Massive collapse of Atlantic cod didn’t leave evolutionary scars
Atlantic cod was once one of the most important fish species in the world. The big, long-lived predator helped feed Europe and North America for centuries. But decades of overfishing in the mid–20th century caused populations to crash, wiping out 30,000 jobs in Canada alone and financially devastating many coastal communities.
Now, a new study provides a ray of hope for the remaining fish. Researchers have found the cod haven’t lost the genetic diversity that would be crucial for their recovery, something that often happens when species hit a so-called population “bottleneck.” The new research—the first to compare whole genomes of cod from before and after intensive fishing—reinforces the notion that more protective management will help beleaguered stocks bounce back, experts say.
“It’s an excellent study, the first of its kind,” says David Conover, a fish ecologist at the University of Oregon. The population in Canada, which has fallen from 4 billion in the 1980s to 1 billion today, for example, “could recover to what it was before, with many large fish,” adds Boris Worm, a marine ecologist at Dalhousie University who was not involved in the research.
Overfishing didn’t just shrink cod numbers—it also shrank their size. The animals, which once matured at 80 centimeters in length, began to reproduce at younger ages as fishing intensified. Now, they mature at about 65 centimeters. One reason size matters is that larger, older fish spawn prolifically and make populations more productive and resilient.
Biologists feared the exhausted populations had permanently evolved these new traits, which would complicate restoring population size. The case of cod “gets cited left and right as the classic example of really rapid, modern evolution,” says Malin Pinsky, an ecologist and evolutionary biologist at Rutgers University, New Brunswick. Lab experiments support that idea, but studies of natural populations—which have looked at just a few genetic markers—are inconclusive.
So in the new study, Pinsky and colleagues at the University of Oslo examined the entire genomes of Atlantic cod. They analyzed DNA from the scales and ear bones of fish from Canada and Norway, taken decades before overfishing started. The team then compared DNA from those samples with that of 46 modern fish from Norway and 24 from Canada.
Pinsky and colleagues found no sign of sweeping changes in the cod genomes, they report this week in the Proceedings of the National Academy of Sciences. That suggests evolution probably hasn’t removed the genetic diversity the cod need to grow bigger and mature later. “That’s really exciting,” Pinsky says, because it implies that less fishing can help cod bulk up to their former size.
Cod populations in the United States remain overfished. In Canada, they are showing early signs of recovery. But in the northeast Arctic, where fishing was curtailed and the size of the fleet reduced, cod are starting to thrive again. Over the past decade, they have even begun to mature at a later age. That’s “a really interesting and hopeful sign” that’s consistent with the new findings, Pinsky says.
So if the cod haven’t lost genetic diversity from overfishing, what stunted their reproduction and growth? One possibility, Pinsky says, has to do with ecology. When large males have been removed from a population, younger fish can mature sooner and at a smaller size, because they don’t have to compete with big bruisers.
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 selected for many small genetic changes that the study was unable to identify. In any case, Pinsky says the study proves one thing: Cod doesn’t have to remain the poster child of population collapse from overfishing.