The otters caught vast numbers of crabs, and with fewer crabs to eat them, grazing invertebrates such as seaslugs in turn became more abundant and nibbled algae off seagrass leaves, ultimately reinvigorating the entire seagrass ecosystem. “I always thought of sea otters as predators of the outer coast,” Tinker says. “But Elkhorn Slough demonstrates an entirely new keystone role for otters in estuaries that we had no idea about.” Tinker is now studying the relationship of sea otters with saltmarshes in the estuary. He has found that as the otters eat burrowing crabs, the saltmarshes get healthier.
These findings suggest that as sea otters recolonise other estuaries in California, they could play a crucial role in restoring imperilled habitats there too. Sea otters also help humans in other ways. Because they consume the same shellfish that we like to eat, and these shellfish can be sensitive to marine pollution, the otters effectively act as sentinels for the health of our coasts. “Sea otters are like a mirror, showing us the impact of things we’re putting in the water,” explains Melissa Miller, a wildlife pathologist at the California Department of Fish and Wildlife.
A few years ago Miller discovered that 21 sea otters had died of microcystin intoxication. The toxin was produced by an algal bloom that originated in a freshwater lake and travelled downstream into the ocean, where it was absorbed by the shellfish that the sea otters ate. The discovery was a wake-up call for the water authorities, which prompted by scientific data provided by the otter researchers took steps to clean up the lake.
RIVAL APPETITES – The relationship between sea otters and people is not always an easy one. For while many view the otters’ return as a great environmental success story, ecological changes triggered by these endearing animals come at the expense of shellfish fisheries. When sea otter populations were historically low, the booming stocks of abalone, clams, crabs and urchins created a welcome harvest for North America s native peoples and commercial fishers.
Now, in all the places where otter numbers are expanding, from Alaska to California, their appetite causes conflict with the people who have become dependent on shellfish for their economic as well as cultural value. “By the time sea otters arrive, it takes about two years before you can’t fish there any more,” says Mike Featherstone, president of the Pacific Urchin Harvesters Association. “In the second year, you find nothing but broken urchins. We know there will be no urchin fishery eventually. Otters are like a rat in the sea – they eat everything, but luckily for them they are cute.”
Tensions are running particularly high in the abalone fishery. In British Columbia northern abalone is listed as Endangered, and so is black abalone in California. Abalone fishermen blame the demise of abalone on sea otters, even though overfishing is thought to be primarily responsible for the plight of these invertebrates. Researchers have demonstrated that the otters do not drive abalone to extinction – instead they influence their preys abundance, size and behaviour. In the absence of otters, abalone grew big and settled in the open where human fishers could easily harvest them.
But where otters hunted them, the invertebrates avoided predation by moving to greater depths and hiding in crevices; over time the abalone even get thinner to fit narrow cracks. In California, a study led by Tim Tinker showed that black abalone actually occurred at higher densities in the very places where sea otters had been present longest. One possible explanation for this unexpected result is that as otters eat urchins and promote kelp growth, they indirectly provide more food for the grazing abalone.
“Abalone and sea otters are not enemies: they truly co-evolved and depend on each other,” says Lilian Carswell of the US Fish and Wildlife Service, an expert on sea otter ecology. While shellfish harvesters may lose out to otters in the short term, the long-term benefits that otters provide will eventually outweigh the pain that some coastal communities feel now. “It is shocking for people to see how the marine environment changes when sea otters first turn up,” Carswell acknowledges. “But the hope is that over time people forgive, forget and move on.”
ANCIENT EVIDENCE – In British Columbia Iain McKechnie, an anthropologist at the University of Victoria, is looking into the past for evidence that First Nations communities and sea otters lived in balance long before the fur trade. By examining sea otter bones dating back 5,000 years found in archeological middens, he has concluded that these native peoples hunted otters and kept them away from specific shellfish-harvesting areas near village sites.
“These people would actively hunt otters to prevent parts of their clam-digging beaches from being targeted, as the otters would otherwise eat all of the clams there, just like a deer that gets into your garden,” McKechnie says. Perhaps these traditional management practices could now guide First Nations people as they find ways to live with the burgeoning population of sea otters. Today, while people adjust to the reality of the sea otters’ spectacular return, the animals also face a variety of other threats. A large oil spill would be devastating, despite state-of-the art facilities to rescue oiled otters.
“It would be a terrible tragedy and the proportion of oiled otters you can save is small,” says Carswell. Then there is the threat posed by rising ocean temperatures, leading to more frequent algal blooms that put sea otters at higher risk of exposure to domoic acid, which can be fatal. In California, meanwhile, one of the main causes of death in sea otters is predation by great white sharks. Shark attacks have dramatically increased over the past 10 years in the south of the otters’ range. “This is now a real impediment to sea otter recovery” says Carswell.
Yet despite these problems there is real excitement about the sea otters’ recovery and their massive potential as ecosystem engineers. Carswell dreams about what might happen if one day they recolonised their entire historic range. “When sea otters return, there are all kinds of ecological connections that we couldn’t possibly have imagined,” she says. “So what else are we missing? What other magical connections might be restored if sea otters return to some of their former haunts?’