An increase in water temperatures shifts the habitat of fish species that like the cold northwards and further limits their distribution area.
Climate change is having a deep impact on living conditions in the oceans. The average water temperature of the seas is rising, Polar ice is melting, water bodies are acidifying, water layers are more stable and mix less well, and low-oxygen zones are expanding. The effects of these changes are a source of increasing stress for fish stocks. Spatial shifting of populations and altered species composition within marine ecosystems are to be feared.
Although not all the consequences of climate change for aquatic ecosystems and fish stocks can be predicted in detail there are already clear signs that these developments will influence and change the fishing industry and aquaculture considerably. Whilst some regions will have to suffer losses, others might benefit. These developments have enormous potential for conflict, and disputes between the affected states could increase. After all, the fish and seafood business is an important economic factor throughout the world. Annual turnover amounts to a good 85 billion US$ and 520 million people are directly or indirectly employed by the industry. Particularly in maritime low-wage countries fishing is of great importance for it offers income and food to the world’s poorest. Nearly 52% of global fish landings come from the marine fishery (and of that 90% is caught in the exclusive economic zones of coastal states and only 10% on the high seas), just under 8% come from freshwater, and 40% from aquaculture.
Climate change is affecting the fish industry throughout the world and its impact is as varied as it is complex. Some effects such as the increased occurrence of extreme weather events are already clearly being felt. Others are creeping up on us more slowly and so perhaps seem less threatening. However, this impression is deceptive for it is often these developments in particular that have a decisive effect on the distribution and size of marine fish stocks and can alter potential catch volumes. Climate can have both direct and indirect effects and the connections are not always immediately recognizable. One direct consequence of the rising temperature of the oceans is the rise in sea level. The volume of the seas is increasing because the ice on the pole caps and the glaciers is melting and the water volume is expanding. Among the indirect effects is a more stable layering of the water. Organisms in the upper water sections can then be cut off from nutrient supply from the depths which affects primary production at the surface and thus impacts the feed of billions of fish larvae and many plankton-eating fish species. In addition to rising temperature it is probably the acidification of the sea and the changed oxygen conditions within the water that will have particularly drastic consequences for marine life.
Temperature increase reduces oxygen content in the sea
Ocean warming as a consequence of climate change is no longer a hypothesis but can now be considered a reality. The average temperature of the world’s oceans at the surface rose by 0.7° C in the second half of the nineteenth century. The increase was less pronounced in the lower water layers: + 0.004° C. The rise in temperature is most noticeable in the North Atlantic waters, including the North Sea and the Baltic. With the help of climate models it was calculated that annual average temperatures in northern Germany are expected to rise by 1.1 to 2.2 degrees by the year 2050. On the German Baltic coast the temperature is expected to rise by about 0.5 to 1.1 degrees within the next 30 years, and similar developments are also to be expected in other marine regions.
Because relatively warmer water can absorb less oxygen climatic warming also reduces the oxygen concentration in the open sea. Based on calculations by climate researchers the oxygen content of the oceans could decrease by one to seven per cent by the end of this century – depending on the region. The effects on marine ecosystems and local species communities are at present difficult to predict. There could be spatial population shifts which would alter or destroy traditional food chains. If oxygen is lacking at the water’s surface this also affects communities living at greater depths. During recent decades, the number and extent of low-oxygen zones on the seabed has increased. Partly, for example in the Baltic, due to the geomorphology of the water body. But partly as a result of changed environmental parameters, increased nutrient input and lack of oxygen at greater depths. The total area of such “dead water zones” in the oceans is currently thought to amount to nearly 250,000 square kilometres. A reduced oxygen concentration within the water can already have a detrimental effect on fishing yields because oxygen dependent species usually leave such areas quickly. The remaining species that are more tolerant of oxygen deficiencies and can cope better with the changed conditions are then often subject to greater pressure from predators and are depleted at an above average rate.
Acidification a threat to the development of calcareous organisms
A further significant effect of climate change is ocean acidification. There is a permanent gas exchange between the atmosphere and the ocean. When the CO2 content in the atmosphere rises it releases carbon dioxide to the ocean up to the point where the CO2 partial pressures in the surface water and in the atmosphere are equal again. Because humans continue to burn vast amounts of fossil fuels such as petroleum or coal more and more carbon dioxide enters the atmosphere and thus also the oceans. In the water, the CO2 is converted into carbonic acid which although it is relatively weak nevertheless reduces the pH-value of the sea water due to the enormous amounts that are introduced from the atmosphere: it acidifies. As a result of the transformation into carb
onic acid the carbon dioxide in the ocean is “caught” in the seas which act as a gigantic “CO2 trap”. The oceans take up more than seven gigatonnes (a figure with nine zeros!) CO2 every year, equivalent to about a third of man-made CO2 emissions. Altogether, 38,000 Gt of carbon are already stored in the world’s oceans, about fifty times the carbon content in the atmosphere.
