"Biologists once thought that humans did little to affect the course of evolution in the short term. But a recent study of cod in the Baltic Sea reveals how overfishing and selective harvest of the largest fish has caused genetic changes that favor slower growth and smaller size."
"For centuries, codfish in the Baltic Sea, which lies between Sweden and Eastern Europe, have been a regional dietary staple. Fishers once hauled in nets filled with cod more than three feet long, weighing as much as 85 pounds.
Cod are a keystone species, an apex predator that kept herring and sprat in check and provided nourishment for seals and other predators. But starting in the mid-1990s, cod numbers in the eastern Baltic began to collapse, leading herring and sprat populations to increase and, in turn, overgraze zooplankton. Baltic cod fishing in the east has been banned since 2019 to allow the stocks to recover.
The numbers of cod aren’t the only thing that has shrunk — so has their average size. Eastern Baltic cod are roughly half as big as they were in 1996, and their median weight has dropped to roughly a fifth of what it had been in that year.
A recent study blames the change on heavy fishing pressure, which selectively removed larger fish, which are more profitable, from the population. That not only reduced cod numbers, it also changed the fish’s genome so they no longer grow as large or as quickly. Slower-growing fish — which stay smaller longer, and so avoid trawlers’ nets — have an advantage in the face of intense fishing pressure."
Jim Robbins reports for Yale Environment 360 August 4, 2025.
Advertisement 
