Climate change robs fish of oxygen
The rise in temperature as a result of climate change reduces the oxygen content of the seas and thus also puts the fish in distress: If their need for oxygen is not met, they grow less and have a shorter life expectancy. Researchers therefore expect increased fish migration to the poles, where the oxygen conditions are even better.
Hans Pörtner and Rainer Knust from the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven discovered the threat to fish from temperature-related oxygen deficiency while observing the eelpout (Zoarces viviparus) in the German Wadden Sea. The demersal fish live on the coasts of northern Europe and grow up to one meter in length. In the North Sea, however, there are mostly specimens with a maximum body length of 25 centimetres.
Counts from the last fifty years have now shown that one year after a particularly warm summer there were fewer adult eelpout than after cooler summers. Lab-grown fish raised in warmer temperatures also stayed smaller than usual. Even average temperatures above 17 degrees Celsius have negative effects on fish populations.
Pörtner and Knust explain this with the reduced oxygen concentration in warmer water. At the same time, an increased metabolism in the fish leads to a greater need for this vital gas. Large specimens in particular react sensitively here because they can no longer adapt to stress so well and therefore die first.
The water temperatures in the North Sea have risen by 1.13 degrees Celsius over the past forty years. At the same time, particularly cold surface temperatures occurred less and less frequently in winter. Different calculations warn of an additional temperature increase of 1.6 to 3 degrees Celsius in the northern regions of the North Sea in the future. The water near the coast could become even warmer within the next hundred years: In their calculations, scientists add 3 to 3.9 degrees Celsius to current temperatures.
According to the researchers, the lack of oxygen will either wipe out sensitive fish populations or encourage them to retreat to cooler regions, such as those still found in the Arctic Ocean. (tk)