Sea otters and abalone: A special synergy
Decades of conservation work have brought southern sea otters back from near-extinction. This year, their numbers topped 3,000 for the first time since fur traders decimated the population in the 19th century. But as the animals move into new territory along the California Coast, they’re encountering another food source: endangered black abalone.
Fishing groups have worried that sea otters may keep abalone from rebounding—and dashing their chance of reopening a commercial abalone fishery in California. But last month, a paper published in the scientific journal Ecology found the recovery of sea otters doesn’t prevent the recovery of black abalone. In fact, the two species benefit one another.
The paper, authored by University of California, Santa Cruz Professor Peter Raimondi with researchers from the University of California, Davis and the U.S. Geological Survey, investigated abalone populations in 12 places along the Central Coast—some with otters, some without.
If otters were harming abalone populations, the researchers reasoned, there would be fewer abalone where there were more otters. Instead, the data showed exactly the opposite pattern: Where there are more otters, there are more abalone.
The result may not be intuitive, but that reflects the complexity of ocean ecosystems. Raimondi and his colleagues aren’t quite sure why otters and abalone benefit one another, but they have some ideas.
First, both species thrive in the same habitat: rich kelp forests over rocky bottoms. And second, while abalone are food for sea otters, otters may also provide food for abalone. Sea urchins, a staple of the otter diet, can wipe out kelp forests, leaving very little food for other animals.
By keeping urchin populations in check, otters improve the health of kelp forests, indirectly giving abalone more to eat.
A holistic approach
Lilian Carswell, Southern Sea Otter Recovery Coordinator for the U.S. Fish and Wildlife Service, isn’t surprised by the finding. “People are beginning to recognize that otters have a variety of effects that we hadn’t really appreciated,” she says.
Scientists have long suspected healthy abalone and otter populations might benefit one other. In 2012, David Jessup, former director of the Marine Wildlife Veterinary Care and Research Center in Santa Cruz, pointed out that otters and abalone coevolved as predator and prey over hundreds of thousands of years.
“It would be extremely disadvantageous to sea otter survival if they could drive abalone to rarity or extinction,” he wrote in a 2012 post on seaotters.com. “It would be a remarkable evolutionary failure.”
The population decline that prompted officials to list black and white abalone under the Endangered Species Act in 1997 had nothing to do with sea otters, Jessup added. At that time, there were still too few otters in the wild to affect the abundance of abalone. Instead, a combination of overfishing and a fatal disease called withering syndrome caused abalone numbers to plummet.
There’s been a long-term misunderstanding about abalone population levels, Carswell adds. In the early 20th century, she explains, California settlers found enormous piles of shellfish up and down the coast. They assumed this incredible abundance was the norm, and abalone fisheries sprang up to take advantage. But scientists now know the unusually high numbers of abalone and other shellfish were due to the absence of otters.
“The [abalone] baseline was already very skewed, totally out of whack, because otters had been removed,” Carswell says.
Andrew Johnson, the Aquarium’s Sea Otter Research and Conservation manager, says people enjoyed the abalone harvest for decades in the absence of sea otters. “It was unregulated, they overexploited it, and they got their benefit from it,” he says. “But now things are coming into a better balance.”
A perfect pairing
In the long term, Raimondi’s paper suggests, both abalones and otters can thrive along the California coast.
Relationships between animals are complex, and ecosystem management works best when scientists and managers take multiple species into account. In contrast to the old idea that otters reduce abalone numbers, Raimondi’s study suggests an important synergy between the two species.
As sea otters recolonize their historical range, they improve ecosystem health. Johnson hopes the paper will inspire productive conversations, helping abalones and sea otters recover together.