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California Sea Otters, Adrift 
by Robin Meadows

Sea otters once ranged all along the Pacific Rim, from Baja California, Mexico, to Japan. But beginning in the mid-1700s, these marine mammals were hunted commercially for their soft, dense fur. Otters were easy targets because they rest in large groups called rafts. To stay together, they tangle themselves in kelp or hold paws with their neighbors. Rafts may have hundreds of otters, and one in Alaska was reported to have at least 2,000 otters. By the time sea-otter hunting was finally banned by the 1911 International Fur Seal Treaty, it was almost too late. Except for perhaps a thousand scattered in Alaskan waters, the otters were all gone.

Or so it was thought. In 1938, a small population of the California sea otter (Enhydra lutris nereis), a subspecies that lives off the central coast of California, was rediscovered off Point Sur, 15 miles south of Carmel. But instead of flourishing under legal protection like those in Alaska and another population in the Pacific Northwest, the California otters limped along for decades. Listed as threatened in 1977, the California population has reached only a precarious 2,800 or so, and disease-ridden otter carcasses are washing up on beaches flanking their range. Alarming though this is, a new study spearheaded by Smithsonian’s National Zoo scientists suggests that disease may not be the otters’ only—or even their worst—problem.

The last big scare for the California sea otter was in the mid-1970s, when the rediscovered population reversed its slow but steady increase. The culprit turned out to be gillnets. Otters dive as deep as 100 meters (about 330 feet) for sea urchins, abalones, and other prey that live in kelp forests and on the sea floor, and back then gillnets were set in such shallow waters that they caught otters as well as fish. Once California banned nearshore gillnet fisheries in 1985, the otter population resumed its gradual climb and grew from about 1,300 to about 2,300 over the next decade. But the otters began declining again in the mid-1990s and although they are now slowly rising again, they are still in trouble. This time the reasons for their woes are far more complex, making another easy fix unlikely.  

Pioneering Parasites

Clues as to what’s wrong come from the 3,100 beach-cast sea otter carcasses that have been collected by the state of California since 1968. These beach-casts do not necessarily reflect the population as a whole, in part because just as many carcasses sink or wash out to sea. And more than two-thirds of them are too decomposed to necropsy. Overall, only about 15 percent of California sea otter deaths are explained.

That said, fresh carcasses provide an amazing degree of insight into the causes of death. Roughly half of the beach-cast otter deaths are attributed to infectious or parasitic diseases, a discovery that made a huge media splash because sea otters are not natural hosts for many of these parasites. Up to 18 percent of the necropsied otters have brain infections of Toxoplasma gondii, a parasite whose primary hosts are domestic cats and other felines. Others have brain infections of Sarcocystis neurona, a parasite whose primary host is the Virginia opossum (Didelphis virginiana). These two parasites spread via the feces of their hosts, and together theykilled nearly a quarter of otters autopsied between 1998 and 2001

The idea that land is the source of the California otter’s ills is bolstered by several other observations. For example, some otters have industrial chemicals such as DDT and PCBs in their blood, and a 2003 spike in otter deaths was attributed partly to toxic algal blooms, which can be triggered by fertilizer runoff. This all adds up to the widely accepted “dirty ocean” hypothesis, which posits that otters are dying because they are swimming in a soup of nasty pathogens and toxic compounds pouring from the land into the sea.

Dire as this hypothesis sounds, it is also somewhat comforting. Although the implication is that we are killing sea otters, the flip side is that we can also help them by cleaning up our act. In keeping with this viewpoint, a 2006 bill called AB 2485  requires labeling kitty-litter packages to discourage pet owners from flushing litter that is used, and possibly parasite-laden, down the toilet.

However, some of the new parasites killing sea otters come from the ocean. One of the best examples is thorny-headed worms (Profilicollis spp.). These parasites typically infest mole crabs and shorebirds, and were quite rare in sea otters 40 years ago. But now thorny-headed worms are infesting otters in massive numbers, ultimately perforating their intestines and killing 14 percent of those autopsied between 1997 and 2001.


About 85 percent of California sea otter deaths remain a mystery, leaving plenty of room for non-disease causes. One big unknown is the impact of the “live fish” fishery that has boomed in California since the early 1990s, coinciding with the onset of the latest sea otter decline. This lucrative fishery caters to sushi restaurants, Asian food stores, and other markets that place a premium on freshness. Targeting rockfish and other bottom-dwelling species, the fishery sets box traps on the sea floor where otters hunt for prey. Box traps are essentially cages that let fish swim in but not out, and they are big enough to catch and drown otters.

