Cod fish in the North Sea are decreasing – can we reverse it?

A man on a boat carrying a large fish caught on a hook.

Generation through generation, fishing after fishing, fishing changes the evolution of fish. This phenomenon, called fisheries-induced evolution, is well documented, although it affects countless fish species differently. For North Sea cod, that has meant early-strains thrive, while slower-mature fish are pushed out of the gene pool. This means that fish populations are evolving towards smaller sizes. a The last paper Models what it might take to reverse this impact through conservation, and what it means economically to do so.

“Overall, fishing is one of the main drivers of change in marine ecosystems,” Hannah Schenk, a postdoctoral researcher at the German Center for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig and one of the paper’s authors, told Ars.

Fishing increases mortality rates among fish – especially large fish, which are caught in greater numbers because they are more likely to remain in fishermen’s nets. In turn, this puts selective pressure on the species: fish that mature faster (but stay smaller) gain an advantage. These small, early genes then frequently pass on their genes, affecting the entire population over time.

“There is a trade-off between these two [factors]Once the cod has matured, it grows less. Therefore, when it occurs early, it usually does not reach such a large size as if it had not been spawning.

of fish and money

The new research focuses on North Sea cod, which has been well studied in terms of mortality, growth, etc., and shows signs of fisheries-induced development. The team began working on the project in 2019 and began to combine an evolutionary model with an economic model. The biological model monitors growth, mortality, reproduction, evolutionary changes, hunting effects, and other factors. The economic model visualizes factors such as fishing costs and consumer preference.

They also tuned the various algorithms in place to improve fish stock management – in this case North Sea cod – and economic benefits. “We first developed a model that basically captures all the essential components without being unnecessarily complex,” said Schenck.

The data for the code came from the International Council for the Exploration of the Seas (ICES), which presents it regularly Inventory valuation data On different types, including cod. For the economic model, the researchers relied on multiple sources, such as data From the German Federal Ministry of Food and Agriculture. This provided a price for different types and sizes of fish. More data came from Scientific, Technical and Economic Committee for Fisheries A STECF report that looked at fishing profit margins.

The team used the model to improve evolutionary and economic health by modifying various parameters that can be controlled by conservation goals and regulations. The administration, in this case, is simply catching less fish by setting conservation goals set by the government. Even regardless of development, Schenck added, optimal management plans would include taking fewer North Sea cod out of the ocean. This is despite the fact that the total allowable catch (known as TAC) has already been taken lowered during recent years.


By running models, the researchers showed that fisheries-induced evolution could be reversed if management was considered over a very long timescale—almost a century. This is necessary because evolution takes place slowly.

Conservation targets on a century-long timeline would effectively reverse the development caused by fisheries with only a slight loss of profit over that time. This loss will depend on the ultimate conservation goal and schedule. With an ambitious conservation target (average fish maturity size of 53 cm, compared to the 2019 level of 50.6 cm) set for 2050, there would be an excess loss of 10 percent, for example. The reason this scenario is slightly less profitable is that management will involve reducing harvest at various points in the future to allow stocks to recover before harvesting begins again.

Schenk said it’s difficult to say whether these findings apply to other fish species. This is because many traits are different for different fish. These include previous hunting pressures, speed of development, etc. Going forward, the team hopes to analyze what types of fishing gear — which influence the size of fish caught — and which size of fish would be optimal to reflect fisheries-induced evolution in populations.

Nature, 2023. DOI: 10.1038 / s41893-023-01078-9 (About DOIs)

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