Massive collapse of Atlantic cod didn't leave evolutionary scars - Science Magazine

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.

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.

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.

That suggests evolution probably hasn’t removed the genetic diversity the cod need to grow bigger and mature later.

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.

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.