Overfishing Has Not Caused Major Genetic Changes in Cod Populations

A "first-of-its-kind" study by researchers at Rutgers University and the University of Oslo has found that overfishing is not likely to be causing evolutionary changes and early maturation in the Atlantic cod fish population. The research study is published in the Proceedings of the National Academy of Sciences.¹

There're not always plenty of fish in the sea

Overfishing, in its most simple form, is the fishing of wildlife from a body of water at a rate that is too high for the species to breed and recover, ultimately leading to a depletion or the full collapse of a species. It is recognized as arguably the greatest threat to ocean conservation.

A 2018 report by the Food and Agriculture Organization of the United Nations stated that the percentage of stocks fished at unsustainable levels had increased from 10% in 1974 to 33.1% in 2015.

Many of the fish species that are vulnerable to overfishing include those that are popular for human consumption – such as cod or tuna – or fish that are slow to mature, such as deep-sea creatures.

Can we reverse the effect of overfishing?

Studying the impact that overfishing has on aquatic creatures, including their genotype and phenotype, is incredibly important for conservation efforts. Many populations of fish now mature at an earlier age, an example being the Atlantic cod (Gadus morhua) which is found in various locations in the Northwest Atlantic Ocean. Populations of fish that age earlier are less productive and are more vulnerable to climate change or the risk of collapse.

"Many fish now mature at an earlier age, but it has been controversial whether this was a behavioral and developmental response to their changing environment or whether it was an evolutionary change," ecologist and evolutionary biologist Dr Malin Pinsky explains. 

Distinguishing between these different types of responses, Pinsky explains that developmental responses are easier to reverse. However, if early maturation in certain fish is an evolutionary response ­– i.e., there has been a genomic change – it would be challenging to reverse. This is because it is possible that the genetic variants responsible for late maturation have now disappeared.

In a new study published in the Proceedings of the National Academy of Sciences, Malin and colleagues at Rutgers University and the University of Oslo set out to answer this question by applying whole-genome sequencing methods (WGS) to Atlantic cod samples obtained before and after intensive fishing.

"A clear signal of evolution caused by fishing would be the loss of genetic variants that had been present before intensive fishing began. For example, imagine a jar of M&Ms sitting on the counter. If you notice on the first day that there were lots of green M&Ms in the jar, but all or most of the green M&Ms were gone on the second day, you would have a pretty good indication that someone really liked to eat the green ones," – Pinsky.

Whole-genome sequencing goes wild

The scientists gathered WGS data from archives of Atlantic scales and ear bones samplesd obtained from two populations of cod that were from either side of the Atlantic Ocean; the northeast Arctic population sampled near Lofoten, Norway in 1907, and the Canadian northern cod population sampled near Twillingate, Newfoundland in 1940.

The genomic data was compared with modern data from cod located in the same regions. Collectively, 113 genomes from these two populations were sequenced.

Pinsky explains that this is the first work to use WGS to determine whether evolution in wild population has occurred due to overfishing. When questioned as to why, he emphasizes that WGS is rarely applied to wild populations due to the technical challenges associated and the cost of building high-quality genomes. "These barriers are rapidly dropping, however, and I hope that there will be many similar studies in the future," he notes.

An opportunity for change and ocean conservation

"Hallmarks of fisheries-induced evolution would include a loss of genetic variants at particular places in the genome or strong changes in the frequency of particular genetic variants," says Pinsky. In the study, the researchers found a marked lack of such genomic changes. Whilst it cannot be definitively ruled out, it is unlikely that evolution has led to the phenotypic changes observed in the Atlantic cod, according to the study data.

In which case, what is causing the fish to mature earlier? There are other potential explanations: the cod may have altered their growth, development or behavior to match the environment without the need for genetic evolution. "For example, when big old fish are present in the population, other fish may need to grow bigger and older in order to compete successfully for mates. If fishing removes those big and old individuals, it may become a lot easier for younger and smaller fish to reproduce, without any need for evolution," says Pinsky.

The research calls for action; evolution cannot be used as an excuse as to why populations have not recovered from overfishing – at least not in this population. It highlights an opportunity to make an active change and an active difference in ocean conservation.

"We are now working to understand whether rapid evolution is common across a wide range of species in collaboration with the Smithsonian National Museum of Natural History and doing DNA sequencing from fishes [that are] 100 years old that were collected in the Philippines, plus resequencing modern fishes from the same locations," Pinsky concludes.

References:

1.      Pinsky ML, Eikeset AM, Helmerson C, et al. Genomic stability through time despite decades of exploitation in cod on both sides of the Atlantic. Proc Natl Acad Sci USA. 2021;118(15):e2025453118. doi:10.1073/pnas.2025453118.