A recent study has revealed alarming declines in the body size of cod in the eastern Baltic Sea, with their average length plummeting from 40 cm in 1996 to just 20 cm by 2019.

Thorsten Reusch, a marine ecologist, pointed out that the selective removal of larger fish has inadvertently favored smaller, faster-maturing individuals, leading to significant evolutionary changes in the cod population.

The research indicates that while trawling aims to protect smaller fish, it actually disrupts the genetic and size structure of the cod population by favoring their survival.

This study underscores the necessity for fisheries management to account for long-term generational impacts on fish populations and genetic diversity, highlighting the importance of sustainable practices for biodiversity conservation.

Ecologically, the shift towards smaller fish is troubling, as these fish mature earlier but produce fewer offspring, raising concerns about the overall decline in population numbers.

Despite a complete fishing ban enacted in 2019, current data shows no signs of recovery in the size distribution or overall population of eastern Baltic cod, which is a distinct population that diverged from others 7,000 to 8,000 years ago.

The findings published in the journal Science Advances may help explain the lack of recovery in cod size since the ban, suggesting that both genetic changes and environmental factors are at play.

Marine biologist Prof. Stefano Mariani emphasized the significance of genetic analysis while also noting that environmental factors likely contribute to the shrinking cod size, necessitating ongoing monitoring of genetic diversity.

The results indicate that the evolutionary changes favoring smaller, faster-maturing cod could hinder future recovery efforts, complicating the path to restoring the population.

Researchers utilized otoliths and high-resolution DNA sequencing to analyze the age and growth patterns of cod, revealing that human activities have induced rapid evolutionary changes contributing to their reduced size.

The decline of cod has also affected its role as a top predator in the Baltic Sea food web, as it can no longer fulfill this ecological role, particularly with other fish species like herring and sprat also facing overfishing.

To effectively protect the cod population, a significant reduction in fishing pressure is essential, adhering to sustainable usage principles, although practical implementation has been lacking.