Endangered Sea Otters Have a New Problem: Overpopulation

There are so many of the marine mammals in some parts of California that the habitat simply can’t hold any more—but other otters face a panoply of threats.
(Photo: Mark Boster/'Los Angeles Times' via Getty Images)
Dec 30, 2015· 2 MIN READ
John R. Platt covers the environment, wildlife, and technology and for TakePart, Scientific American, Audubon, and other publications.

A year ago, conservationists warned that the recovery of California’s iconic sea otters had stalled. The number of sea otters in the region grew by just five animals in 2014, leading some to worry that these incredibly cute marine mammals were in peril.

Well, not so fast.

California’s sea otters are, as many have pointed out, a conservation success story. They were once thought to be extinct in California, but about 50 sea otters were rediscovered in 1938. Since then, intense conservation efforts have allowed the population to grow to about 3,000.

Yes, the population growth rate has slowed in recent years, but now we know why, and it’s not because of bad news. According to research presented this month at the Society for Marine Mammalogy’s biennial conference, sea otters are doing so well that they have reached the “carrying capacity” of much of their habitat. That means there are as many otters as the environment can feed. In the central part of their range, they simply can’t grow their population any further.

Noted sea otter researcher Tim Tinker of the United States Geological Survey called this “a major OS upgrade to sea otter science” and a change from the previous perception that sea otters were a species “on the edge,” with many threats affecting their survival.

Tinker presented the synthesis of 15 years’ worth of sea otter studies by more than three dozen researchers. Collecting the data was easy, he said. Sea otters stay within a mile of the shore and can be observed with a simple telescope. This allowed researchers to study their behavior, body conditions, reproductive success, and other factors. Even their diet could be easily recorded, because otters bring all of their food to the surface and eat while floating on their backs.

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The new research puts to bed previous suspicions that limited genetic diversity—a side effect of their close call with extinction—or other external factors may have limited the success of sea otters’ recovery. Tinker called these “flawed assumptions.”

However, the success story does have its limits. Tinker pointed out that sea otters have only reached carrying capacity within the central portion of their range, around Monterey, California. The otters living on the northern and southern edges of the range still face threats.

Most notably, Tinker reported a tenfold increase in the number of otters being killed by sharks. The sharks don’t eat the otters, but for some unknown reason they have started biting and killing them. As a result of these predator attacks, Tinker said, sea otter populations in the northern and southern edges of their ranges are now declining by 1 percent to 2 percent per year.

Additional threats could emerge. Another presentation, by Max Tarjan, a doctoral candidate at the University of California, Santa Cruz, reported that many male otters don’t have access to breeding females. Male otters disperse into new territories when they mature, while female otters remain close to their birthplaces for their entire lives. As a result, Tarjan reported, 80 percent of male otters never mate or sire pups, something that could affect population dynamics in the future.

Meanwhile, a presentation by Monterey Bay Aquarium senior research biologist Teri Nicholson reported that the current El Niño cycle could result in an increased number of sea otters suffering from neurological conditions caused by marine parasites.

For now, however, sea otters are doing about as well as they can—and that’s a conservation story worth celebrating.

Reporting for this story was made possible by a fellowship from COMPASS, funded by the Packard Foundation.