Humans reach 'super predator' status, and that's a problem, study says

Humans are throwing ecosystems out of whack by not only killing a large number of animals, but by killing adults and top carnivores in particular, a study suggests. One answer is to act more like animal predators.

If humans hope to fish the oceans more sustainably, they are going to have to start fishing like fish, a new study suggests.

That means harvesting younger, smaller fish to leave more of their elders to continue maturing and reproducing. And it means fishing quotas that are more in line with what nonhuman predators consume.

Humans have assumed "super predator" status, note the researchers conducting the study. Population growth increases demand and technologies have increasingly reduced the personal risk involved in hunting animals on land or in the water. As "super predators," humans also have a penchant for killing adult animals in higher proportions than their nonhuman prey do.

The persistent loss of adults in their reproductive prime can weak havoc on the survival prospects for an animal population, researchers say. And if the adults are top-level predators in an ecosystem, their reduction or loss can shift the nature of the ecosystem.

Past studies have focused on humans' direct or indirect roles in driving species extinct or pushing them to the brink of extinction. This study takes a different tack. It compares patterns in human hunting and fishing with those of nonhuman predators for similar adult prey.

On land, "humans are the only predators who turn large carnivores into prey," says Chris Darimont, an ecologist at the University of Victoria in British Columbia and the lead author of a formal summary of the findings appearing in Friday's issue of the journal Science.

Worldwide, humans kill carnivores at an average annual pace nine times higher than the pace at which carnivores killed each other, the study's team found. Carnivores kill about 2 percent of their populations annually, usually during competition for dominance; humans kill 18 percent of the carnivore population.

By contrast, humans kill the same proportion of large herbivores like deer, elk, or moose that carnivores do – 5 percent for carnivores and 6 percent for humans.

For fishing, humans gather up about 14 percent of available adult-fish biomass each year, with rates for some species vaulting to 80 percent or more. That 14 percent rate is 14 times higher than the rate at which piscine predators feast on adult fish, Dr. Darimont says.

Throttling back human predation to truly sustainable levels on land and at sea will require what Darimont calls major transformations.

Perhaps the most radical changes involve fishing.

Targeting younger fish alone won't do the trick, adds Thomas Reimchen, a biologist with the University of Victoria and the study's senior author. "It's shifting the extraction rate to juveniles at quotas that are representative of what other predators use."

Fisheries-management efforts already are heading in this direction, notes Chris Dorsett, vice president for conservation and policy at the Ocean Conservancy in Washington, in an e-mail.

These efforts increasingly are based on the health of marine ecosystems, which can be restored and sustained through approaches including fishing-quota reductions and marine protected areas.

From a biological perspective, the catch-younger, catch-fewer approach the new study suggests "makes a lot of sense," says Deborah Brosnan, a marine scientist with the University of California at Davis and Virginia Tech.

Larger fish produce more eggs over longer periods of time than do smaller fish, let alone juveniles that haven't yet reached reproductive age. Culling increased number of juveniles would, in theory, leave enough survivors to support healthier fish populations.

But putting theory into practice would be difficult, she suggests.

Two thirds of global fish stocks already are depleted, with some regions, such as the Caribbean, essentially devoid of what once were its largest fish, she notes. That would make it tough to establish a baseline for natural predation rates for a new management scheme.

Moreover, in Asia alone, some 300 million people rely on fish as their primary source of protein. The number rises significantly when the rest of the world is included. With continued population growth, those numbers will increase.

And the biggest fish earn the biggest bucks. Focusing on juveniles would yield less money per ton of fish caught. And quotas likely would have to be reduced dramatically to bring them closer to fish-predation rates from nonhuman predators.

Such an approach would "put us in line with most other predators – not because predators are thoughtful creatures, but because that's what they can handle," she says. "What would be intriguing is to see how we get from where we are now to restoring a system where we could actually act like a normal predator."