Baltic cod is getting smaller. Evolution before our eyes

For many people, fresh fish on the plate has become an inseparable part of summer holidays by the Baltic Sea. Unfortunately, for yet another year in a row, that fish won’t be Baltic cod. Since 2020, a fishing ban on this tasty species has been in place. The problem isn’t just the dwindling number of fish but also the ever-decreasing size of individual specimens. What’s behind these changes?

Natural process or the result of human activity?

About 30 years ago, some Baltic cod specimens weighed up to 40 kilograms. Today, an average-sized fish easily fits on a dinner plate. These changes are an example of natural selection – the evolutionary favouring of individuals with specific traits while others are eliminated.

However, calling it “natural” can be misleading. The fact that Baltic cod are shrinking in size is, unfortunately, a direct consequence of human activity. Genetic changes in the fish are linked to the intense exploitation of the species. Cod are simply adapting to their environmental conditions. This would be unremarkable, if not for the anthropogenic causes and the astonishing pace of change – not hundreds or thousands, but merely a few dozen years.

For the first time, we’ve proven in a fully marine species that evolutionary changes in the genome caused by intensive exploitation have pushed the population to the brink of collapse, says Dr. Kwi Young Han, a biologist at the GEOMAR Helmholtz Centre for Ocean Research in Kiel and the lead author of the study.

Otoliths and a genetic journey through time

Researchers working on the GEOMAR’s Baltic Sea Integrative Long-Term Data Series project used otoliths to examine changes in the genotype of Baltic cod. These are small calcium carbonate structures from the inner ear of the fish. The study analysed otoliths from 152 cod caught in the Bornholm Basin between 1996 and 2019.

After conducting chemical analyses and DNA sequencing, scientists discovered that cod which grow more slowly now have the upper hand. Those that grew faster have nearly disappeared. Fish reaching maturity at a smaller size also performed better in regions of intensive fishing.

Smaller size – greater risk

A smaller Baltic cod may escape fishing nets more easily, but evolutionary changes bring other risks. Fish of this kind are more vulnerable to climate change and have a harder time defending themselves from predators. They also produce fewer offspring, which could have a particularly negative effect on the population’s size in the long term.

A cod that fits on a dinner plate is no reason to celebrate. As Professor Thorsten Reusch, head of the Marine Ecology Research Division at GEOMAR, said: we are witnessing evolution happen before our very eyes, driven by human actions. Fascinating from a scientific perspective – yet deeply concerning from an ecological one.

Will the Baltic cod fishing ban help?

Since 2020, a summer fishing ban on cod has been in force in the Baltic Sea. Initially, the restrictions introduced by the European Union were meant to be temporary, but it turned out they did not lead to the species’ recovery.

The population of Baltic cod is still considered critically low, not only due to overfishing but also because of other factors: a high number of predators feeding on cod, and climate change negatively impacting food availability.

Researchers emphasize that genetic changes in cod are serious and could be permanent. Even if the population is rebuilt to its original size, that doesn’t necessarily mean the genetic damage can be undone. It’s possible that genetic variants linked to faster growth and later maturity may be lost forever.