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Found across the North American and Asian coasts of the Pacific Ocean, the sea otter is one of the smallest, and perhaps the most endearing, marine mammals in the world.

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As with most members of the Mustelidae family, the sea otter has incredibly dense fur, made up of approximately . Although useful for insulation in absence of blubber, the sea otter’s characteristic coat made it the target of the maritime fur trade in the 18th century, forcing the species to the brink of extinction.

The subsequent implementation of international protective legislation in 1911 resulted in a significant recovery of sea otter populations across the species� endemic range, reducing instances of hunting and persecution. However, pollution, oil spills, climate change and predation continue to pose a substantial threat to the vitality of sea otters, with the species therefore retaining an endangered status.

1. Appearance

The sea otter is the largest member of the Mustelidae family, which includes weasels, badgers and ferrets. Newborn sea otter pups measure in at a mere and weigh just 1.4 to 2.3 kilograms. Northern sea otters than their southern counterparts, with northern males reaching a height of 152 centimetres and a weight of 45 kilograms, while southern males grow to around 120 centimetres in length and weigh 32 kilograms. is observed within the species, as females are moderately smaller.

Sea otters typically sport a dark brown undercoat, with lighter brown guard hairs, and have webbed feet to aid with swimming. Their nostrils and ears are able to close when submerged underwater. With of skin, the sea otter has the densest fur of any mammal. Comprised of two layers, an undercoat and long guard hairs, this water-repellent coat traps air to aid with buoyancy and insulation in absence of blubber. Sea otters maintain an exceptionally clean coat, often to help retain its waterproof, insulating properties. 

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Since sea otters reside in coastal, shallow waters, their diet , such as sea urchins, clams, muscles, abalone, crustaceans, snails, squid, and octopuses. Within newly reoccupied habitats in central California, sea urchins, abalones and rock crabs constitute the principal prey of local sea otter populations, whereas those found in soft-sediment habitats tend to consume clams and crabs. Sea otters found within the Aleutian archipelago of Alaska, however, regularly consume bottom dwelling fish, which constitute up to 50% of their diet during certain seasons.

Despite its relatively small stature, a sea otter’s from small snails, clams and limpets, to kelp crabs and giant pacific octopuses. When foraging for abalone, urchins and clams, sea otters over smaller options, with Californian populations Pismo clams smaller than 7 centimeters in diameter. Sea otters have been found to consume upwards of and have an incredibly high metabolism, with a daily food intake of 25-40% of their body weight in order to keep them warm.Ìý

Whilst foraging for food underwater, these marine mammals use to act as a pocket for food storage. Clams, which burrow in soft sediment, are . Males at a maximum depth of 82 metres, whereas females only descend to 54 metres. Typically feeding in the morning and afternoon, sea otters float on their backs at the surface of the water to eat. They are amongst the few animals that have been observed using tools to access food sources; for prey such as clams and abalone, sea otters are to strike and break open hard shells.

3. Habitat & Behaviour

Sea otters are a costal species and therefore use a range of , foraging primarily in waters less than 30 metres in depth and within a kilometre of the shoreline. with rocky, intertidal habitats that support kelp beds, sea otters utilise kelp as anchorage to avoid floating away with ocean currents when resting, indicating the importance of kelp canopy as a habitat component. Some have even observed that dense kelp canopy can help otters with , evading shark bites with greater ease. Nevertheless, the species has also been known to spend time in areas with soft-sediment, devoid of kelp, where the substrate consists of mud, silt or sand rather than rocks.

Habitats that offer protection from ocean winds and swells, such as barrier reefs, kelp forests, rocky coastlines, inlets, bays and estuaries, are of critical importance to sea otters as well. Although this marine mammal tends to remain within a small home range and displays high site fidelity, some sea otters, predominantly males, do carry out migrations and long distance movements at certain times of the year.

Sea otters are almost exclusively aquatic, though they may come ashore to rest or sleep. Known for their characteristic floating, sea otters are incredibly buoyant due to the air trapped within their dense coats, allowing them to sleep, feed, play, and even nurture their young while floating on their backs. These mammals are highly social and are only weakly territorial, with instances of aggression or fighting . Males and females float in separate groups of between 10 and 100 individuals, , often holding hands to prevent separation.Ìý

Sea otters are the only species of otter to give birth whilst in the water, with females typically delivering a litter of one. The coats of newborn pups trap sufficient air to prevent them from diving or becoming submerged underwater, with mothers often when leaving to hunt and forage. Infants are taught to swim and search for food at four weeks old, and remain dependant until the age of six to eight months.

