Overfishing has led to a collapse in the eastern Baltic cod population, but over the past three decades the size of the fish themselves has also been [1]dramatically and mysteriously shrinking . From a report:

> Now scientists have uncovered genomic evidence that intensive fishing has driven rapid evolutionary changes that have contributed to these fish roughly halving in average body length since the 1990s. The "shrinking" of cod, from a median mature body length of 40cm in 1996 to 20cm in 2019, has a genetic basis and human activities have left a profound mark on the population's DNA, the study concluded.

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> [...] The dramatic shrinking of cod has been a source of concern for several decades, but it was not clear to what extent the phenomenon has been driven by environmental factors such as hypoxic conditions caused by algal blooms, pollution and more extreme marine seasonal temperature changes. [...] The study used an archive of tiny ear bones, called otoliths, of 152 cod, caught in the Bornholm Basin between 1996 and 2019. Otoliths -- a bit like tree rings -- record annual growth, making them valuable biological timekeepers.



[1] https://www.theguardian.com/environment/2025/jun/25/cod-shrinking-size-overfishing-study