FI-I-1: Distribution of thermophilic marine species

The picture shows the two arms of a man stacking boxes filled to the brim with sardines. Click to enlarge
Thermophilic sardines are conquering the North Sea thus gaining in relevance to fisheries.
Source: Photograph: © ermess /

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents


FI-I-1: Distribution of thermophilic marine species

The effects of climate change on fish stocks are visible already today in the North Sea in terms of the encroachment of southern European thermophilic species on northern latitudes. In the most southern research areas nearest the German North Sea coast, almost every catch now turns up a southern species. Likewise, the most northern research areas show a similar increase.

Six curves show the percentage of catches with at least one Lusitanian fish species for different boxes in a time series from 1987 to 2013. The time series for Box C runs close to the 0 axis with a maximum value of just under 20 per cent in 2014. The curve for Box H is similar, with a maximum of around 5 per cent in 2003.
FI-I-1: Distribution of thermophilic marine species

Six curves show the percentage of catches with at least one Lusitanian fish species for different boxes in a time series from 1987 to 2013. The time series for Box C runs close to the 0 axis with a maximum value of just under 20 per cent in 2014. The curve for Box H is similar, with a maximum of around 5 per cent in 2003. The other three boxes show a partly fluctuating course, but a rapidly significant increasing trend. In Box A, 100 per cent was reached in 1998 and maintained until 2017. In Box E, the values have been above 90 per cent since 2007. In Box K, the highest value to date was over 50 per cent in 2015.

Source: Thünen-Institut für Seefischerei (GSBTS: German Small-scale Bottom Trawl Survey)

Thermophilic fish species in North Sea and Baltic Sea

Rising water temperatures, changed currents and rising CO2 concentrations in sea water affect the living conditions for all marine organisms. It is worth remembering that water masses in the North Sea do not warm up in direction of north to south. In fact, the warming process takes place in complex spatial patterns. In the North Sea it has been observed that the stocks of psychrophilic (cold-loving) fish, molluscs and crustaceans have a tendency to move to cooler zones as warming increases. Their organism requires a specific range of temperatures, which is no longer available to them in a warming habitat. Besides, they follow plants, plankton and other marine organisms which they feed on and which prefer water at colder temperatures. At the same time, new species emanating from more southerly seas have encroached on the North Sea.

The brackish water of the Baltic Sea, with its mixture of freshwater and seawater, has given rise to unstable ecological equilibria. The high variability of environmental conditions offers only few, very tolerant fish species adequate conditions for living and reproduction. In view of the higher tolerance of species it is to be expected that the consequences of climate change in the Baltic Sea will have less pronounced impacts on species shift than in the North Sea. Nevertheless, changes to the productivity of fish stocks in the Baltic Sea are likely to occur.

Changes in the distribution of fish stocks and in species composition confront marine fisheries with new challenges. In this light, the spatial shift of North Sea fish populations towards cooler zones may entail economic losses to the fishery operators concerned, if new ranges of well-known species become harder to reach thus requiring much greater expense. It is hard to predict with any certainty to what extent such economic and ecological effects might be offset against the distribution and abundance of other species. At any rate, under current circumstances, it would not be worthwhile to target catches of these other species.

An important foundation for future adaptation of fishery management is the exact observation of the spatial shift in fish stocks and the observation of changes in the species communities. Under the ‘German Small-scale Bottom Trawl Survey’ (GSBTS), standardised catches are carried out every year in designated areas of the North Sea. The objective is to assess the natural variability of fishing quotas for various species of fish and to record medium to long-term changes in the composition of fish species.

If you analyse the catch records of recent years up to 30 years ago in five research areas within the German Bight, you will see that more and more frequently specific south-European species originating in Portuguese seas have been identified in catches. Typical representatives of this group of species are for instance the tub gurnet (Chelidonichthys lucerna), the red mullet (Mullus surmuletus), the yellow sole (Buglossidium luteum) and the Mediterranean scaldfish (Arnoglossus laterna) as well as the anchovy (Engraulis encrasicolus) and the sardine (Sardina pilchardus). In the most southerly fishing zones nearest the German coast (Box A and Box E), every catch turns up at least one of these species, whereas in the late 1980s such catches would have been rare. Beginning in the mid-1990s, a little further north, approximately in the latitude of Denmark’s Esbjerg (Box K), representatives of the above-named species appeared, and this subsequently happened with medium frequency, albeit not continuously. Likewise, there have been more and more catches, also in the northern North Sea (Box C) which suggest a potential distribution, also in these areas, that goes hand-in-hand with increasing temperatures.

Concurrently with the encroachment of southern European species, a decline in psychrophilic species has been observed. As a case in point, the codfish has almost completely disappeared from the southern North Sea. This is not just a consequence of intensive fisheries, but also – due to the detrimental effect of the seas warming in these latitudes – on the basic food source and the physiological processes this fish species undergoes.

Apart from climate change, there are other factors such as commercial fisheries responsible for the shift in fish stocks. However, the intensified warming does seem to play a vital role in the spatial relocation of fish stocks. Mild winters have enabled some southern fish species to survive winters and reproduce in the North Sea.



FI-I-2: Occurrence of thermophilic species in inland waters

WW-I-7: Water temperature in the sea