4. Ecological Importance

Despite their small stature, sea otters are regarded as a keystone species, meaning that they have a profound effect on marine ecosystems and play a crucial role in maintaining the vitality of their habitats.

As mentioned, sea otters rely heavily on kelp forests to provide anchorage when floating in rafts and for foraging. A large proportion of their prey sources reside within kelp forests, such as urchins, crabs, and barnacles. If left unchecked, sea urchin populations can grow rapidly and overgraze on kelp, consuming it faster than it can be replenished and eventually destroying the forest habitat. This creates “urchin barrens�, where nothing but sea urchins are able to grow due to their abundance. Kelp forests, found within coastal ecosystems, sequester significant amounts of atmospheric carbon and prevent it from entering the atmosphere. These marine forests are capable of storing than those found on land, with research suggesting that seaweed forests sequester tens of millions of metric tons of carbon yearly within the deep ocean globally. The degradation or loss of kelp forests could thus have grave effects on the levels of atmospheric carbon, highlighting the importance of the trophic function that sea otters serve.

In addition to kelp forests, sea otters tend to hunt for prey, such as crabs, within seagrass meadows and eelgrass beds. Seagrass also sequesters atmospheric carbon, with the flowering plant able to capture carbon .ÌýBy controlling the abundance of crabs within these habitats, crabs are prevented from overpopulating and consuming too much of their prey, which include snails and slugs. Rather than feeding on seagrass, these mollusks graze on algae and other epiphytes that grow on seagrass, . By removing surface algae, grazing mollusks allow the seagrass to absorb sunlight and grow with greater efficiency, enhancing its ability to act as a carbon sink.Ìý

In addition to serving as blue carbon ecosystems, kelp forests and seagrass meadows are the preferred habitat of a range of marine species that rely on them for shelter and sustenance, including sea otters. These marine mammals are therefore regarded as an indicator species for coastal ecosystems, as the health, vitality and population dynamics of sea otters are indicative of the health of cohabiting species and the ecosystems they share.

5. Threats

Once widespread across the , from the coasts of Japan through to Russia and Alaska, down to California and Mexico, sea otters were estimated to have a global population of 150,000 to 300,000 at the start of the 18th century.

Once Russian explorers reached Alaska in 1741, sea otters became the target of an extensive commercial harvest that lasted 150 years until the species was left at the brink of extinction. By 1911, once the International Fur Seal Treaty was implemented, it is hypothesised that a mere 2,000 sea otters remained across 13 remnant colonies. The consequence of this near extirpation was , a marine ecologist who observed a dramatic loss of kelp forests in absence of sea otters when diving in Alaska. Although the maritime fur trade no longer poses a threat to sea otters, these marine mammals are still vulnerable to pollution and oil spills, disease, entanglement, climate change, and predation.

At present, oil spills are regarded as the most significant anthropogenic threat to sea otters. With no blubber to provide insulation, sea otters rely heavily on their dense coats to keep them warm. When exposed to oil, however, their fur loses its insulating properties and renders sea otters extremely vulnerable to hypothermia and other health complications. Oil accidentally ingested while grooming can further cause illness and gastrointestinal disorders, disrupting the sea otter’s ability to absorb nutrients and often leading to malnutrition and death. Inhalation of oil has also shown to cause lung damage in the species.

In 1989, the tanker vessel Exxon Valdez spilled approximately into the ocean in Prince William Sound, Alaska. Within seven months of the event, nearly 1,000 deceased otters were recovered in the spill area, however it is estimated that the resulting from the spill ranged from 2,650 and 3,905. Research conducted in the years since the disaster suggests that the Exxon Valdez oil spill had continuous and enduring consequences on sea otter populations, with an increase in mortality rates seen in otters that were four to five years old, or older, at the time of the spill. The study further found that until at least 1996, otters born after the event were also affected negatively. This suggested that, although the direct accounted for the majority of long-term spill effects, sea otters were also affected indirectly through maternal influences or exposure to lingering oil residue.Ìý

Other marine pollutants, such as from the land, also have detrimental effects on sea otter populations, their habitats, and their prey sources. Run-off occurs when rainwater washes over urban or agricultural areas and collects pollutants as it traverses through waterways. These pollutants include: toxic chemicals used in agriculture, such as pesticides and fertilisers; heavy metals, such as mercury, lead and cadmium; tire dust; nutrients; sewage; and pathogens, such as bacteria and parasites. In addition to having direct, harmful effects on the health of sea otters, these pollutants can of their prey sources, such as bivalves, resulting in the intoxication, infection, and eventual death of sea otters.

