The far-reaching influence of Alaska's sea otters

Sea otters (not to be confused with Eurasian otters) once lived from the Baja Peninsula up the west coast of North America in a long arc to the Russian Far East and Japan. But they fell victim to a raging torrent of exploitation because of their coat.

Sea otters have the densest fur of any animal on earth, with up to a million hairs per square inch (155,000 per sq cm). A human head, for comparison, has 700 hairs per sq in (110 per sq cm). Pups are born so fluffy they cannot sink due to the air trapped in their coats.

Since sea otters are unable to dive deeper 300ft (91m) and find their food mainly on the seafloor, they are forced to live close to shore. They rarely cross deep channels because the depth would prevent them diving for food during the journey. This restricts them to a small home range, something with consequences when too many are killed in one area.

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When an expedition from Russia to Alaska returned home in 1742, it held in its cargo 900 sea otter pelts. After people saw the quality of the fur, a commercial rush was on. The pelts' astronomical value was a tempting lure. Four sea otter hides could buy a house in Victoria, British Columbia, in the early 1900s.

The commercial harvest continued until sea otters gained protection in 1911 at the signing of a fur seal treaty between Russia, the US, Britain and Japan. Records for the total harvest are scarce, but fur traders likely killed close to a million sea otters during a century-and-a-half of exploitation in Alaska.

When the slaughter stopped, there were 13 small populations left, adding up to barely a thousand animals. More than 99% of all sea otters had been killed. Most of the surviving animals were off the Aleutians and the Alaska Peninsula, with one outlying population hanging on in Central California. For the first time in more than a million years, Pacific coasts fell quiet to the rhythmic sound of otters cracking the shells of bivalves on rocks they balanced on their bellies.

It should come as no surprise that the first step in the sea otter's recovery was to stop killing them. After the 1911 treaty, the population immediately stopped falling. By the mid-20th Century, it had rebounded to around 30,000. This was a good start, but it was still only a tenth of historic numbers.

Without the ability to disperse far on their own due to their reluctance to cross deep channels, sea otters were unlikely to refill their original range. So in the mid-1960s, biologists decided to relocate otters from the Aleutian Islands to suitable habitat in Southeast Alaska, British Columbia, Washington, Oregon and California.

The reintroductions were a huge success. The population grew at rates well north of 20% in some areas. Southeast Alaska, for example, where just over 400 otters were returned, now has 25,000. There are over 125,000 sea otters alive in the Pacific Ocean today. The success is widely regarded as a triumph in marine conservation.

But although tourists and many coastal indigenous tribes are happy at the animals' recovery on south-east Alaska's coast, you don't have to be there long before the conflict between divers and sea otters comes up.

In the mid-1960s the region became home to several lucrative dive fisheries. For three decades, Alaskans donned masks and flippers and descended to the ocean floor close to the tideline to pluck abalone (a large edible sea snail), geoducks and sea cucumbers from rocks washed by Alaska’s productive waters.

At first, the divers made a good living. But as more people took part, harvests declined. By the late 1980s, the dive fisheries were struggling. The Alaska Department of Fish and Game started restricting some harvests and then closed the abalone fishery completely in 1996. Pinto abalone became so scarce in south-east Alaska they were considered for endangered species listing.

The divers thought they knew exactly who to blame. Sea otters had been reintroduced to the area in the mid-1960s and shellfish started to decline soon after. The "rats of the sea" and their voracious appetites were the problem. (Further south, in British Colombia, such reintroductions have also brought unexpected consequences for the indigenous groups living there.)

Lauren Bell, a doctoral student in the marine science lab at the University of California in Santa Cruz (UCSC), tells me there is little doubt sea otters took a toll on shellfish, but they are likely not the only culprit.

"Abalone crashed during the heyday of the commercial fishery," she says. It seems highly unlikely divers were free from blame.

But complicating the picture further, in the midst of the divers' grumbling, an unexpected ecological benefit of the otter's return started to emerge: their impact on kelp.

Kelp is a type of seaweed with a preference for rocky coastlines bathed by cool waters. The macroalgae is prolific. With sufficient light, a suitable pH, and enough nutrients in the water, it can grow explosively.

Kelp is also crucial to the health of the nearshore environment. "When you take kelp away, it reduces biodiversity," Bell tells me. "The productivity just plummets."

