National Geographic

Sea Otters: Your Defence Against The Algal Apocalypse

When humans dump nutrients into the oceans, whether it’s fertilisers running off from farms or sewage pouring in from cities, the results are usually predictable. The influx of nitrogen and phosphorus quickly becomes too much of a good thing. It fuels the growth of algae that stop sunlight from reaching underwater plants, introduce toxic substances into the food chain, and deplete the water of oxygen. This process, known as eutrophication, transforms clear, teeming coastal waters into green, slimy, choking wastes.

But Elkhorn Slough is different. This huge Californian estuary, surrounded by farmland, receives 150 times more nitrogen fertiliser than it did a century ago. It should be an algal mess but it’s actually a thriving marine reserve. Flocks of sea birds fly over the water, while luxuriant meadows of seagrass grow within it.

The area owes its resilience to a defence system that stops the nutrient overload from triggering an algal apocalypse. It has a living vaccine against eutrophication, and a very cute one at that—the sea otter.

By studying 50 years’ worth of historical data, and comparing areas with or without otters, Brent Hughes from the University of California, Santa Cruz has shown that these adorable animals trigger an ecological chain reaction that safeguards the seagrass meadows and keeps algal blooms at bay.

Sea otters are large oceanic weasels that were extensively hunted for their fur—the world’s densest—in the 19th and 20th centuries. They were flirting with extinction, but a hunting ban and conservation efforts restored their numbers of healthy (if still endangered) levels. They now thrive in several parts of the Pacific Northwest, including Elkhorn Slough.

During their absence, nutrient levels in the slough doubled during the 1970s, causing the seagrasses to disappear. They were at an all-time low in 1984 when the sea otters returned after a hundred year absence. Since then, the seagrasses’ fortunes have reversed. Their meadows now cover a 7-fold greater area, even though nutrients have continued to pour into the estuary.

Sea otter, by Mike Baird.

Sea otter, by Mike Baird.

Hughes discovered why. Sea otters grab shellfish and other prey from the sea floor and smash them open on the surface, using rocks as hammers and their own bellies as anvils. This makes it very easy for scientists to record what they’re eating, and Hughes used decades of such records to show that the Eklhorn sea otters are crab-specialists. “We estimate that they can easily remove 400,000 crabs per year in an area the size of 7 football fields,” he says. “That’s a huge effect, which cascades down to affect the seagrass.”

The crabs eat other animals including an orange sea slug and a shrimp-like isopod, both of which graze on algae. So by killing the crabs, the otters inadvertently protect the slugs and isopods, which in turn protect the seagrass by nibbling away at encroaching algae. This complicated four-part chain reaction (or “trophic cascade”) is what keeps Elkhorn Slough in its current healthy state.

To see what would happen if the otters disappeared, have a look at the video below. The first clip shows the seagrass beds of Elkhorn Slough. “The seagrass is nearly devoid of any algae growing on the leaves, it’s green and healthy looking, and there are large, conspicuous sea slugs consuming the algae,” says Hughes.

The second clip comes from Tomales Bay, a nearby inlet with far lower levels of incoming nutrients but notta lotta otters. In fact, none. “The seagrass looks relatively unhealthy: it’s brown, covered in algae, and slumped over,” says Hughes. “The crabs are four times more abundant and 30 percent bigger than they are in Elkhorn Slough.” (That’s roughly what they were like in the slough before the sea otters returned.)

To check that the sea otters were truly responsible for the difference between the two sites, Hughes ran an otter simulation. His team ringed off small areas of estuary and added fixed amounts of seagrass, slugs and isopods. Then, they added either the small crabs you find when sea otters are around, or the large ones you get when the otters are absent. The bigger crabs did indeed eat more grazers, leading to more algae and less seagrass.

Sea otters are near the top of their food chain, and it’s not surprising that they exert a strong top-down influence upon other local animals. In iconic studies during the 1970s, James Estes established them as classic examples of keystone species—those that are disproportionately influential for their numbers. They protect kelp forests by eating the sea urchins that would otherwise raze them down. With otters, you get underwater jungles of wavy green kelp. Without the otters, you get bare “urchin barrens”.

“The really interesting discovery here is that otters counter the detrimental impact of eutrophication,” says Estes.  In other words, their top-down influence is strong enough to nullify the bottom-up effects of nutrients entering the slough.

It’s the bottom-up effects that scientists usually turn to when explaining the decline of seagrass beds around the world, along with the loss of other coastal habitats like salt marshes or kelp forests. But Hughes’ study shows that the loss of top predators, such as sea otters, matters too. It might even matter more—after all, seagrass can actually do better under conditions that would normally lead to eutrophication, as long as there are otters around.

This has huge implications, says Hughes. Until last winter, sea otters were actually banned from southern California because people were worried that they would compete with local fisheries. That ban has since been lifted, and the otters are free to expand into their historical range, down into Baja, Mexico. As they do, the damaged seagrass beds in the southern estuaries could recover. “With the results from this study regional managers are much better informed on what to expect when sea otters start recolonizing estuaries,” says Hughes.

They might also have other positive effects that we don’t know about yet. “Sea otters are thought to be kelp-dwelling predators,” says Brian Silliman from Duke University. “This paper shows that they fit just fine into seagrass food webs. Where else could they expand to? Marshes? It’s all very exciting to think about.”

