Shrinking Under Pressure

Cod today can fit neatly on a dinner plate but these fish used to be giants, measuring more than three feet in length and weighing up to almost 90 pounds. 

A new study found that decades of intense fishing, combined with environmental change, not only shrank the number of cod in the sea but also played a role in how the fish shrank in size, altering their evolution. 

“Selective overexploitation has altered the genome of Eastern Baltic cod,” said lead study author Kwi Young Han in a statement. “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.” 

Together with herring, cod was the backbone of the Baltic fishery. But fishing for cod was banned in 2019 after the cod population began decreasing dramatically.  

Even so, cod are not reverting to their original size. 

One reason is that when fishermen fished with large nets, the smaller cod escaped, which presented an external pressure to remain smaller, researchers said.  

It is difficult to connect the population’s decrease in size only to evolution and not consider other factors, like pollution or temperature change, they added. For example, warming ocean temperatures might play a role in the shrinking process, but researchers think that the phenomenon is too extensive to be caused only by temperature changes, explained the Smithsonian Magazine. 

To carry out the study and find why cod shrank, researchers analyzed 152 fish otoliths – tiny, bone-like structures in the inner ear – caught in the eastern Baltic cod harvested between 1996 and 2019.  

Otoliths serve as records of a fish’s annual growth, acting as biological timekeepers. 

Then, researchers sequenced the DNA of each fish and identified specific genetic markers related to body growth that showed patterns of directional selection, meaning that their frequency systematically increased or decreased over time. These markers overlap with genes that influence growth and reproductive functions. 

The study also showed that a known change in the cod’s chromosomes, a structural change in the genome that often helps animals adapt to the environment, was affected by directional selection.  

Findings prove that the shrinking of cod has a genetic basis and that human actions, constituting an external pressure, have measurably shaped their DNA, predisposing cod to remain small as genetic variants related to larger body sizes became less common. 

This evolutionary change carries significant consequences, as genetic variants related to faster growth and later maturation might already be lost as cod reach adulthood at smaller sizes and produce fewer offspring. 

A loss of genetic diversity could curb the species’ ability to cope with future environmental changes. 

“What we are observing is evolution in action, driven by human activity,” study author Thorsten Reusch told the Guardian. “This is scientifically fascinating, but ecologically deeply concerning.”