Sea otters, it turns out, are a classic example of how predators exert a strong top-down influence on an ecosystem. Like the wolves in Yellowstone, they completely reshape the territory. This happens because of the sea otter's phenomenal appetite for urchins. (Read more about how sea otters transform their habitats to sink carbon).

Sea urchins can devastate a healthy marine habitat: left unchecked, they will mow down a kelp forest and create a wasted ecosystem known as an urchin barrens.

Otters, with their own impressive appetites, eat enough urchins to prevent this from happening. "It is a pretty clear relationship," says Kristy Kroeker, Bell's PhD supervisor and an assistant professor who runs a marine science lab at UCSC. "Where you see otters, you are more likely to see a kelp forest." Research has shown the arrival of otters can flip an urchin barrens back into a shaded kelp forest in a season or two.

What's more, the benefits of healthy kelp forests reach far beyond its benefits to fish, algae and bivalves.

When otters help a kelp forest grow, they also do a favour for us. Kelp capture carbon as they photosynthesise, just as plants do on land.

Another benefit of an organism that takes carbon directly out of sea water is that it helps with ocean acidification. Research shows kelp forests provide short-term refuges from acidification during their period of maximum growth at the height of the day. The decline in acidity is only temporary, so kelp won't provide a permanent solution to ocean acidification. But it can provide a buffer.

A 2012 study found sea otters indirectly lead to the lock up four to 36 times as much carbon per square metre in kelp in the ocean ecosystem each year as is locked up in their absence. The money from selling this carbon capture via carbon markets could fund an impressive amount of marine mammal conservation, the study noted.

Many experts suspect kelp will have a role to play in a warming world. In addition to its carbon benefits, kelp forests improve the health of the nearshore environment, provide nurseries for young fish, and can be harvested for food. The state of Alaska has realised commercial kelp farms could be important in the state's changing economy and is pumping money into seaweed through grants and incentives.

Whether or not ocean farming takes off in Alaska, sea otters are the lynchpin of any kelp renaissance taking place within the species' range. You need sea otters to keep the urchins in check.

And the story about sea otters' impact on shellfish could also be too simplistic. In Alaska, there have also been signs recently of abalone and other shellfish coming back despite the presence of otters, says Bell. Macroinvertebrates such as shellfish thrive in areas high in kelp detritus. The rocky habitat on which kelp grows provides plenty of cracks and overhangs for abalone and other shellfish to hide.

Another phenomenon that Kroeker has noticed is that nowhere do you find wall-to-wall sea otters: as a result of their fast metabolisms and inability to travel far, otters populate the marine landscape in patches. They can thickly populate one area and be completely absent across an adjacent channel. Neither Kroeker nor Bell thinks otters are free from blame for the shellfish decline. But they both think the story is more nuanced.

"We are wondering," says Kroeker, "if there is a sweet spot where you can have it all."

Coastal tribes are currently the only people in the US permitted to hunt marine mammals under the terms of the 1972 Marine Mammal Protection Act. A condition on the hunt is that animals must be used for subsistence food or traditional crafts. Sea otter harvest by tribes in Sitka Sound has been growing. In some years, Native people have harvested nearly 1,500 otters for subsistence use.

Unpalatable as it is to many people, this harvest could be the key to creating the patchy mosaic Kroeker thinks of as the sweet spot. It wouldn't be hard to keep populations low in some areas to benefit shellfish and high in others to promote kelp.

"The way I’m interpreting it is that you might be able to manage otters on a much more local scale than we think about," says Kroeker.

The patchy mosaic Kroeker envisions would mean the apparent all-or-nothing battle lines between otters and shellfish are in fact softer than we thought.

Such an arrangement requires a different vision of human-otter coexistence. The vision rejects the idea of people and wildlife as belonging to different domains, but instead existing in a coupled system. There is not one area permanently reserved for otters and one area permanently set aside for people. There is a fluid entanglement of both.

It's also an arrangement has worked in the past. The system evolved over millennia and created a cultivated abundance of marine biodiversity. Getting that system back is where the challenge lies. The idea of a coupled system is not just a different kind of management. It’s a whole different kind of ethic.

* This article is based on an extract from Christopher Preston's book, Tenacious Beasts: Wildlife Recoveries that Change How We Think about Animals, published 21 February 2023.

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