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

[highplainsdem]

Source: Guardian

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 dramatically and mysteriously shrinking.

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.

“When the largest individuals are consistently removed from the population over many years, smaller, faster-maturing fish gain an evolutionary advantage,” said Prof Thorsten Reusch, head of the marine ecology research division at Geomar Helmholtz Centre for Ocean Research Kiel and senior author of the research.

-snip-

Read more: https://www.theguardian.com/environment/2025/jun/25/cod-shrinking-size-overfishing-study

[Karasu]

1. ...And conservatives think we can still keep doing the same old unsustainable shit until the end of fucking time.

[multigraincracker]

2. The current human population of Earth is 3 times what it can support.

[Melon]

4. How can that statement be true if the global population is currently supported?

[mike_c]

3. there's not much new about this report other than "cod"

It's been understood for years that harvesting the biggest fish from a population over time drives selection favoring small size. This understanding has never really trickled down to fisheries managers, who see smaller fish as needing to be thrown back to grow some more. If only the small fish survive to reproduce, stocks of big fish decline as small breeding individuals produce more small fish just like themselves.

Here's a citation and abstract from 23 years ago that I assigned in my upper division ecology course: Conover and Munch. 2002. Sustaining fisheries yields over evolutionary time scales. Science, 297: 94-96.

"Fishery management plans ignore the potential for evolutionary change in harvestable biomass. We subjected populations of an exploited fish (Menidia menidia) to large, small, or random size-selective harvest of adults over four generations. Harvested biomass evolved rapidly in directions counter to the size-dependent force of fishing mortality. Large-harvested populations initially produced the highest catch but quickly evolved a lower yield than controls. Small-harvested populations did the reverse. These shifts were caused by selection of genotypes with slower or faster rates of growth. Management tools that preserve natural genetic variation are necessary for long-term sustainable yield."