In addition to marine pollution, shipping traffic and overfishing, climate change is affecting fish stocks in the North and Baltic Seas. The increasing warming of seawater and the melting of large freshwater reservoirs, as in Greenland, but also the acidification of the oceans are changing species composition and fishing conditions. The resulting economic losses can be particularly high for marine fisheries. Inland fishing and fish farming in aquacultures are also affected by the effects of climate change.
Effects of climate change on the sea
With the additional carbon dioxid entry into the atmosphere, mankind has triggered processes that will presumably determine the state of the oceans for thousands of years. Some of the effects can already be observed today - for example the rise in sea level, the warming of surface water and the acidification of seawater. What is new here is the global dimension of the changes. With the oceans and their natural resources, important foundations of human life are threatened. The oceans are a source of biodiversity and food, among other things. The coasts are settlement areas for many millions of people.
The changes in the North and Baltic Seas are particularly relevant for Germany. Their waters will continue to heat up in the future due to the ongoing climate change. For the period from 1969 to 2017, the average annual surface temperature of the North Sea has already increased by 1.3 degrees. For the western Baltic Sea, the mean annual temperature has risen by 0.6 degrees each decade since 1982, with the strongest warming occurring in summer. Between 1980 and 2015, the Baltic Sea warmed by 1.6 degrees at the surface for the western Baltic Sea and 1.9 degrees at a depth of 20 metres.
Depending on the climate scenario, the air temperature in the Baltic Sea region will become 2 to 4 degrees warmer in the future. These changes are also reflected in the water temperatures, which will be 2 to 3 degrees warmer on average at the surface by the end of the century. Summer surface water temperatures of over 18 degrees Celsius could occur up to a month longer than today.
The rise in temperature alone has already led to shifts in the occurrence of species and thus to changes in marine ecosystems. The habitat of the cold-loving species could shift towards the poles and subsequently decrease. Oxygen shortages can also be aggravated by temperature increases caused by climate change. Another consequence could be a trend towards smaller body sizes of fish species, which in turn could have a negative impact on their reproductive potential. The stocks themselves and their distribution will change - both commercially important and non-commercially exploited fish species. Stocks that are already heavily and over fished could become more vulnerable and present fisheries management with even greater difficulties than before.
An increasing amount of carbon dioxide in the atmosphere means that the amount of carbon dioxide in the water also increases, as carbon dioxide from the air is absorbed by the water. In the water, carbon dioxide causes the pH value to change and the water to become more acidic. As carbon reservoirs, the oceans have so far absorbed about one third of the carbon dioxide that has been released into the atmosphere by human activities since the beginning of industrialisation. As a result, the average pH value of the sea surface has dropped from 8.2 to 8.1. This small step on the logarithmic pH scale already corresponds to a 30% increase in acidity.
Due to these changing environmental conditions, fish stocks and populations of other marine organisms are changing. For example, climate change can cause the temporal synchronisation of certain developmental phases to dissolve, so that fish larvae can no longer find the appropriate food supply. Overall, it can be expected that known food webs and competitive situations will change.
Anthropogenic stress factors such as overfishing, shipping traffic or pollutants in the water have proven to have negative effects on the marine fauna. Therefore, the climate impacts cannot be precisely determined or delimited from the other factors.
Marine areas (North Sea, Baltic Sea)
Due to the expected warming, it is foreseeable that the living conditions for cold-loving species will deteriorate overall, particularly in the southern North and Baltic Seas, while warm-loving species could migrate more from the south.
In the North Sea, increasing warming is leading to a shift in the habitats of cold-loving native fish populations to the north (e.g. for cod and plaice). Among others, the Barents Sea is becoming increasingly important as a habitat for cod. However, there are also species that do not migrate immediately to the north, but try to cope with the deteriorating conditions.
Other marine animals, on the other hand, are less able to adapt to climate change. Their populations are expected to decline significantly as a result of the warmer winters, because reproduction cycles or predator-prey relationships are disrupted, for example.
The warming of the water is encouraging new species to migrate into the North Sea, which were previously more at home in more southerly areas. An analysis of the catches of the last 30 years shows that certain southern European species originating from the Portuguese maritime area have increasingly appeared in the catches. The mild winters allow some southern fish species to winter and reproduce in the North Sea.
The brackish water environment of the Baltic Sea, in which fresh and salt water mix, places special demands on the local biocoenosis. Due to these conditions, ecological equilibria susceptible to disturbances have been established. Even slight changes in temperature, salt or oxygen conditions can lead to a significant shift in species composition.
Effects of climate change on inland lakes
Global warming has visibly changed the marine ecosystems. In addition to direct effects, there are also numerous indirect effects that contribute significantly to changes in lakes.
One of the direct effects is that the temperature of the surface water increases with the higher air temperature. The winter ice is receding or not appearing at all. The higher water temperatures also change the thermal structure of lakes.
Both effects together cause indirect effects such as changed light, oxygen and nutrient conditions. This often has a greater effect on the development of phytoplankton and the structure of food webs than the changes directly caused by temperature.
Global warming is expected to reduce the oxygen concentration in lakes. The higher water temperatures cause higher oxygen consumption - at the same time, less mixing of the water can create zones where the oxygen is no longer replenished in time by the incipient circulation. Oxygen-free zones can thus be formed.
If the winters become milder and the ice cover less, the oxygen supply will improve and fish mortality will decrease. On the other hand, such mild winters in very deep lakes can prevent deep mixing and thus the oxygen supply in the deep water. This oxygen deficit in the deep water is then transferred from winter to the following summer, causing the oxygen-deficient areas to spread even further.
In inland fisheries, the effects of climate change have so far played a subordinate role compared to other influencing factors, such as the tourist use of the waters. Prolonged dry periods, which are a consequence of climate change, are increasingly threatening large clam, crab and small fish populations in small and very small waters. The situation is similar in aquaculture, even though here water temperatures influenced by climate change, the duration of ice cover of winter ponds and water flow rates are considered to be important factors influencing production.
For the future, experts do not rule out the possibility that climate change will have an increasing impact on fish stocks, yield conditions and the yields of inland fisheries. For example, heat-loving species that are spread by shipping on canals have better chances of establishing themselves when water temperatures rise. For heat-loving species such as the carp, competitive conditions could improve, while for the brown trout and other species that can only exist in a narrow range of low temperature, rising water temperatures are likely to restrict habitats.
With an adequate water supply and, if necessary, technical support for oxygen supply, the carp pond industry ("warm water ponds") could benefit in principle from the increase in water temperatures in the future. For the water balance of a landscape, large-area earth ponds also offer a possibility for water retention after heavy rainfall events.