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Freshly caught cod: will younger than older fish soon be on the table?
Photo: Jens Büttner / dpa-Zentralbild
The Baltic Sea fisheries are facing a huge upheaval - and for consumers, the standard fish of the German Baltic Sea coast may no longer be available in the future.
According to a study, the cod stock in the western Baltic Sea has collapsed to such an extent that experts believe that a foreseeable recovery is unlikely.
Researchers led by Christian Möllmann from the Center for Earth System Research and Sustainability at the University of Hamburg come to the conclusion in their study that the tipping point for this population has been exceeded. The team has analyzed decades of fisheries data in the region using statistical models, the university said. Scientists from the University of Kiel and the German Center for Integrative Biodiversity Research Halle-Jena-Leipzig were also involved.
The reasons for the development are climate change and overfishing.
The study was published in the journal "Scientific Reports".
"Due to the high catch quotas and previously neglected environmental factors, it is very unlikely that the cod population on the German Baltic coast will recover in the near future," it said.
In fisheries management, a sustainable total biomass is determined annually for certain fish species that can be caught.
So their existence can recover.
"However, this system does not take into account the changing environmental conditions in the region, for example due to climate change." For example, too much cod - in other sea areas called cod - has been caught in recent years.
"Usually one assumes that the stocks can recover if the fishing pressure is reduced," said Möllmann.
"Our analysis shows that this is probably no longer the case." The fishing pressure combined with the warming of the water could have resulted in fewer fish being able to breed, so that fewer and fewer eggs survive.
The cod stock will not recover at all or only very slowly.
Do older fish need more protection?
In order to conserve fish stocks as a whole, researchers are also experimenting with new ways. Researchers in Berlin, together with colleagues from other countries, have now come to the conclusion that the importance of large female fish is systematically underestimated. Instead of releasing the small fish again, they advocate that larger animals should also be protected more as a kind of "megalike fish".
“The number of eggs per gram of female fish increases with the mass of fish, for example in the case of cod or pike.
This means that the systematic removal of the large spawning fish has a particularly negative numerical effect on the total number of eggs released, «says Robert Arlinghaus from the Berlin Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) and the Humboldt University in Berlin.
He is co-author of the study presented in the journal Proceedings of the US National Academy of Sciences.
So far, most of the methods used to calculate the population have assumed that the number of eggs in a female increases proportionally with her weight.
But recent studies have shown that this assumption is wrong for most fish species and that the number of eggs increases disproportionately with weight.
While the importance of large fish for population renewal is often underestimated, the spawning potential of small fish is often overestimated - especially in species in which fertility increases particularly strongly with the size of the fish.
According to the results, this is the case with the Pacific sardine, for example.
One result of such a misjudgment could be that catch quotas are systematically set too high or unfavorable protective measures such as minimum dimensions are used across the board.
A third of the stocks are overexploited
“It has become out of fashion to determine biological principles such as egg numbers for used fish stocks as a function of the mass of the fish.
Our models indicate that knowledge of these relationships is important in order to assess how much greater protection of large fish is worthwhile from a fishing perspective, ”says Arlinghaus.
In the end, both nature conservation and fisheries could benefit.
According to the World Food Organization (FAO), a good third of the world's fish stocks are already overfished.
The total amount of fish catches worldwide has therefore been stable at around 85 million tons per year since the 1990s.
According to the environmental organization WWF, fish is the most important source of protein for three billion people.
apr / dpa