Recent research reveals that overfishing and environmental changes have caused significant genetic alterations in the Baltic cod (*Gadus morhua*), impacting its size and reproductive capabilities. Understanding these genetic shifts is crucial for conservation efforts and highlights the profound influence human activities have on marine ecosystems.
A study published in *Science Advances* demonstrates, for the first time, that a marine species has undergone detectable genomic changes due to exploitation. Dr. Kwi Young Han, the lead author, stated, "Selective overfishing has altered the genome of the eastern Baltic cod. The decrease in average size correlates with a reduction in growth rate. This is the first evidence showing evolutionary changes in the genome of a marine population under extreme exploitation pressure."
Researchers identified genetic variants associated with body growth that underwent directional selection, becoming more or less frequent over time. These genetic regions overlap with genes involved in growth and reproduction. A chromosomal inversion, a structural change associated with environmental adaptation, was also observed, following the same selection pattern.
To reach these conclusions, the team analyzed the otoliths (ear stones) of 152 cod caught between 1996 and 2019 in the Bornholm Basin. These structures, similar to tree rings, record the annual growth of the fish. Using chemical analyses and high-resolution DNA sequencing, researchers tracked how the cod's growth and genetic composition changed over 25 years.
The results show that individuals with rapid growth are nearly gone, while those that reach maturity earlier and at smaller sizes have had an evolutionary advantage under fishing pressure. Prof. Dr. Thorsten Reusch, the research team leader, explained, "When the largest individuals are constantly removed, natural selection favors smaller fish that reproduce faster. We are witnessing an evolutionary process accelerated by human intervention."
This has likely led to the loss of variants associated with faster growth and late maturation. The surviving cod matures earlier, is smaller, and has reduced reproductive capacity. The loss of genetic diversity also means a reduced ability to adapt to future environmental changes.
Data collected in 2025 during the ALKOR expedition shows that, despite fishing bans, there are no clear signs of recovery in the cod's body size. This underscores the need for additional measures to conserve this species and protect the Baltic Sea's marine ecosystem. This research underscores the importance of sustainable fishing practices and the need to protect marine biodiversity.