Humans have changed the genetic makeup of cod

Cod once ruled the Baltic Sea – massive, plentiful, and essential to the region’s fishing economy. They often stretched over a meter long and weighed up to 40 kilograms.

Along with herring, cod once sustained industries, communities, and families. Now, they have vanished – at least in their original form.

Catching an impressive-sized cod is a rare sight – and is no longer permitted. A ban on targeted cod fishing has been in effect since 2019, following a population collapse.

It’s not just the numbers that plummeted. Cod shrank, and the changes didn’t stop at the surface level.

Fishing pressure rewired a species

According to researchers from GEOMAR Helmholtz Centre for Ocean Research Kiel, decades of intense fishing pressure didn’t just reduce the population – it rewired the species itself.

The new study shows that human actions have altered the actual genetic makeup of cod in the Baltic Sea.

“Selective overexploitation has altered the genome of Eastern Baltic cod,” explains Dr. Kwi Young Han, the study’s first author. “We see this in the significant decline in average size, which we could link to reduced growth rates.”

“For the first time in a fully marine species, we have provided evidence of evolutionary changes in the genomes of a fish population subjected to intense exploitation, which has pushed the population to the brink of collapse.”

Changes in cod genetics

The study links specific genetic changes to a shift in how cod grow and reproduce. The researchers found that genetic variants tied to growth became more or less common over time – classic signs of what’s called directional selection.

These variants weren’t random either. They matched up with known genes that influence size and reproduction.

The experts also observed changes in a particular section of the cod genome – a chromosomal inversion – that typically helps animals adapt to environmental stress.

In this case, it followed the same selection pattern, reinforcing that the shrinking of the cod isn’t just bad luck or poor diet. It’s evolution, triggered by how we fish.

DNA from the past

To study this long-term shift, the team turned to something unusual: tiny stones in fish ears called otoliths. Like tree rings, they store growth data year by year.

The scientists used 152 otolith samples collected from the Bornholm Basin between 1996 and 2019 – part of GEOMAR’s Baltic Sea Integrative Long-Term Data Series.

The researchers paired chemical analysis of the otoliths with high-resolution DNA sequencing. They traced how cod growth patterns and genetic traits shifted across nearly three decades of fishing pressure.

Evolution driven by human activity

The outcome? Cod that grew fast and large used to be common. Now, they’ve nearly vanished. The survivors are the ones that grow slower, mature smaller, and escape early capture.

“When the largest individuals are consistently removed from the population over many years, smaller, faster-maturing fish gain an evolutionary advantage,” noted Dr. Thorsten Reusch, head of the Marine Ecology Research Division at GEOMAR.

“What we are observing is evolution in action, driven by human activity. This is scientifically fascinating, but ecologically deeply concerning.”

Cod recovery isn’t guaranteed

The genetic changes found in cod aren’t easily reversible. Cod that grow slowly and reproduce early might be surviving, but they’re also less productive. They mature at smaller sizes and produce fewer offspring.

Furthermore, the genetic diversity of cod is shrinking. This diversity is crucial for helping species adapt to new conditions.

“Evolutionary change unfolds over many generations,” said Dr. Reusch. “Recovery takes far longer than decline, and it may not even be possible. This is evident in our 2025 length data from the current ALKOR cruise: despite the fishing ban, there’s no sign of a rebound in body size.”

Significance of cod genetics

This isn’t just about fish. It’s about how quickly humans can push a species to evolve in real time – and how hard it is to undo that. Genetic changes don’t bounce back when the pressure’s off. They linger, and in many cases, they stick.

“Our results demonstrate the profound impact of human activities on wild populations, even at the level of their DNA,” said Dr. Han. “They also highlight that sustainable fisheries are not only an economic issue, but also a matter of conserving biodiversity, including genetic resources.”

The science is clear. Overfishing doesn’t just take fish out of the sea. It leaves a permanent mark on the species. And it might have already cost us more than we can get back.

The full study was published in the journal Science Advances.

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