The fish that is getting smaller to slip through nets
The average size of eastern Baltic cod is shrinking as a result of overfishing, scientists have claimed.
The fish are thought to have evolved to slip through nets, according to a study which directly links human influence to changes in their DNA.
The research, published in the Science Advances journal, also blamed decades of overfishing for the dwindling numbers of the species.
'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, senior author of the study and head of the marine ecology research division at Geomar Helmholtz Centre for Ocean Research Kiel.
'What we are observing is evolution in action, driven by human activity. This is scientifically fascinating, but ecologically deeply concerning.'
Researchers examined tiny ear bones, called otoliths, of 152 cod caught in the Baltic Sea's Bornholm Basin between 1996 and 2019.
The otoliths record annual growth, similar to tree rings, and showed the genomes of fast-growing cod had nearly disappeared, while slower-growing, smaller fish had a higher survival rate.
Overall, the average body length of the fish had roughly halved from 40cm to around 20cm since the 1990s.
Dr Kwi Young Han, a biologist and first author of the study, said: 'For the first time in a fully marine species, we have provided evidence of evolutionary changes in the genomes of a fish population subjected to intense exploitation, which has pushed the population to the brink of collapse.'
The EU banned the fishing of eastern Baltic cod in 2019 following years of period of overfishing. However, scientists warned its population may never fully recover.
Prof Reusch said: 'Evolutionary change unfolds over many generations. Recovery takes far longer than decline, and it may not even be possible… despite the fishing ban, there's no sign of a rebound in body size.'
Dr Han added: 'Our results demonstrate the profound impact of human activities on wild populations, even at the level of their DNA.
'They also highlight that sustainable fisheries are not only an economic issue, but also a matter of conserving biodiversity, including genetic resources.'
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