Otters are curious animals and in aquarium studies, they readily entered these traps. When they die at box-trap depths, their carcasses are likely to stay underwater rather than washing ashore. This is because, unlike whales and other marine mammals insulated by a layer of blubber, sea otters are insulated by a layer of air held by their exceptionally fine fur. When the fur is no longer groomed, the air bubbles out and the carcass loses its buoyancy. Box-trap-drowned carcasses that do eventually wash ashore are likely to be too badly decomposed to necropsy.

What’s in a Diet?

Malnutrition has also been a suspect in the species’ decline, and the latest research confirms that poor diets could be a major cause of otter deaths. “Conservation biologists and resource managers usually overlook this possibility due to lack of familiarity with nutritional sciences,” says National Zoo comparative nutritionist Olav Oftedal, who is lead author of the first sea otter nutrition study. Completed in 2007, the four-year study was sponsored by the Monterey Bay National Marine Sanctuary and the Marine Mammal Commission. Oftedal’s co-authors included National Zoo conservation biologist Katherine Ralls and Tim Tinker, a sea otter expert at the University of California, Santa Cruz.

Sea otters are big eaters, consuming 25 percent of their body weight or more each day. This adds up to a lot of seafood, considering that females weigh about 45 pounds and males weigh about 65 pounds. Not surprisingly, California’s sea otters are infamous for overfishing their favorite prey, especially energy-packed sea urchins.

The idea that California otters are running out of high-quality prey sprang from several lines of evidence. Although the population size is now growing again, the increase masks a troublesome pattern: Most of the current increase is in the southern tip of the range, while otters in the center of the range are declining. Ralls says these central otters are at such high densities that their habitat may no longer be able to support them. In addition, the otters’ range is expanding at both ends, which could mean that they are traveling farther in search of food. Adult females are also increasingly dying in the summer, when they have to eat even more than usual to nurse their pups. Emaciation or starvation contributed to the deaths of a quarter of all necropsied otters.

Oftedal, Ralls, Tinker, and their colleagues compared sea otter nutrition in the Monterey Bay National Marine Sanctuary’s shrinking population to the thriving populations at Glacier Bay, Alaska, and San Nicolas Island, which is off the coast of southern California. “San Nicolas Island otters are as big and fat as otters anywhere where the population is growing fast,” says Tinker. “Monterey has the scrawniest sea otters we’ve found anywhere. You can see their ribs and vertebrae when they’re diving.”

He should know. Tinker previously led a three-year study of Monterey otters that included watching 63 radiotagged adults to see what they ate. This work set the stage for the otter nutrition study with Oftedal and Ralls that was based partly on observations of about 1,600 feeding bouts, which lasted from one to four hours each and totaled about 58,000 otter dives. This extensive dataset revealed that Monterey otters have very different eating habits from the other otters studied.

In each of the thriving populations, all of the otters had very similar diets: mostly sea urchins in southern California’s San Nicolas Island, and mostly clams in Alaska’s Glacier Bay. In contrast, across the declining Monterey population, otters ate a wide variety of prey including crabs, sea urchins, clams, mussels, abalones, turban snails, sea stars, sand dollars, and worms called fat innkeepers (Urechis caupo). But each Monterey otter ate only a few kinds of prey, resulting in six main diet types ranging from mostly abalones and crabs, to mostly sandy-bottom prey such as clams and worms, to mostly snails. “This extreme dietary specialization suggests that competition for food is intense in this population,” says Oftedal.

Indeed, the biggest, most accessible prey have been depleted in the middle of the California sea otter’s range. “By specializing, they get good at catching and eating their prey fast so they can get enough,” says Tinker. Even so, these otters are working very hard for their food. Sea otters eat every few hours around the clock, and the Monterey otters devote 40 percent of their time to foraging and feeding. Sandy-bottom specialists, for instance, can go through hundreds to thousands of mole crabs, sand dollars, and other small prey each day. Otters at the prey-rich San Nicolas Island have it much easier, spending only half as much time feeding. “All it takes are a few big red urchins, which are the size of pineapples, and a big abalone, and they’re full,” says Tinker.

San Nicolas Island otters are also exceptionally well-nourished. Sea urchins are full of fat and an energy-rich diet is just what the otters—as marine mammals without blubber—need to stay warm in the cold ocean. To find out how well the Monterey otters’ needs are met by their unusual diets, the team determined the equivalent of food-label “nutrition facts” for each of the six diet types.

The first step was gathering prey samples from a variety of coastal habitats. For example, collectors waded in tide pools for prey such as snails and abalones, dug in sand flats at low tide for prey such as mole crabs and clams, and dove into kelp beds for prey such as crabs and sea urchins. The next step was processing the prey like an otter to ensure that the nutritional analysis was as accurate as possible. Otters have specialized methods for getting at the flesh in each type of prey. For example, they smash snails against rocks on their chest, rip the arms off sea stars, and pull the pinchers and carapace off crabs.

