The Biggest Marine Animals Risk Extinction

Scientists have been sounding the alarm about a “sixth great extinction” as species disappear at an unprecedented rate.

Much of the research, though, has focused on the disappearance of terrestrial animals because of climate change and the human destruction of habitat.

Now a study published Wednesday in the journal Science examines how marine life will respond. The researchers found that the bigger the animal, the more likely it is to go extinct. The reason? Most likely us, said study coauthor Douglas McCauley, an ecologist at the University of California, Santa Barbara.

“In terms of the fossil records we have, we can look back much farther into time to see what our oceans looked like and how marine animals fared during times of mass extinction,” McCauley said.

So that’s what the researchers did: They compared the average body sizes of more than 2,400 species of marine-dwelling vertebrates and mollusk skeletons gathered over the past 500 years with fossil records that stretch back 445 million years.

What is happening in the world’s oceans is unprecedented.

“In previous extinction periods, we found that there was either no correlation between an animal’s size and the likelihood of it going extinct or we have found a negative correlation—where smaller animals were actually more likely to go extinct,” said study coauthor Noel Heim, a postdoctoral researcher at Stanford University’s School of Earth, Energy, and Environmental Sciences.

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Those previous extinctions usually coincided with catastrophic events, such as an asteroid hitting Earth or extreme volcanic periods that wiped out swaths of life indiscriminately. But the culprits threatening the world’s wildlife this time around—humans—are much more calculated in the species we target, Heim said.

“The best place to see the correlation is on land. Look at wherever humans start showing up across Australia, America, Eurasia, and you’ll start seeing populations of large megafauna start to go extinct,” Heim said. “There’s a pattern to it, and there appears to be a similar linkage to the oceans.”

Examine fishing data, and the examples start popping up. Historically, humans have hunted larger marine animals (such as whales, sea lions, and walruses) and big fish species (such as tuna) for food, oil, clothing, and tradable goods. The tactic led to the extinction of the Steller’s sea cow—an oversize version of the manatee—which was hunted in the 1760s by sailors in the North Pacific.

Heim noted a similar trend with whale sharks in the South Pacific, where illegal hunting of the species continues. In one study, it was found that in under a decade adult whale sharks in the region shrank more than nine feet and the population declined by 40 percent.

“Take a look at Atlantic cod,” Heim said. “When we fish, we take the biggest individuals, and over time as we more intensely fish, the individual cod left are smaller [and] end up maturing younger, and that can actually shrink the average species size.”

Our selective hunting and exploitation of larger marine species could have an outsize impact on the ecosystems as well.

“Inherently, larger ocean species eat more food, migrate farther distances, and overall have a bigger ecological footprint,” McCauley said. “So if we remove the bigger animals like whales, sharks, and fish from the environment, it’s likely to have a bigger impact on the rest of the species.”

In the world’s oceans, human impact has been limited compared with what has occurred on soil. In the past 500 years, researchers have recorded 514 animal extinctions on land. There have been only 15 extinctions of marine animals during that time.

“If we keep going at our current pace and keep our current ocean management practices, we are setting a course to create a sixth mass extinction—and what’s being seen on land will soon follow in the sea,” McCauley said.

There’s hope. Recent expansions of marine-protected areas, such as the quadrupling of the size of Papahānaumokuākea Marine National Monument by the Obama administration last month, can help preserve ocean life.

“We need to start rethinking how we protect our oceans,” McCauley said. “Most of the marine reserves we have are small, golf-course-sized parks that are great for sea stars and sea snails, but for sharks, rays, whales, and migrating fish species, much of the ocean remains a danger zone.”