Effects of climate change on the North Sea and Baltic Sea

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Warming leads to an increase in North Sea fish species

Climate models predict a 2-4ºC rise in water temperature along with a rise in sea levels in the current century. This will have major implications for species, ecosystems and food webs.

The MarBEF project MarFISH examined archaeological evidence from the waters around Denmark (the Kattegat, Skagerrak, the Belt Sea and Bornholm) during a warm period from 7000-3900BC and showed that, during this period, there were several warm-water fish species present.

These species were: smoothhound (Mustelus sp.), common stingray (Dasyatis pastinaca), anchovy (Engraulis encrasicolus), European sea bass (Dicentrarchus labrax), black sea bream (Spondyliosoma cantharus) and swordfish (Xiphias gladius).

These species currently have a more southerly distribution and their presence near Denmark in the past was presumably caused by the warmer temperatures at that time. However, fishermen in this area are now capturing commercially important quantities (tens and thousands of tonnes) of some of these species. This is mainly the case for small-sized southern species while large, northern species have shifted their distributions to more northern and deeper waters. These changes have also been seen in scientific fisheries surveys which annually monitor the species composition of the North Sea fish community[1].

Warming leads to shifts in fish phenology

During the substantially colder climate in the 17th century, the herring Clupea harengus membras fishery in the North East Baltic Sea (Gulf of Riga) mostly took place during the summer months (June-July). This was probably because the fish migrated later to the spawning areas close to the coast where they were caught. In contrast, nowadays, in much warmer climate conditions, the coastal trapnet herring fishery in spawning grounds takes place a few months earlier[1].

Examples of other effects on phenolgy

Climate change and eutrophication

Climate change will also have many non-thermal impacts on fish populations. For example, changes in the strength, direction and location of ocean currents can affect the probability that fish eggs and larvae survive and grow. Moreover, as temperatures rise, the ability of the ocean to retain oxygen will decrease. In many coastal areas in Europe (e.g., bays, straits, estuaries) the combination of rising temperature and decreasing oxygen, will lead to eutrophication, especially in areas which already also receive high levels of nutrients. This will reduce the habitat size of bottom-living fish species such as cod and flatfishes. These species will become less abundant and widespread as coastal areas experience longer and more frequent anoxic periods[1].

Climate change and salinity

In some areas, climate change could even influence the salinity of the seawater. This could happen because precipitation and the discharge of freshwater from rivers and lakes in, for example, north-eastern Europe, could change. In the Baltic Sea, some climate oceanographic models predict that the salinity will fall even further because climate change in this area will increase precipitation. The salinity in this area is already so low that some fish species have adapted their physiology to be able survive.

If climate change leads to a fall in Baltic Sea salinity, this will reduce the number of marine fish species, even though one might otherwise predict that the increasing temperature should allow warm water-adapted species to immigrate. The Baltic Sea example shows that it will be important to consider multiple aspects of climate change, especially in coastal areas, if we are to estimate how marine biodiversity will change in future[1].

Climate change and sea level rise

Another impact of climate change will be the rise in sea level due to melting of land-based glaciers and the expansion of seawater as it warms up. Both factors will cause flooding of existing coastal lowlands. The newly flooded coastal areas will provide more fish habitat, especially for benthic juveniles stages which are common in coastal areas[1].

See also