July 7, 2025 — A new study reveals that decades of overfishing have altered the evolution of cod in the eastern Baltic Sea.
The research, published in the journal Science Advances on June 25, aimed to answer a question that had puzzled scientists for decades: What’s behind the dramatic size change in eastern Baltic cod?
These fish used to be enormous. In 1996, the biggest Baltic cod grew more than three feet long. By 2019, however, their sizes had been cut in half, and the cod’s weight was but a fraction of its previous glory. Now, the average cod can sit in a person’s cupped hands.
For decades, fishers in the Baltic Sea caught cod relentlessly, using large nets. Smaller fish could escape more easily, presenting an external pressure to remain smaller. But directly connecting the population’s decrease in size to evolution—and not other environmental factors, such as pollution or temperature change—is notoriously difficult for scientists.
Regulators banned fishing of eastern Baltic cod in 2019 due to a population collapse, but their size still shows no signs of bouncing back. In the new study, scientists find that overfishing did not merely remove the biggest individuals—it changed the genetic composition of the cod population, predisposing them to remain small.
“Human harvesting elicits the strongest selection pressures in nature,” Thorsten Reusch, a marine ecologist at the GEOMAR Helmholtz Center for Ocean Research Kiel in Germany and co-author of the paper, tells Emily Anthes of the New York Times. “It can be really fast that you see evolutionary change.”