Sea otters on the rise, but shark bites stall range growth

Can sea otters survive the recovery of great white sharks?

 

One hundred decoy sea otters bobbed in the Pacific Ocean, off the California coast and just south of Half Moon Bay. U.S. Geological Survey research wildlife biologist Tim Tinker and his sea otter research team were practicing aerial counts of the marine mammal as part of a study published in 2014. At the end of this particular count, they came up one short. Tinker found it washed up on some rocks a distance away, with a huge, half-moon bite mark. Somewhere nearby in the Pacific Ocean, a shark had tried it out for a snack, finding plywood and foam instead of blubber.

In a spring survey of threatened California sea otters published in September, Tinker and his team counted a three-year average of 3,272 sea otters, for the first time above the minimum three-year average of 3,000 required by the U.S. Fish and Wildlife Service to consider them healthy enough to delist. But currently, the sea otters are experiencing large-scale ripple effects from a marine ecosystem in flux. They’re getting a boost from an unexpected sea urchin boom, but are also suffering from a huge jump in mortality from great white shark bites, who are also growing in number due to their pinniped food source expanding.

15085420569_2482a0dc3b_o-jpg
Sea otters form "rafts" when resting, shown here.
Lilian Carswell/USFWS

“What we seem to have here is an ecosystem of populations that are recovering from different abuses, and are coming back at different rates, ” says Lilian Carswell, U.S. Fish and Wildlife’s southern sea otter recovery and marine conservation coordinator. 

Sea otters were hunted almost to extinction for their fur in the 1800s, and gained Endangered Species Act protection in 1977. As an apex predator and keystone species, they help structure and enrich the marine environment around them. Their eating habits are a big part of that: Since sea otters aren’t insulated by blubber, they compensate by having a very fast metabolism, fueled by huge quantities of sea urchins, clams and abalone. The otters control urchin numbers, Carswell says, keeping them from forming what she describes as “roving hordes” on the seafloor, where they attack the holdfasts of kelp, causing the plants to detach and disintegrate. Kelp forests provide breeding and feeding grounds for many marine species and are important sequesters of carbon. Sea otters use the long blades as anchors when napping and to wrap their pups up to keep them from floating off while the adults forage. 

Along the coast, California sea otters have reestablished a small part of their historic range that stretched from Baja California to the Pacific Northwest. Lately, a strange trickle effect has boosted their population. Tinker and others hypothesize that sea star wasting disease—proliferated by warming waters—has wiped out huge numbers of sunflower sea stars, a main predator of sea urchins. Tinker describes communities of the unrestrained urchin populations, called urchin barrens, as a purple shag carpet covering the ocean floor. This has given sea otters an extra food source in their central range, one that had previously been at carrying capacity. The unusual bump in sea urchins has been good for this year’s sea otter juveniles, but it’s not expected to last. The expansion of the sea otter population is key for their long-term viability, but it has been hampered by a curious roadblock: Mortality from shark bites at both ends of their range along the coast.

Due to a 1994 California ban on gillnets that killed shark as by-catch and an increase in sharks’ food sources—sea lions and elephant seals—great white sharks are thriving, too. Researchers think that juvenile sharks are conducting “investigation bites,” like the one given to Tinker’s decoy sea otter. The sharks don’t actually eat the sea otters; they give them one bite, and when they don’t find blubber, they swim off. The injured otters usually die, though, and wash back up on shore. “That’s significant because range expansion is part of their recovery and that’s been curtailed by the shark gauntlet,” Tinker says.

At this point, Carswell, Tinker and other researchers are waiting to see what will happen—whether diseases might affect the sea otters or urchins, and if the shark-bite deaths will persist. Tinker remains optimistic that the sea otters will continue their upward trajectory since large-scale killings caused by the fur trade and the use of gill and trammel nets have now been abolished. Because the habitat and food sources are still there, Tinker says, it’s not a matter of if, but rather when and how the otters return.

Anna V. Smith is an editorial intern at High Country News. She tweets