Ocean Fishing May Spread 'Runt' Genes

Atlantic silversides are about the size of a French fry and are built for speed.

Atlantic silversides are about the size of a French fry and are built for speed. Through fishing, David Conover's team was able to create a population of genetic runts in just four years. Conover Fish Ecology Laboratory hide caption

itoggle caption Conover Fish Ecology Laboratory

Unnatural Selection: Full

Speed Ahead

  

David Conover's Atlantic silversides aren't the only animals that can experience "rapid evolution" due to natural factors or human actions. Here are some other examples:

  

Guppies

  

In a river on the Caribbean island of Trinidad, researchers have caused small fish called guppies to mature earlier and grow bigger by moving them from one part of a stream to another. The difference? One stretch contained predators that liked to eat guppies, the other didn't. After just 11 years, the guppies in the waters without predators had evolved to mature earlier and weigh 10 percent more than the fish in the area with predators. Researchers reported the rapid burst of guppy evolution in the journal Science in 1997.

  

Bighorn Sheep

  

Hunters seeking big trophies appear to be downsizing wild sheep horns.

Bighorn sheep, which once roamed the Rocky Mountains from southern Canada to Colorado, are now found only in small areas protected by inaccessible habitat or refuges. Males are known for their head-to-head combat, which involves smashing together their curly horns. The sheep with the biggest horns often wins.

  

But sheep with big horns are also sought by hunters. And in one study, researchers found that the horns in one Canadian population of sheep became 25 percent smaller in just 31 years. That's because hunters killed the largest rams before they reached their breeding peak, giving an advantage to males with smaller horns. Researchers have seen a similar trend among elephants in Africa, where poachers often select animals with the largest tusks.

  

Antibiotic Resistance

  

Perhaps the best-known example of rapid evolution is antibiotic resistance.

  

Since the widespread introduction of antibiotics about 50 years ago, scores of microbes have evolved resistance to the chemicals. One reason is that bacteria and other microbes are constantly shuffling their genes. So eventually, a genetic combination arises that allows the microbe to survive an antibiotic blast. That resistant strain can then spread, creating a big challenge for doctors.

  

For instance, recent years have seen the spread of antibiotic resistant strains of tuberculosis and several other deadly bacteria. — Virginia Hughes and Meagan White

A couple of times a year, biologist David Conover leaves his office at Stony Brook University and goes fishing — in his own private ocean.

Conover's "ocean" is actually a dozen huge barrels of salt water. They sit on a concrete floor, surrounded by pumps and pipes, in the university's marine lab on Long Island in New York.

Swimming around in the tubs are little fish called Atlantic silversides. Each fish is about the size of a French fry.

"They are very torpedo-shaped," Conover says, standing next to a tank during a recent visit to the lab. "They are a schooling fish that are built for speed, not comfort."

Mystery: Protected Fish Aren't Rebounding

Conover doesn't fish for silversides because he likes to eat them. Rather, the fish are part of an experiment aimed at solving a mystery out in the real ocean, one that could have big implications for anyone who likes to eat fish.

The mystery involves some popular table fish, like the northern cod off Canada. Northern cod were once so in demand that they were badly overfished. Populations plummeted.

So, about a decade ago, the Canadian government basically banned fishing for northern cod. The idea was to give the fish some time to have babies and get their numbers back up.

Conover says the trouble is that the cod — and some other overfished stocks — haven't bounced back.

"That's not supposed to happen, according to theory," Conover says. The remaining fish "are supposed to find plenty of food, and be able to grow fast and have lots of offspring."

The fishing industry depends "on that rebound happening," he adds. But "in many stocks its not happening. So something else is going on."

Overfishing and Evolution

About a decade ago, Conover wondered if that "something else" could be evolution.

That may sound strange, but here's how biologists think about it.

Start with the idea that modern fishing fleets are really good at what they do. Decade after decade, they fill up their nets with the biggest, most valuable fish. That means those fish don't get much opportunity to reproduce, and — in a relatively short time — there are fewer and fewer fish left that carry the genes for "bigness."

So what remains in the ocean? Well, a bunch of genetic "runts." Little, scrawny fish that have gained an evolutionary advantage because they can slip through the nets and survive to have babies.

The problem, biologists say, is that runty fish often don't have very many babies. And the babies that do appear are more likely to pass on the genes for "runtiness." So the new fish are less likely to carry the genes for things that fishermen want, like fast growth and size.

To see if catching all the big ones really could really make that happen — force evolution to favor little fish — Conover built his ocean eight years ago, and then started fishing.

Armed with long-handled nets, his team of researchers plays the role of the fishing boats. They scoop up silversides and then carefully measure each one, shouting out the length to a record keeper as they work.

Then, they throw back some of the fish. These are the lucky ones that get to pass along their genes.

Conover has now fished his way through eight generations of silversides.

Revenge of the Runts

He's found that, as predicted, if you take the big fish out of a school and leave the little ones, you'll end up with a population of runts. Moreover, it can happen very fast — in just four years, or four generations.

Conover says those results suggest that fish out in the ocean can undergo the same kind of rapid evolution seen all the time on land.

"We know that pests evolve rapidly in response to pesticides," he says. "We know that diseases can quickly evolve responses to the antibiotics that we develop. All we're saying with fisheries is that the same process happens here. So we are undermining the capacity of the population to rebound from fishing."

Conover says that's a message that should worry officials responsible for managing fisheries.

"Most fisheries scientists had not accepted the premise that evolution was something that happened fast enough that they had to worry about it," he says. The conventional wisdom, he says, was that "evolution happened slow, it takes thousands of years, so we can effectively ignore it."

But not anymore, says Steven Berkeley, a biologist at the University of California, Santa Cruz. The danger, he says, is that fishing is taking all the "good" genes — the ones we want — from heavily fished stocks.

Preserving Good Genes

"If a farmer did this, he'd be out of business pretty quickly," Berkeley says. "He would be selecting for the poorest egg-laying chickens, or slowest growing cattle. So it is exactly the opposite of what we'd be doing if we were farming fish."

Berkeley says one way to save the good genes is to set up more marine reserves, where fish are protected from nets. And he says that if there is some good news from Conover's experiment, it's that it is still not clear if fishing can cause permanent evolutionary change.

If given more time, Berkeley says overfished species may still bounce back.

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