With the rise in water temperature by one to two degrees, important commercial fish species in the North Sea have decreased in size by around 16 percent over the past 40 years up to 2008. The case is an example of the effects of warming water bodies on fish growth.
It is clear that fishing, which often focuses on catching larger fish, favors the stock of smaller fish. This is because fish that become sexually mature quickly and reproduce at an earlier age when they are smaller survive. This can be passed down through generations and result in affected fish species becoming smaller over time.
Now, to study the effects of climate change on fish size, one can look at fish that are not affected by the fishery or at fish in a controlled environment where the water temperature is manipulated. The latter has shown that fish actually attain a smaller size in this case. There is already a name for this: the "temperature size rule".
Researchers are trying to explain the phenomenon. The currently most common theory sees a connection to oxygen supply in the reduced growth of fish. The gills that a fish uses to breathe grow slower than the rest of the body. Once a fish has reached a certain size, it can only take in enough oxygen to live - but no longer to grow. And with rising water temperatures, the aquatic animals consume more oxygen. Researchers disagree as to whether this is the reason why fish no longer grow as large as under normal conditions due to warming waters.
The associate professor of animal ecophysiology at Deakin University in Australia, Timothy Clark, has therefore carried out long-term experiments together with a team and exposed fish to warmer water than normal. They would have tried to supply the fish with additional oxygen, which is said to have finally disproved the theory. "While fish metabolism increases as the water warms, we found that the gills grow sufficiently to keep up with the increased oxygen demands as the fish get larger," he writes in an article for the network The conversation".
According to the researchers, the reduced growth must have another reason. Fish grow faster in warmer water and reach sexual maturity at an earlier age and smaller body size. "It's possible that once the fish start reproducing, they put their energy into reproduction rather than further growth," the researcher said. Whether the reduced energy supply in body growth is related to an adaptation to reproduction cannot be said with certainty.
For Clark, however, one thing is certain: "Fish cannot continue to shrink forever." After all, there is a minimum size that each species must reach in order to maintain a viable population. According to forecasts, the effects of global warming on the ecosystems in water will increase significantly. And the phenomenon apparently also occurs in other animal species. In a study, researchers from the USA came to the conclusion that migratory birds have continuously become smaller and lighter over the past 40 years. According to a report by the SWR from the end of 2019, this is also directly related to climate change, as is the case with fish. For the researchers, this is a cause for concern because: "If their entire habitat gets too warm, the species will die out."
Sources: The Conversation, SWR