Baltic cod are declining — and now researchers say the damage is genetic

world’s marine fish commonly caught for human consumption are considered overfished (Image: FT)

In a remarkable discovery, scientists have found that heavy overfishing not only lowered eastern Baltic cod stocks but has also induced an irreversible biological shift in the species. In a paper this week in Science Advances, scientists present the first evidence of genetic transformations that cod overfishing has brought about evolutionary changes, leading to smaller, slower-growing fish that now make up the majority of the population. The shift in the species' genetic makeup is making it far more difficult — and possibly irreparable — to recover, even with fishing bans, the Financial Times reported.

Cod have decreased in size and breed quickly

Eastern Baltic cod, previously used to being large, are now significantly smaller. Mature cod today are only 20 cm on average, compared to 40 cm in the 1990s. Not only is the change not merely the result of younger fish being caught, but researchers have demonstrated that the population has genetically adapted to severe fishing pressure. The larger, faster-growing cod were more likely to get caught in nets, and the ensuing gene pool is dominated by fish that mature at smaller sizes earlier — a classic case of human-driven evolutionary selection.

A 23-year-old genetic analysis reveals profound changes

The group, headed by Kwi Young Han of the Helmholtz Centre for Ocean Research in Kiel, Germany, studied 152 cod fish caught between 1996 and 2019 in the Bornholm Basin, a large fishing ground off Sweden and Poland. By chemically and genetically unravelling otoliths — small ear stones that record a fish's growth — researchers could track long-term biological changes. They found strong evidence of "directional selection," a type of evolutionary pressure whereby particular genetic traits (in this case, slow growth and early spawning) become more prevalent over generations.

The cost of survival: fewer offspring, less flexibility

Though adapting to become smaller helped cod avoid fishing nets, it had a price. Big fish produce more eggs, and thus recovery of the population is weaker and slower. Moreover, decreased genetic diversity makes the species less capable of coping with other stressors like climate change, ocean acidification, and predation. "We're worried about these populations and others like them crashing or even becoming extinct," said Malin Pinsky, a marine biologist who researches Atlantic cod and wasn't part of the study.

Overfishing may be pressurizing gene alterations in other ocean animals

Researchers warn that cod may not be the only sea creature to have evolved as a result of extreme human pressure. The same evolution has been seen in Atlantic salmon and suspected in over-fished populations such as tuna, anchovies, and herring. The long-suspected overfishing-evolutionary phenomena have been hypothesized for decades — this test is one of the first to provide conclusive genetic proof.

A warning to fisheries management everywhere

Eastern Baltic cod have been prohibited from being caught since 2019 by the EU because of collapsing stock levels. But this research indicates that the ban was too little, too late to prevent long-term biological injury. "Cod live for over 30 years," said Rick Stafford, Bournemouth University professor of marine biology, "but with the levels of fishing currently taking place, it is unlikely to catch cod older than five years." The discovery puts a dark shadow over the global fishery industry: Can marine ecosystems actually recover after decades of fishing, or have we already irreparably altered the components of these species?

With the world reeling from declining fish populations and increasing environmental catastrophes, the Baltic's message is clear. We are not merely fishing for fish — we are rewriting evolution.