Overfishing has caused cod to halve in body size since 1990s, study finds | Fish
Overfishing has led to a collapse in the population of the cod of the eastern Baltic, but in the last three decades, the size of the fish themselves has also decreased spectacular and mysteriously.
Now, scientists have revealed genomic evidence that intensive fishing has led to rapid evolutionary changes that have contributed to these fish about half in the length of the average body since the 1990s.
The “narrowing” of the DCO, with a mid -mature body length of 40 cm in 1996 to 20 cm in 2019, has a genetic base and human activities left a deep mark on the DNA of the population, concluded the study.
“When the most important individuals are always withdrawn from the population for many years, smaller and faster fish have an evolutionary advantage,” said Professor Thorsten Reusch, head of the Marine Ecology Division of the Geomar Helmholtz Center for Ocean Research Kiel and the main research author.
“What we observe is the evolution in action, motivated by human activity. It is scientifically fascinating, but ecologically deeply worrying. ””
The dramatic narrowing of the COD is a source of concern for several decades, but it was not clear to what extent the phenomenon was motivated by environmental factors such as the hypoxic conditions caused by algae proliferations, pollution and more extremely seasonal marine temperature changes.
“It was very difficult to prove that it was an evolution that had happened,” said Dr. Kwi Young Han, the first author of the study, who finished his doctorate at Geomar.
The study used an archive of tiny ear bones, called Otoliths, of 152 COD, captured in the Bornholm basin between 1996 and 2019. Otoliths – a bit like tree rings – recorded annual growth, which in fact of precious biological timers.
Scientists have combined annual growth data with metrics and CODS body size genetics to assess if there had been a genetic change in the population over 25 years under fishing pressure.
Between 1996 and 2019, the median length of a mature cod in the data set increased from 40 cm to 20 cm. The median weight in 2019 (272 grams) was only a fifth of the median weight of a mature cod captured in 1996 (1,356 grams).
The analysis has revealed systematic differences between rapid and slow growth fish and that the variants of genes that make large body size more probably became less common over time, indicating evolutionary pressure.
The trawling is intended to be selective in size, with minimum bonding sizes legally designed to protect the smallest individuals and allow fish to reach maturity and make before being captured.
However, this may have had the involuntary consequence of producing strong selective evolutionary pressure in favor of small fish, which would be more likely to escape the nets.
“The demographic argument is that each individual should at least reproduce once before being captured,” said Reusch. “Although it seems logical in terms of maintaining a healthy demography of fish stocks, it has the potential to completely spoil the genetic and size structure.”
The results, published in the journal Science Advances, could help to explain why there has been no rebound in body size since the collapse of the stock caused a prohibition of complete fishing of the cod of the Eastern Baltic in 2019, which remains in place.
Professor Stefano Mariani, marine biologist at John Moores University of Liverpool, who was not involved in research, said that genetic analysis could not explain the whole extent of the narrowing that has been observed, with environmental factors probably also playing an important role.
But he said that showing that “humans’ activities can accelerate evolution” was a “striking” result which highlights the importance of monitoring the basin of fish populations, as well as simply following the number of fish.
“It would be really good to try to maintain diversity because as soon as you cut a certain section of diversity, it is like losing insurance for the future where it could have an advantage,” he said.
