Warm water can absorb less oxygen, so fish need more energy to grow. The consequences for North Sea creatures are considerable.
Plaice on shore leave in Hamburg. Photo: dpa
Their stocks are already suffering from overfishing and massive water pollution. Now climate change is adding to the problem: By up to 29 percent, the body size of North Sea fish is said to have shrunk in the past 40 years due to rising temperatures. That is the conclusion of a study by the Scottish University of Aberdeen.
According to the researchers, the research, published in the Global Change Biology Journal, provides the first empirical evidence of the link between fish shrinkage and global warming. "The effect of global warming on fish is much more extreme than had been assumed," summarizes study leader Alan Baudron. Affected, he says, were 80 percent of the fish studied, including the popular herring, plaice, sole and the haddock, which is threatened with extinction.
The warmer climate was identified as the main reason for the shrinkage because it was the "only synchronous factor to which all affected species were exposed," Baudron says. He doesn’t consider other influences because the feeding habits and habitats of all species were completely different.
The reason why the fish are shrinking: The warmer the water, the less gas it can absorb. Vital oxygen is therefore becoming increasingly scarce underwater. That’s why "more energy is needed to absorb oxygen," explains marine biologist Rainer Froese of the Geomar Research Institute in Kiel. "So there is less energy left to grow."
Since fish cannot tolerate fluctuating temperatures well, "many species react to temperature increases by migrating to colder regions". The balance between the species is thus considerably disturbed: More and more fish compete in cold regions, while they disappear from warmer areas. Baudron also suspects that the reduction in size leads to a lower reproductive capacity.
This is problematic for the fishing industry: over the past 50 years, yields have dropped by about a quarter, according to the study. Marine expert Iris Menn of Greenpeace warns, "Fish is the only source of protein for about a billion people." As the effects of climate change add to massive overfishing, marine life is more at risk than ever. The quota reduction under the EU fisheries reform from 2015 can only help to a limited extent, because stocks are being given too little time to recover, warns Menn.