In 1996, from infectious diseases, such as peritonitis, protozoal encephalitis, and toxoplasmosis, primarily in breeding adults. Caused by a parasite known as Toxoplasma gondii, toxoplasmosis is a disease that has plagued sea otter populations for decades as a significant cause of fatalities as well as a contributing factor to the stagnant recovery rates of sea otter populations in California. Felines have been identified as the definitive hosts of toxoplasmosis, shedding Toxoplasma gondii oocysts in their faeces, which is then washed into waterways and eventually reaches the ocean. These oocysts are either ingested directly by sea otters or contaminate the marine invertebrates they regularly consume.

Being a coastal species, sea otters often fall victim to destructive fishing methods that are prone to high bycatch rates and overfishing. Between the mid-1970s and early 1980s, a considerable number of sea otters were caught and drowned in gill nets and trammel nets within California. In 2003, research suggested that sea otter fatalities increased in the summer as commercial fin fisheries became more active in coastal regions.Ìý

As sea otter reintroduction initiatives and range expansions continue to prove successful in California, with the once remnant population of 50 Californian sea otters having risen to approximately 3,000 individuals in 2018, concerns over potential conflicts with shellfish fisheries have also .

The reintroduction of sea otters to ecosystems that have been without the species for decades has naturally resulted in the reestablishment of historic feeding interactions. For commercial fisheries that have come to rely on these habitats in the interim, the reemergence of sea otters serves as a source of competition for common prey.

In Southern California, to the range expansion of sea otters, resulting in the proposal of a species management plan in 1968 to avoid overlap with the lucrative abalone trade. Upon the enactment of the in 1972, and once sea otters were given a status of “threatened� under the in 1977, the proposed management plan was rendered invalid as environmental conservation took precedence. Nevertheless, the potential perception that conservation measures negatively impact economic interests could result in decreased public support for the preservation of sea otters.

As ongoing studies continue to shed light on the effects of climate change on marine ecosystems, some to sea otter populations include ocean conditions becoming increasingly warm, acidic, and deoxygenated.

Warming oceans have shown to support the pervasiveness and range expansion of dinoflagellate phytoplankton, harmful algal blooms, and cyanobacterial blooms, which produce toxins such as saxitoxins, brevetoxins, ciguatoxins, domoic acid, and okadaic acid.

Although it is unclear whether the direct effects of toxin exposure on sea otters are acute or chronic, studies have shown that these and induce shellfish poisoning in mammals that consume the contaminated bivalves. This leads to detrimental neurological, gastrointestinal, and cardiovascular symptoms.

Research has further indicated that as oceans become increasingly acidic due to excessive carbon dioxide absorption, saxitoxins will increase in toxicity as a result of . Domoic acid exposure has been identified as a primary cause of death in southern sea otters, and has also been linked to the prevalence of cardiomyopathy in sea otter populations.

Northern sea otters in Alaska appear able to identify and avoid consuming clams that have accumulated low levels of saxitoxin in their tissue, however this decline in available prey sources will likely have a significant impact on sea otters in the future. This is especially true of populations relying on soft-sediment habitats, such as in the Kodiak archipelago of Alaska, where over 50% of a sea otter’s diet consists of butter clams. Bivalves that form their shells from calcium carbonate, such as clams, oysters and muscles, have also been decreasing in numbers due to ocean acidification. The increased acidity of seawater reduces the availability of carbonate ions, making it extremely difficult for , and even causing some bivalve shells to dissolve.

A on the health, body condition, and causes of death of southern sea otters has suggested that the most common primary cause of death amongst the subspecies is shark bite mortality. Primary predators of sea otters include orcas, great white sharks, bald eagles, coyotes, and brown bears, with orcas having been linked to significant sea otter population declines in 1998 across the Western Gulf of Alaska and Aleutian Islands. With the preferred prey species of these macropredators, which include Northern fur seals, harbour seals, and sea lions, declining in numbers, the less preferred choice of sea otters become a primary target despite their lack of blubber.

6. Conservation Efforts 

With the enactment of the International Fur Seal Treaty of 1911 came an almost immediate end to the drastic population declines that sea otters had suffered for decades prior, marking the start of a long journey towards the species� recovery. The subspecies Enhydra lutris nereis, known as the Southern or Californian sea otter, is listed under Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), while E. lutris lutris and E. lutris kenyoni are listed under Appendix II.

