Overfishing has caused cod to halve in body size since 1990s, study finds

Overfishing has caused cod to halve in body size since 1990s, study finds | Fish | The Guardian

Evolutionary change driven by intensive fishing led cod to ‘shrink’ from average 40cm length in 1996 to 20cm in 2019

theguardian.com

TL;DR

Scientists have discovered that decades of intensive trawling in the eastern Baltic have not only collapsed cod stocks but also driven a real-time evolutionary shift: from 1996 to 2019 the median mature cod shrank from 40 cm to 20 cm. By analyzing otolith “ring” data alongside genetic markers, the team showed that fishing pressure consistently removed the largest, slow-growing individuals, tipping the gene pool toward smaller, faster-maturing fish.

This size-selective netting, meant to spare juveniles, has ironically backfired by favoring scrawnier survivors who escape the mesh—an unintended evolutionary bottleneck that could explain why cod haven’t bounced back despite a 2019 fishing ban. The findings highlight the need to track not just fish numbers but genetic diversity if we want stocks that can adapt and recover.