The final step was analyzing nutrient levels by diet type and comparing them with those recommended for pet cats and dogs. Although not ideal, these domestic carnivores were the closest match the scientists could find for sea otters. “To our knowledge, our study is unparalleled for a wild carnivore,” says Oftedal.

The nutritional analysis showed that compared to domestic carnivore and San Nicolas Island otter diets, many of the Monterey otter diets are shockingly deficient. “They are all pretty bad on energy,” says Ralls. With the exception of red sea urchins, which are close to one-third fat, Monterey otter prey are extremely low in fat. The prey with the second-highest fat content, red abalones, are still much lower at about ten percent. And many of the otters’ prey have hardly any fat at all—snails, clams, and sand dollars are less than four percent fat. Moreover, the Monterey diets are also low in vitamins. For example, all six of the diet types are low in vitamin B1, and several are low in vitamin A as well.

Worst of all is the snail diet, which was eaten by a tenth of the Monterey otters studied. In addition to being low in fat and vitamins, the snail diet is remarkably high in calcium. Otters eat snails by smashing them open, and fragments of the high-calcium shells stick to the snails’ flesh. Too much calcium can interfere with the absorption of other minerals such as phosphorus and zinc, and otters on the snail diet got ten times more calcium than those at San Nicolas Island. “The snail-based diet appears to be replete with nutritional error,” says Oftedal.

Besides causing otter deaths directly, malnutrition may be linked to diseases. “Deaths from disease could be a symptom of inadequate nutrition,” says Ralls. It makes sense that weak, malnourished otters are more likely to succumb to diseases than strong, healthy ones. And there is some evidence that otters on the snail diet are more likely to die of the parasitic brain diseases toxoplasmosis and sarcocystis. Diets may also expose otters to “new” parasites not found in their preferred prey. For example, San Nicolas Island otters don’t eat mole crabs but some of the Monterey otters do, thereby exposing themselves to the intestine-perforating worms that parasitize the crabs.

To see if diet is an underlying cause of otter diseases, Ralls wants to know what the beach-cast otters ate. Otter prey have chemical “fingerprints” in the form of specific fatty acids and stable isotopes, which can be analyzed in otter fat and whiskers, respectively. The plan is to watch what radiotagged otters eat, then analyze their fat and whiskers. If the chemical fingerprints reflect the observed diets, the scientists will know that analyzing fat and whiskers is a reliable way of showing whether particular diets are really linked to particular diseases.

Habitat Expansion

If poor diets are the primary threat to the California sea otter, cleaning up the ocean won’t be enough. “Let’s say we get rid of toxoplasmosis but the real problem is that the otters don’t have enough to eat. Then they’ll just get the next disease,” says Ralls.

In that case, the otters’ greatest hope would be expanding into new, prey-rich waters, but the otters face barriers at both ends of their range. The barrier at the northern end is white sharks (Carcharodon carcharias), which don’t eat otters but do bite them. “The sharks are going for seals but sometimes get sea otters or even logs by mistake,” says Tinker. White sharks frequent elephant seal beaches and sea lion rookeries, which are common at the northern end of the otter’s range. Shark bites cause about 60 percent of beach-cast otter deaths in the ten-mile stretch of coast north of Monterey Bay.

The barrier at the southern end of the range is legal. The otters at San Nicolas Island were translocated there 20 years ago by the U.S. Fish and Wildlife Service (USFWS) in hopes of starting a second population. California otters are vulnerable to oil spills, and having two populations would help keep a spill from wiping them all out at once. However, the shellfishing industry opposed this translocation vigorously for fear that the sea otter would outcompete them for shellfish, and in concession the USFWS designated the southern third of the state’s coastline as a “no-otter” zone in 1987. Any otters that ventured south of Point Conception, which is near Santa Barbara, were to be caught and relocated.

But nothing went as planned. While the otters at San Nicolas Island are doing well, the population has never grown big enough. Many of the 140 translocated otters left and today there are only about 40 living there. In addition, maintaining the “no-otter” zone proved to be troublesome for the USFWS, so the agency stopped enforcing it in 1993. The USFWS is now in the process of declaring the translocation a failure, which could mean getting rid of, or at least reducing, the “no-otter” zone. “Containment of the southern sea otters is not consistent with the requirement under the Endangered Species Act to avoid jeopardy to the species,” wrote the USFWS in a 2001 policy notice in the Federal Register.  

Regardless of the law, sea otters have already moved south of Point Conception. Letting them continue this natural range expansion may be the best thing we can do for them. It’s not a quick fix and it’s not a sure thing. And it feels passive. But “the real danger is that we’ll take action that won’t actually help the sea otter,” says Ralls. “People want to do something but it’s important to do the right thing.”

—Robin Meadows is a contributing editor to ZooGoer.