It is particularly worrying that the speed of the acidification process is today over one hundred times faster than in the last 65 million years. Since the beginning of the industrial age the pH value of the oceans has fallen by 0.1 units which is approximately equivalent to a 30% increase in the degree of acidity (pH value is a logarithmic measure). Due to the acidification of the seas it is increasingly difficult for calcareous organisms such as corals, mussels, crabs or numerous plankton species to build up robust skeletal structures. The ecological consequences of acidification are alarming. It endangers not only the continuity and growth of the coral reefs, the most species-rich ecosystems of the oceans, but also the survival of numerous plankton organisms, many of which are calcareous species. And with that an important element in the marine food web which serves as the first food for many fish larvae is in danger of being lost. Fishes, particularly during earlier development stages, can also be damaged directly by acidification which can, for example, cause tissue damage in cod larvae.
Climate change shifts the distribution boundaries of fish species
The impact of climate change on fish stocks is already noticeable in some regions, usually as a shift in distribution boundaries when for example a species that likes warmth suddenly moves into temperate latitudes. The rise in water temperature causes fish species that one would otherwise sooner expect to find in more southern marine regions to migrate to the North Sea. Examples of this are striped red mullet, gurnard or Norway lobster. Some species do not only migrate there for the summer but can now even live permanently in the North Sea because the winters there are milder than in the past. Climate change does not only shift distribution boundaries of individual fish species, however, but also influences the drifting of fish larvae, growth and survival rates, and the linking of regional populations. The example of the spreading of mackerel stocks northwards in the Atlantic proves that these developments also have a direct impact on fisheries and can lead to disputes.
In the tropics and at temperate latitudes of the oceans productivity is mainly limited by the availability of nutrients whereas in Polar and subpolar regions it is sooner light supply and temperature that act as limiting factors. That is why the effects of climate change are currently less noticeable on the equator than at higher latitudes. There climatic warming has already led to spatial shifts of fish swarms in the direction of the poles or into deeper water layers. What the concrete effects of climate change will be in a particular region depends on numerous factors. Some species communities and ecosystems could benefit because the immigration of new species will raise biological diversity in the short term. Others will perhaps be harmed because the competitive conditions between the different species will change. It is almost impossible to predict all the consequences of climate related structural changes within the marine food webs.
Change to sustainable fishing urgently needed
Climate change does not only pose a problem to fisheries but also to aquaculture, especially to farms in temperate latitudes that are dependent on cold water. For them, the optimum locations could shift further in the direction of the poles. However, as almost everywhere there are likely to be winners and losers in aquaculture, too. Higher temperatures offer economic advantages in many places, because the growth phase of the fish is extended and energy costs are reduced. A disadvantage is, however, that toxic algal blooms can develop in warmer water, diseases can spread more easily and the frequency and strength of weather catastrophes, such as storms or flooding, can increase. In the course of these considerations one should not overlook the fact that aquaculture – and even more so the fishing sector – is not only a victim of, but also a contributor to, global climate change. Climate experts have calculated that on average, for every tonne of live weight landed by the fishery about 1.7 tonnes of CO2 are released. The deep-sea fishery is particularly emissions-intensive because it often has to cover great distances to the fishing grounds.
The effects of climate change are still only beginning to be felt in the fishing and aquaculture sectors. However, the rapid changes in chemical, physical and biological conditions in the oceans will probably become more and more apparent in the coming years. A general forecast is currently difficult but, according to experts, from a global viewpoint the impact on fisheries and aquaculture is likely to be negative, and regionally, perhaps even severe. Even if it is possible to limit global warming to 2 degrees above the preindustrial level by 2050 (which many climate experts in the meantime doubt) based on calculations of the Intergovernmental Panel on Climate Change (IPCC) annua
l losses of 17 to 41 billion US dollars can be expected. There are at present no visible signs of any determination to halt climate change by means of resolute, uniform action. And so it is all the more urgent that we prepare for the consequences of the coming changes and push forward the transition to sustainable fishing. While better monitoring and management of fish stocks do not stop climate change they can at least limit some of the non-climatic problems facing the seas. The problem of climate change is additionally exacerbated by other anthropogenic factors such as increasing habitat loss, marine pollution, and overfishing. And there is not much time left for decisive action.