No major changes in cod DNA from long-term fishing pressure

“This is an important research milestone – and perhaps a slightly surprising one”, says the marine scientist Olav Kjesbu.

He is a co-author of a new study, together with other researchers at the Institute of Marine Research (IMR), University of Oslo (UiO) and Rutgers University in the United States. The study has been published in the prestigious American scientific journal PNAS.

Cod are reaching sexual maturity more quickly

The age when cod reach “puberty” has fallen since industrial fishing for cod started in the 1950s. In parallel, the cod that are caught have become a bit smaller. This fact has been known since at least 1990, when it was documented by Terje Jørgensen of the IMR.

Since then, the reasons for this have been the object of much research and discussion.

Some researchers have argued that high fishing pressure has caused genetic changes in the population, because fish that reach sexual maturity earlier are more likely to pass on their genes. This is referred to as fishery-induced evolution.

The researchers have now tested this hypothesis by analysing DNA samples of Atlantic cod from both before and after the start of industrial fishing.

“There were no major losses in genetic diversity and no major changes that suggested intensive fishing-induced evolution”, concludes the lead author Malin L. Pinsky on Rutgers’ website.

Old and new samples were analysed

The oldest samples analysed in the study were the scales of cod caught in the Lofoten fishery as long ago as 1907. These were compared with tissue from the otoliths of cod caught in 2011 and 2014.

The DNA analysis was carried out at the University of Oslo’s Centre for Ecological and Evolutionary Synthesis (CEES). The centre also obtained the otoliths of Canadian cod that were caught off Newfoundland in 1940 and 2013. The researchers analysed the whole genomes of 113 individuals – almost 11 billion gene sequences in total.

This led them to conclude that the genome of the Atlantic cod on both sides of the Atlantic Ocean had remained stable over the past century.

Fisheries management and climate affect how cod mature

So what is the alternative explanation for why the cod are reaching puberty earlier than in the past? Most likely it is related to “phenotypic plasticity”. That means the same genes can express themselves differently under different conditions.

To use an analogy: if you eat more, you put on weight, but your genes don’t change.

“Phenotypic plasticity means that the cod are sensitive to changes in fishing pressure and climatic conditions. This may explain the changes we have seen in the population, and more research should be done on this in the future”, says Olav Kjesbu.

The positive thing about this is that if the stock is managed sustainably and has good environmental conditions, the cod may become bigger again. This would be more difficult if earlier sexual maturation had been “coded into” its DNA.

To return to our analogy: by eating less, you can lose weight.

“Indeed, in Norway we have observed a slight rise in the average age at which cod reach sexual maturity in recent years. That is probably due to a combination of good management, combined with a favourable climate for the cod”, says Kjesbu.

In Canada, the cod population collapsed around 1990. It has still not recovered, but it is coming.

Gold dust in our archives is generating new knowledge

More than a century ago, scale samples were used to ascertain the age of fish. Later, scientists went over to using otoliths, and they are still used today.

The IMR has an archive of several million otoliths and fish scales, carefully sorted into paper envelopes and cardboard boxes. The scale samples from 1907 are some of the oldest ones in the archive.

“We found that the DNA in these samples is often amazingly well-preserved, which enables us to perform detailed genetic analyses across time periods”, says Bastiaan Star, a researcher at the University of Oslo, to Titan.uio.no. Star believes the study would not have been possible with the methods available just 20 years ago.

Marine scientist Jane Aanestad Godiksen did the “detective work” of selecting representative samples from the archive, which she then sent to Oslo for analysis. The study is part of a larger genome project at CEES, which also involves the analysis of other material from the IMR archive. An earlier study was published in 2016.

“I think it is fantastic that some people at the start of the last century saw the value of preserving these structures, which means we can study things today that they had no idea about at the time”, says Godiksen.

“Imagine what will they know in another 100 years.”