In the US, sea otters are awarded legislative protection under the Marine Mammal Protection Act of 1972 and were added to the Endangered Species Act in 1977, whilst in Canada they are managed under the Species at Risk Act. As such, it is illegal to purchase, sell or possess any part the endangered species or items made from its body parts.

Such conservation initiatives allowed sea otter populations to gradually increase in numbers, reaching a global estimate of 30,000 individuals by the mid-20th Century. Nevertheless, this figure represented a mere and sea otters remained absent from over 3000 kilometres of their former coastal range. Research indicated that the reluctance of sea otters to disperse from their home territory and traverse deep channels would render natural range expansions unlikely, therefore biologists initiated relocation initiatives in the mid-1960s. Healthy otters from the Aleutian Islands were transported to Southeast Alaska, British Colombia, Washington, Oregon and California. These reintroduction efforts were deemed extremely successful, with recorded population increase rates of 20% in some regions.

In Southeast Alaska, the initial population of 400 to 450 introduced sea otters has grown to approximately 25,000. A further 8,000 now inhabit the coast of British Colombia, having begun as a population of just 89 released individuals. This recovery is widely seen as an incredible achievement in marine conservation, as a species on the brink of extinction with fewer than 2,000 individuals now consists of a population of .Ìý

Unfortunately, in Southern California and Oregon. Of the small groups of sea otters transported from Alaska to the Oregon coast, none appeared to stay as scientists theorised the mammals had attempted to swim back to their home range and died. On San Nicholas Island in Southern California, a population of 125 introduced sea otters was reduced to just 12, however the remnant sea otters survived and have now grown to have a population of 150.

At present, Southern sea otters occupy a mere 13% of their historical range, remaining , and consist of a population of . Although this is a significant recovery, given that a single remnant colony of 50 sea otters remained in central California in 1911, the reintroduction of sea otters to Northern California and Oregon is regarded as and support the sustainability of sea otters by connecting southern and northern subspecies.

Research has indicated that there is sufficient suitable habitat to support reintroduction efforts, as the region represents the most extensive remaining gap in the once continuous historic range of the species. Now collected from numerous successful reintroduction initiatives, scientists and conservationists can more accurately and efficiently determine areas along the Californian coast that would most benefit from the presence of sea otters, and where threats to reintroduced populations are minimal.

Despite the numerous benefits that sea otters bring to coastal marine ecosystems, reintroduction and range expansion initiatives have faced that ascribe decreases in the availability of shellfish to the reemergence of sea otters. In California, abalone and have expressed concern about the potential competition they face with expanding sea otter populations. A in Alaska attributed a $28 million US dollar loss between 2005 and 2011 to sea otters. Yet, of a historic coexistence between humans and sea otters, in the form of indigenous and archeological knowledge, as proof that conservation initiatives can account for the interests of all parties involved. 

At present, the Marine Mammal Protection Act and the Endangered Species Act , as long as they are used for sustenance or traditional handicrafts. This practice is monitored by the U.S. Fish and Wildlife Service through their , and the Marine Mammal Protection Act further requires the implementation of species recovery plans to ensure populations remain at viable levels. Some have to serve as a form of population control for the species, as Native tribes have harvested up to 1,500 sea otters in some years and such a system has proven to support marine biodiversity over millennia of human-animal coexistence.

This holistic approach to conservation is evident in the ethos of organisations such as the , which aims to use scientific and socio-economic policy assessments that reflect the interest of Native tribes, governments, environmental groups and fishery representatives to determine the most effective strategies for the restoration and protection of sea otter populations in Oregon. If such initiatives prove successful in Oregon, they can be adapted and applied to other states and countries in order to minimise conflict between humans and sea otters, to better protect the species against the threats they still face, and to support the full recovery of sea otter populations throughout their entire endemic range.

How to Help

• Celebrate . Taking place between 21-27 September, Sea Otter Awareness Week is the perfect opportunity to inform those around you of the ecological importance of sea otters, the threats they face, and the small steps that people can take to help protect the species.Ìý
• Opt for eco-friendly household cleaners. When buying household cleaning products at the market, opt for non-toxic and biodegradable cleaners that are better for the environment and marine ecosystems.
• Clean up pet waste properly. Felines carry the parasite, Toxoplasma gondii, that causes toxoplasmosis in sea otters. When disposing of cat faeces, avoid contaminating waterways that will end up in the ocean. Avoid purchasing “flushableâ€� kitty litter.Ìý

Featured image: Lee Jaffe/Flickr.