively stable since 2000. The longest time series can be found for the North Sea, where sampling began in 1965. Sampling takes place by trawling in the ICES statistical rectangles in a particular quarter or particular quarters of the year. Apart from identifying the types of species, the biomass and the number of specimens per length class are also registered.

The exchange of data on species involves the use of AphiaIDs, the species codes used by the World Register of Marine Species (WoRMS). While the taxonomic resolution has changed over time, it has been at the same (high) level since 1986. A total of 603 fish taxa (not species, as both synonyms and the occurrence of genus and higher taxonomic levels add to the list of taxa) have been identified from these trawl surveys. About 8% of the AphiaIDs used were not valid. Several epibenthic non-fish species have been recorded in the surveys but are excluded from calculations. Species are classified according to biogeographical affinity using the classification from the RECLAIM project.

Based on catchability issues, the data are summarised to give presence/absence scores. The basic data used for this method therefore indicate only the presence or absence of a given species within a specific rectangle in a given year (i.e. whether or not the species was found in that rectangle in that year). Because of the fluctuation in the occurrence of fish species in each single rectangle, data analyses have been aggregated to a higher level. The MSFD marine sub-regions have been used for this aggregation. The ratio of the number of Lusitanian species to the number Boreal species (L:B ratio) is calculated by ICES statistical rectangles. Ratios are averaged across statistical rectangles to MSFD sub-regions (see ‘Data sets uncertainty’).

The relationship between observed shifts in fish species distribution and trends in SST is analysed to investigate if these changes can be related to climate change. Significance is tested using the Mann-Kendall test (p<0.05) for monotonic trends. The p-value indicates the probability that there is no significant correlation between changes in fish species distribution and changes in SST over time and that changes are based on random fluctuations only. A p-value of under 0.05 supports the alternative hypothesis, i.e. that there is a significant correlation.

Policy/environmental relevance

Changes in fish distribution are increasingly being observed across the globe. Many of these changes are described as poleward and have been attributed to ocean warming. While some previous studies of warming-induced distributional shifts in marine fishes have successfully confirmed the presence of changes, few have quantified the magnitude or orientation of the shift, nor predicted future responses.

Under the EU Common Fisheries Policy (CFP), the share of the catch of each fish stock is split among management areas using a fixed allocation key known as ‘Relative Stability’. In each management area, Member States get the same proportion of the total catch each year. That proportion is largely based on catches made by those member states in the 1970s. Changes in distribution can, therefore, result in a mismatch between quota shares and regional abundances within management areas, with potential repercussions for the status of fish stocks and the fisheries that depend on them. Assessing these changes is thus crucial to ensure adequate management and sustainable exploitation of our marine resources.

The main aim of the marine indicators set is to support and evaluate efficiency of the Marine Strategy Framework Directive (MSFD) and the UN Agenda 2030 (SDG 14), as well as the Maritime Spatial Planning Directive (MSPD) and other EU and international polices. The objective is to illustrate long-term trends where possible.

EU policies aim to achieve climate target goals as a key priority. After objectives under the Kyoto Protocol for the period 2008-2012 were achieved, the EU Adaptation Strategy was endorsed. In 2021, the European Commission adopted a new, more ambitious EU Adaptation strategy within the European Green Deal. Mitigation and adaptation to climate change are built into sectoral policies in EU funds, such as the Biodiversity Strategy for 2030, the Marine Strategy Framework Directive (MSFD) and the Water Framework Directive (WFD).

Climate change is considered in the MSFD, as it is a pressure on the marine environment and needs to be considered in the directive’s programmes of measures and when determining good environmental status (GES). The MSFD provides a qualitative descriptor (D1) for determining GES in relation to biological diversity: ‘Biological diversity is maintained. The quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climatic conditions’. The D1C4 criterion relates to the distributional range of species: ‘Distribution patterns should be in line with prevailing physiographic, geographic and climatic conditions’. This is in line with objectives under the European Green Deal, aiming to protect biodiversity and ensure sustainable fisheries sectors. Thus, it is also linked to the Farm-to-Fork Strategy, the Common Fisheries Policy and other EU environmental policies such as the Birds Directive, the Habitats Directive and the regulation on Invasive Alien Species (IAS).

Accuracy and uncertainties

Methodology uncertainty

The methods used are considered rather robust, as they transform the catches to presence or absence of species rather than using fish species abundance. Using abundance would most likely result in more noise in the data, as the number of specimens caught in a trawl haul is greatly influenced by the actual physical conditions on that day (e.g. current direction and time of day).

Changes in the numbers of Lusitanian versus Boreal species (L:B ratio) have been related to mean yearly water temperatures measured as SST anomalies. As experienced during data preparation for the ICES WKFISHDISH report, the use of several surveys for the same rectangles and sub-divisions introduces the risk of mixing data with biased catchability and, therefore, only one survey has been chosen for each ICES statistical rectangle. The current survey is chosen based on the best temporal and spatial coverage. A list of the surveys used can be found in https://www.eea.europa.eu/data-and-maps/figures/overview-of-the-different-surveys-2.

The indicator is based on the evidence that species distribution shifts are poleward as climate warms. This may not be suitable for some marine regions, such as the Mediterranean and Black seas. For these regions, it may be more useful to use a different species classification system or one that investigates distribution shifts bottomward (increasing depths), however, it should be noted that distribution shifts may also occur upwards (decreasing depths) during seasonal/reproduction migrations or, for example, due to reduced oxygen concentration levels at deeper depths.

Data sets uncertainty

Catch in these trawl surveys may not represent exactly the fish fauna that is present in a given area, as several factors influence the catchability of a certain fish. Thus, data collected are evaluated in light of some constraints in the way the survey has been carried out, namely:

• The trawls used in the surveys are benthic trawls, so it is less likely that they have the same catchability as, for example, for pelagic species such as mackerel and herring. However, pelagic species are caught to some extent, and even species such as Atlantic blue fin tuna and swordfish appear in the species list. However, it is important to emphasise that there is no reason to expect catchability to change over time in the same habitat and if the same gear is used.

• It is most likely that the catchability of species varies between different surveys because of differences in the gear used. However, the use of only one survey for each statistical rectangle ensures that the catchability should be the same over time.

• Due to potentially different survey catchabilities between statistical rectangles within an MSFD sub-region, the L:B ratio for a sub-region is calculated as the mean of the ratios.

• The data coverage in space and time is limited but has been quite stable over the last 15 years. However, at the sub-region scale there are temporal coverage differences. This could lead to a biased estimate of trends, particularly for the Celtic Seas region. Future work on this indicator would model the L:B ratio in space and time to fill gaps in survey coverage.

• Future assessments will also investigate the relationship between fish distribution and sea water temperature at the seabed.

Data sources and providers

Metadata

DPSIRTopicsTagsTemporal coverageGeographic coverage

TypologyUN SDGsUnit of measureFrequency of dissemination

References and footnotes

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