Reference: Hughes, Eby, van Dyke, Tinker, Marks, Johnson & Wasson. 2013. Recovery of a top predator mediates negative eutrophic effects on seagrass. PNAS http://dx.doi.org/10.1073/pnas.1302805110

There are 11 Comments. Add Yours.

  1. Tony
    August 26, 2013

    great article! Elkhorn Slough’s one of the wildest places I’ve seen along the california coast to see wildlife. One question though, what makes Otters all that different from seals, which also feed within the grasses but are found all down the california coast? Are seals not able to break open shellfish in the same way? thanks

  2. Lionel Gambill
    August 26, 2013

    This is doubly good news. The demise of sea otters was due to human greed. Otters were attracted to humans and often tried to snuggle against the legs of the humans who were clubbing them to death for their fur. Let’s hope and pray that that ugly history does not repeat itself.

  3. Frank
    August 26, 2013

    who would have guessed. So much to learn about how the environment Works.

  4. Karen
    August 26, 2013

    Excellent article! Elkhorn Slough is a very special place.

  5. Tuvaak
    August 27, 2013

    Sea otters are mustelids (weasels), and as such would not snuggle against people clubbing them. They are cute, but only from a distance. If you look at their teeth they are no longer ‘cute’ but rather menacing and they can be quite aggressive. It is dangerously risky to be up close to these aggressive, wild animals. When skinned out, aside from their flippers, they do not look that different than their cousin the wolverine.

    Their diet differs from seals in a couple of ways. Sea otters consume 20% – 30% of their body weight in seafood every day, much of which is the same seafood people eat. Like many wild animals, they waste quite a bit, harvesting up to twice what they actually consume and wasting the rest. They also are nowhere near as good of swimmers as seals, which is why you see them eating slow moving shellfish rather than eating finned swimmers that seals will eat.

    • Tony
      August 27, 2013

      wow, didn’t realize that otters are related to wolverines and weasels. interesting stuff!

  6. Chris Phoenix
    August 27, 2013

    Tuvaak – look at the Russian tame-fox study to see how quickly a wild animal can evolve into a tame one, or an even wilder one, under intense selection pressure. The fact that sea otters are weasels says very little about their reaction to humans.

    The sea otters alive today are descended from those who were most avoidant of humans. It is no wonder that they will avoid us, after generations of selective killing of the ones that did not avoid humans.

    I wonder whether, prior to the fur trade, sea otters were hunted by Native Americans or not. If not, they might very easily have evolved behavior patterns that let them obtain fish scraps by being close to humans, in much the same way that seagulls have learned to follow fishing boats.

    • Tuvaak
      August 27, 2013

      Alaska Natives such as myself have hunted sea otters long before the arrival of Russians to Alaska. We ate sea otter, made tools from their bones, we used their fur for clothing, shelter, and bedding. My peoples hunted sea otters out of the same areas where we hoped to obtain sea foods, because kooshdakhaa (otters) eat the same foods we eat. Before I say anything more, I would like to emphasize that I would not hunt sea otters in California with their low otter populations caused by how people treat their environment down there.

      Sea otters are large wild animals, unlike seagulls, and I certainly would not want one trying to get fish scraps from me. Besides, the only parts of the fish I discard are the bones, viscera, and fins, none of which a sea otter would find desirable. There are plenty of uses for everything else on a fish.

      When hunting sea otters, it is easy to know when their inua (spirit) has left them as they fight until the very end and are among the most ferocious animals. Then again, most of the other game I hunt are much more docile – moose and deer do not have the same fight as a weasel. Yes, I do use their fur, and yes I do eat them, I wouldn’t waste any animals that gives up their life at my hands as such would not be respectful, nor would any other Alaska Native who honored their heritage.

  7. Robert S-R
    August 30, 2013

    “Notta lotta otters.” Bravo, sir. By the way, I went ahead and subscribed to your YouTube channel, and it’s great seeing the sneak peeks of what you’ve got in the works!

  8. Gabrielle Page
    September 6, 2013

    Bravo the comments from the hunter about having respect for the creature you are hunting and using for a purpose not for vain-glorious trophy hunting. Growing up in the wilds of BC Canada I am very familiar with the urge some people have to blast the hell out of God’s creatures for spurious reasons. Any damn fool can kill another sentient being with a gun but it takes a thoughtful human person to kill for survival.

  9. Tuvaak
    September 10, 2013

    When I hear/read comments from individuals about the killing of animals, it makes me think that they might have a different perspective than mine out there. Animals have life, as do plants. With the exception of most plants which obtain food from the sun, life requires another life to be given up for life to be sustained. Even vegetarians require killing plants to survive. I’m not advocating that anybody should cease eating plants because they were life, rather I think it is important to respect all the life that is given up to sustain our life, weather it is plant or animal, when we eat/wear it.

    Blasting the hell out of God’s creatures for spurious reasons is horrible. Respectfully being a good steward of all the resources God has endowed us with is otherwise. All of our actions eventually will be accounted for, one way or another; it is our choice if we want to ensure that when they do it will be pleasant or otherwise. Yes, we will have to account for any spurious killing on our part, as we will have to account for the times we idly sat by and let others (such as an overabundant segment of a population of sea otters) ravage the environment.

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