Atlantic cod in 1952, before overfishing eroded Canadian population
Peter Stackpole / The LIFE Picture Collection via Getty Images
By Erik Stokstad
Atlantic cod was once one of the most important fish species in the world. The large, long-lived predator has helped feed Europe and North America for centuries. But decades of overfishing in the mid-20th century caused populations to collapse, wiping out 30,000 jobs in Canada alone, and financially devastating many coastal communities.
Now a new study offers a ray of hope for the remaining fish. Researchers have found that cod have not lost the genetic diversity that would be crucial to their recovery, something that often happens when species hit a so-called ‘bottleneck’. The new research – the first to compare whole genomes of cod from before and after intensive fishing – reinforces the idea that more protective management will help bounce back beleaguered stocks, experts say.
“This is an excellent study, the first of its kind,” said David Conover, a vice president at the University of Oregon. Boris Worm, a marine ecologist at Dalhousie University, for example, adds that the population of Canada, which fell from 4 billion in the 1980s to 1 billion today, “was able to recover to what it was before.”
Overfishing has not only shrunk cod numbers, but also its size. The animals, which had previously grown to 80 centimeters in length, began to reproduce at younger ages as fishing increased. Now they are about 65 inches tall. One of the reasons why size is important is that larger, older fish make much spawn and populations more productive and resilient.
Biologists feared that the depleted population would permanently develop these new traits, which would complicate the restoration of the population size. The cod case “is cited left and right as the classic example of rapid, modern evolution,” says Malin Pinsky, an ecologist and evolutionary biologist at Rutgers University, New Brunswick. Laboratory experiments support this idea, but studies of natural populations – which looked at only a few genetic markers – are unconvincing.
In the new study, Pinsky and colleagues at the University of Oslo therefore examined the entire genome of Atlantic cod. They analyzed DNA from the scales and ear bones of fish from Canada and Norway, which were taken decades before overfishing began. The team compares DNA from the samples with those of 46 modern fish from Norway and 24 from Canada.
Pinsky and colleagues found no sign of sweeping changes in the cod genome, they reported this week in the Proceedings of the National Academy of Sciences. This suggests that evolution is unlikely to remove the genetic diversity needed by the cod to grow later. “It’s really exciting,” says Pinsky, because it implies that less fishing can help the cod to their previous size.
Cod population in the United States remains unconvinced. In Canada, they show early signs of recovery. But in the northeastern Arctic region, where fishing is limited and the size of the fleet is declining, cod are starting to thrive again. Over the past decade, they have even begun to mature at a later age. This is a very interesting and hopeful sign ‘consistent with the new findings,’ says Pinsky.
So if the cod did not lose genetic diversity due to overfishing, what hindered its reproduction and growth? One possibility, says Pinsky, has to do with ecology. If large males are removed from a population, younger fish can grow faster and smaller because they do not have to compete with large bruises.
Evolution could still have played a subtle role, argues Nina Therkildsen, an evolutionary geneticist at Cornell University. She and other scientists say overfishing may have opted for very small genetic changes that the study could not identify. Either way, Pinsky says the study proves one thing: cod does not have to remain the poster child of the population dump from overfishing.