Cute, Furry, and Catalytic: Sea Otters
In 2009, a lone sea otter appeared on the Oregon Coast, where they’d been extinct since 1906. It floated twenty yards from the Whale Watching Center in Depoe Bay for a few days, and then vanished.
Unfortunately for marine biologists, tourists, and several species of kelp, the otter wasn’t an advance scout for a larger population seeking new real estate. Single otters often venture far from their home range and then return. The visitor was probably from the Northern California population.
Sea otters are adorable. They float on their backs cracking crabs and sea urchins to the delight of wildlife watchers in Monterey, the Olympic Peninsula, British Columbia, and Alaska. But these furballs are also agents of massive change. By their presence or absence, otters send ecological shock waves through coastal ecosystems. As sea otters rebound from nearly being hunted into extinction, these changes are gathering steam. Many are unpredictable. We should watch sea otters, and not just because they’re cute.
Until hunters decided their fur was desirable, sea otters once ranged from Baja to Japan. By 1910 they were extinct in Oregon, Washington, and British Columbia, with isolated populations hanging on in the Aleutians, Prince William Sound, remote areas of California, and the Russian Far East. With their main predator gone, populations of sea urchins, abalone, and crabs exploded. Carpet-like herds of sea urchins ate entire kelp forests, creating vast “urchin barrens”. In estuaries, crab populations feasted on sea slugs, which control algae parasites that devour seagrass and eelgrass. Juvenile fish need seagrass, eelgrass, and kelp for camouflage, so fish stocks declined. Since kelp beds moderate ocean swell, intertidal communities changed, shifting toward hard-shelled, impact-resistant species. Commercial fisheries had a field day with abundant abalone, urchin, and crab populations. These effects make sea otters a keystone species, whose presence or absence creates a cascade of far-reaching ecological changes.
In the 1970s, otters were reintroduced to the Northwest coast of Vancouver Island, the Central Coast of British Columbia, Glacier Bay, the Olympic Coast, and the Oregon Coast. The Oregon population didn’t survive. The others did. There are roughly 150,000 otters today, half the pre-trapping population, but in isolated subgroups. Given the link between otters, kelp forests, and estuaries, it seems that more sea otters would be a good thing. It is. But oceans are dynamic places, and the return is far from simple.
The effects of otter reintroduction vary. But two things are clear: they eat, and they have sex. The Vancouver Island population has increased at 17% per year and expanded from the South Brooks to Clayoquot Sound. Lacking blubber, an otter eats continuously to keep warm, chowing down the equivalent of 15-20 adult Dungeness crabs per day. Where otters have been reintroduced, a crash in populations of urchins and crabs usually follows, often 50-100% in the short term. Effects on commercially valuable abalone are inconclusive, but dive-based studies indicate that they retreat into deep crevices, out of the reach of both otters and fishermen.
Being Cute Doesn’t Make You Popular
This voracious appetite doesn’t endear otters to fishermen. The combined otter and human crab harvest pressure forced a Dungeness crab closure in Glacier Bay in 1999. Similar dynamics exists with urchin divers and Washington’s Olympic population. In 2013, shellfish fishermen sued the US Fish & Wildlife Service to keep an otter-free zone on the California Coast, and an Alaska Senator introduced a “bounty on sea otters” bill in March 2013.
The flip side is that smaller the urchin and crab populations allow the regrowth of nearshore kelp and seagrass. This helps fish populations, including many commercial species. Kelp also sequesters carbon: by some estimates, sea otters are responsible for 8.6 billion kg of carbon storage, worth $408 million on the European Carbon Exchange.
As otters expand their range and their prey adapt, will the populations of abalone, crab, and urchins reach a dynamic equilibrium, or will otters eat their way through an area and move on? It’s unclear. There may be a variation in patterns. Freed from urchin barrens, kelp comes back successional waves. The first to return are bull kelp and acid kelp, which are then outcompeted by giant kelp, stalked kelp, and laminaria. But the rate of kelp regrowth varies drastically, possibly with wave exposure and how quickly urchins and crabs migrate in to replace the casualties of the initial otter feeding binge. On exposed shores, where surf limits urchins and some kelp remained, the results of otter reintroduction were moderate. South of Monterey Bay, where the Sheepshead Wrasse also keeps the urchin population in check, the results are also milder.
Otter, kelp, and urchin populations may also be shifting around in a localized, patchwork-style movable feast. We don’t usually think of sea urchins as fast, but if you drop empty urchin shells on a carpet of sea urchins, they’ll clear out of the area for two weeks.
There’s a lot we don’t know about otters. Why did the Oregon reintroduction of the 1970s fail? Did they migrate north and join the Olympic population? With few kelp forests to hide in, were they eaten by sharks or Orca?
Threats have also evolved. No longer hunted for fur, otters are vulnerable to disease. Infectious diseases account for 60 percent of otter mortality. These illnesses are linked to pollution, runoff from coastal communities, a chemical used in boat paint, and banned but persistent chemicals like PCBs and DDT. Since otters live in regional populations without much intermixing, genetic isolation may be increase vulnerability.
A second risk is oil, which accumulates in the shellfish that sea otters eat. Twenty-five years after the Exxon Valdez, Prince William Sound’s otters still show exposure to oil. The Valdez spill was a visible cataclysm, but oil enters the sea in many subtle ways: stormwater from coastal towns, rain off coastal highways, and small leaks from many ships. Since humans live near the coast in ever-greater numbers, we’ll need to find solutions to these problems. Because otters are visible and reasonably stationary, they can serve as early warning systems for ocean health.
The next time you a sea a sea otter placidly floating on his back, remember that cuteness is only part of his game. He’s also unleashing a massive ecological cascade.