Written by Amy Barker
If you have an interest in marine conservation you are likely to have heard the story of the southern sea otter (Enhydra lutris nereis), the ‘keystone’ species that restored an entire ecosystem along the California coastline. For those of you who have not encountered this story before, or whose memory is a little patchy, I will recap the basics. The sea otter distinguishes itself from other marine mammals in its evolutionary strategy of extremely dense fur as opposed to blubber in order to survive in freezing ocean temperatures. With one million hairs per square inch they have the densest fur of any mammal, a distinction which sadly left the species highly sought after by hunters. They were left almost extinct by the 1920s, and the ecosystem within their former range was in turn left devastated by their decline. Sea otters are major predators of sea urchins and other invertebrates which feed on the giant kelp, which forms underwater ‘forests’ of vital habitat for a great abundance of biodiversity. These forests also share an important function with forests found on land – they absorb vast quantities of carbon dioxide, and unlike those on land, they also protect the coastline from storm surges. Without these top predators, numbers of kelp feeders grew exponentially, and the kelp forests quickly disappeared, along with the diversity of species that were dependent on it. Sea otter numbers are still not what they were, but along the California coast and particularly in the Monterey Bay, where a large proportion of the species resides, their numbers are now sufficient to maintain the forests that dominate the coastline once again. Their total population has grown from around 50 in 1911 (when the hunting ban began) to around 3000 today, although their range remains a fraction of what it was.
You may have heard of this story as one of the more striking examples of a keystone species at work, or you may know it better as one of the great conservation success stories. It seems to support a popular philosophy in conservation that put simply is ‘if you leave nature alone it is sure to bounce back’. Although it is astonishing how the kelp forests in Monterey Bay appeared to resurrect alongside the sea otter, it is important to remember that there is no guarantee of the continued persistence of either. Sea otters today face threats, both new and old to them, that are totally unrelated to the hunting ban. There is much more to this story, and extensive research has propelled our understanding of the species and informed further conservation efforts. We know more about their ecological role and the new threats that they face, and new strategies are being employed to ensure their continued success. So, here is a reprise therefore on the classic tale of the Southern Sea Otter, a species that is so much more than a ravenous urchin consumer (which is important too of course!).
But first I should say that I feel particularly invested in their story as my grandparents live in Pacific Grove, a small town on the Monterey Bay. I have been lucky enough to see otters in the wild and up close at the Monterey Bay Aquarium every year for as long as I can remember, on visits to my grandparents. I have grown very fond of them as has the rest of my family, and still watch them with the same awe and excitement as I did when my family first pointed them out to me. I really feel that even putting the ecological argument aside, the otters are worth saving just as the endearing and unique beings that they are!
Keystone in other ways?
Sea otters have been discovered more recently to exhibit their capacity as a keystone species in food chains other than the famous chain involving those voracious sea urchins. One example can be found in Elkhorn Slough: an estuary and tidal slough in Monterey Bay and within Monterey county, and a small hotspot for sea otters, being only 7 miles in length. Research by Hughes et al (2013) discovered that the extent of eel grass was significantly correlated with sea otter densities. This is because the otters feed on crabs which eat the mesograzers which in turn eat the algae that cover the eel grass, inhibiting its ability to photosynthesise. Similar to the kelp forests, the eel grass creates important habitat for a great many species, and the otters appear key to its maintenance. I was fortunate enough last summer to help out with PhD research by Kat Beheshti on the eel grass in Elkhorn Slough, and this research will be important in informing the ongoing restoration that takes place there. Kat explained that the otters eat the small crabs “like popcorn”, which conjured an image that not only made me laugh but also made me appreciate the significance this grazing must have on the ecosystem! Because the otters do not have any blubber, they need to consume a great volume of food to keep up their body temperature, as well as devoting hours a day to grooming their dense fur. I can imagine an otter reaching for another crab before they have barely finished chewing their last – there is such an urgency to their feeding! Even a small number of otters can dramatically change the size of a crab population. Kat has her own exciting website named ‘Just Slough It’ where you can find out more about this. The more we discover about the irreplaceable role sea otters play in the Monterey Bay ecosystem, the more frightening it is to reflect on the idea that they had almost gone for good, and even more horrifying to imagine the prospect that their existence may not be guaranteed into the future as threats such as climate change extend out of our control.
Challenges to the otters: old and new
Southern sea otters do not appear to have any consistent predators, and many instead succumb to disease and parasites. However, sea otters can be mistaken as prey by great white sharks (particularly juveniles), and although they are not eaten by the sharks, often die of shark bites. This also appears to be a common cause of death. According to the Monterey Bay Aquarium, which has conducted sea otter research for decades, the greatest threat to the sea otter is oil spills. This is because oil quickly disrupts the insulating property of their fur, resulting in risk of hypothermia. Furthermore, the population is relatively concentrated in their California range, and the far-reaching effects of an oil spill could envelop a great proportion of the entire population. This is a frightening prospect, and as with many threats to wildlife, prevention is infinitely more effective than any cure we could offer.
Recent censuses have revealed that the otter population appears to have reached a kind of standstill, or a halt in the steady increase in numbers observed since their protection began. Researchers are still unsure as to why this is the case. Aquarium researchers have been conducting post-mortems on sea otter carcasses as well as tracking and observing living otters to try to illuminate the cause of this standstill in population growth.
A very recent problem in Elkhorn Slough is that otter numbers appear to be at capacity. On my last visit to Elkhorn Slough earlier this summer, I was approached by a graduate student from the Center for the Blue Economy at the Middlebury Institute of International Studies at Monterey, who was conducting a survey. He was part of an investigation into the economic value of the otter at Elkhorn Slough, and the potential economic value the otter could bring if reintroduced to a similar habitat. He mentioned that this is becoming a pressing need as Elkhorn Slough is at or near capacity in terms of its otter population. Elkhorn Slough is particularly popular as a release site for otters rehabilitated at the Monterey Bay Aquarium, which has been running an otter program since 1984. Elkhorn slough has an abundance of prey for the otters and offers protection from both storms and great white sharks. However, there aren’t an inexhaustible number of Elkhorn Slough type havens for sea otters along the California coast, and the challenge in future will be to find more locations as attractive to otters.
Can a Darth Vader costume save the sea otter?
Conservation strategies to protect the sea otter are forever being monitored and updated as more research is done, and the evolution of the sea otter program at the Monterey Bay aquarium is a good example of this. As well as taking on injured or otherwise vulnerable adults, the aquarium also rescues lost or abandoned sea otter pups, and in the hope of releasing them back into the wild attempts to teach them everything they would’ve learned from their mothers. The staff at the aquarium used to take on this ambitious task themselves, by, amongst other things, strapping into their wetsuits and diving gear to show the young otters how to dive for their food and how to get at their food using stone tools too. The otters did appear to learn this way, however it left them feeling far too comfortable around humans when they were released back into the wild. Although the adjective that seems to come most readily to those who encounter an otter is ‘cute’, the otter is a top predator, and can be dangerous company. Otters and humans in close proximity to one another is unsafe for both parties, and so the aquarium had to change their strategy.
The aquarium puts on short documentary films in their auditorium every day on an interesting variety of subjects, and the one I saw the most this summer (because my 8 year old cousin couldn’t get enough of it) was about a rescued otter pup called ‘Luna’. Initially the staff took on all feeding and grooming care of Luna, but in order to mitigate any bond or association forming between Luna and her human carers, the staff wear what is jokingly described as ‘Darth Vader’ attire: masks and clothing that obscure all their features. Luna then became part of what is now their very successful surrogacy program, in which pups are introduced to a female in the exhibit who (hopefully) adopts the role of mother, teaching the pups how to be otters. Luna was ‘adopted’ by Rosa, who has fostered more pups than any other otter at the aquarium. Luna then went on to raise her own pups out in the wild – and so saving one otter really is worth every investment the aquarium makes!
The more we know about otters, the more effective our conservation measures will be; however, we will need continued research as otters face new challenges. It is clear the ecosystem depends on the otter in more ways than one, and along with their uniqueness and their irresistibility in the eyes of all who encounter them, we have every reason to prioritise their conservation into the future.
Amy Barker is studying Conservation Biology and Ecology at the University of Exeter, and has interests in the broad field of wildlife conservation, and more particularly in the fields of animal behaviour, cognition and ethics. She also has a very active interest in art, and has recently become an illustrator for the wildlife magazine ‘Life’ run by students of Exeter and Falmouth Universities. She hopes to begin a career in the field of conservation after finishing her degree.
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