By Emily Williams, Miami University, Project Dragonfly

The southern sea otter, or California sea otter, is one of three endangered subspecies of sea otter found in a small area near central California. Despite government protection, this species has been endangered for over sixty years, unable to make significant population growth in almost thirty years. Habitat expansion and population growth are greatly threatened by two main factors, their natural predator, the great white shark, and the isolation of kelp cover, found only in patches along the California coast. This inspires conversation for more calculated conservation efforts, such as reintroduction techniques in suitable locations, for continued efforts of recovering the southern sea otters and how community involvement is essential. The San Francisco Bay is the largest estuary on the west coast. If southern sea otters could establish a population there, would it be enough to help the species? Found solely in central California, should southern sea otters be reintroduced to the San Francisco Bay to help boost numbers and further protect this species? Analysis of habitat suitability and area threats may be able to determine the probability of successful reintroduction for this species. 

The smallest marine mammal in North America, southern sea otters (Enhydra lutris nereis) are the only fully aquatic otter species, spending their entire lives in the ocean. Mixed with deep browns in color and a charismatic face with a sharp diamond nose and large black eyes, adults average four feet long and weigh between 50-70 pounds. Otter pups barely weigh five pounds at birth.  With an appetite for seafood, southern sea otters hunt for most invertebrates, snacking on sea urchins, crabs, snails, mussels, clams, and more. While they must dive to hunt for these small bottom-dwelling ocean creatures, the sea otters always return to the surface to eat, floating on their backs and using their chests and stomachs as dinner plates. 

With the thickest fur of any animal, grooming is essential to maintain up to one million hairs per square inch. Most marine mammals have a blubber layer to keep warm, but the southern sea otter does not; they depend on insulation provided by their dense, water-resistant fur to maintain a stable body temperature. Grooming sustains this warmth by trapping air bubbles close to the skin and preserving heat. Southern sea otters also have loose flaps of skin under their forearms to store their food, especially while hunting, to keep their paws free. Their two small paws are handy tools to prime each square inch of hair and catch their prey. Their two large, webbed flippers help propel them through the water.

The California coast is an ideal location for southern sea otters to flourish, home to the two primary ecosystems this species prefers: kelp forests and estuaries. Southern sea otters are considered a keystone species because they play a fundamental role in preserving the health and stability of the ecosystem. Kelp forests are large brown algae stalks that group together like a forest in shallow waters close to the shore, creating a dense ecology network. The sea otters and these environments share a mutual relationship, both benefiting from the presence of the other. This environment provides food and shelter for thousands of species of marine life, from marine mammals, like sea otters, to their prey, the invertebrates, and hundreds of fish species. The kelp can face threats of over-predation from sea urchins if that population is not maintained, so the presence of sea otters feasting on urchins plays an important role in ecosystem stability. Kelp forests, like plants in any ecosystem, balance ocean chemicals by absorbing carbon dioxide and producing oxygen. 

Similarly, estuaries face threats of an overabundance of algae growth on eelgrass populations. Eelgrass populations are vital to estuary stability, providing habitats for nurseries, increasing water clarity, and reducing wave energy. Algae threats are maintained by sea slug populations, which face threats of over-predation from crabs. Sea otters then preserve the sea slug populations by controlling the crab populations. Sea slugs are as important as sea otters, providing a mutually beneficial relationship with the ecosystem. This balance is essential for the entire health of ecosystems on California’s coastline. 

After a serious population decline as a result of being overhunted in the 1800s, the southern sea otters were not protected under law until the North Pacific Fur Seal Treaty of 1911. This important act championed two historic wildlife protection laws, the Marine Mammal Protection Act of 1972 and the Endangered Species Act of 1973. These crucial policies of law-abiding protection prohibit the hunting, capturing, killing, or harassment of a marine mammal, as well as any endangered or threatened species and their ecosystems. 

A multitude of facilities along the peninsula is partnered with the California Marine Mammal Stranding Network, with over seventeen institutes along the west coast involved in southern sea otter rescue as a network of collaborators, from marine mammal centers, wildlife centers, museums, universities, and laboratories. The California Department of Fish and Wildlife, Monterey Bay Aquarium, and the Marine Mammal Center are the primary responders for stranded sea otters, although the Monterey Bay Aquarium is the only facility to rescue, rehabilitate, and successfully release southern sea otters.

The Monterey Bay Aquarium has a successful extensive southern sea otter rehabilitation program which has been ongoing for over thirty years. A key contributor to sea otter recovery in California, the Monterey Bay Aquarium rescued their first southern sea otter pup in 1981, three years before it was scheduled to open to the public. The Sea Otter Program at the Monterey Bay Aquarium was successfully rehabilitating and releasing southern sea otters through 2001, before the development of the surrogacy program. The Sea Otter Program has successfully rehabilitated and released dozens of pups into Elkhorn Slough, a seven-mile-long estuary off the coast of Monterey Bay, California.

Data on this population group between 2002 and 2016 found that the released individuals from the program and their continued offspring accounted for 55% of the otter population in Elkhorn Slough, going from just 20 individuals in this area to nearly 150. Not only is this a vital process in creating healthier ecosystems that, in turn, have a better ability to endure the effects of climate change, but the evidence of successful reproduction from this population of sea otters supports the importance of rehabilitation and release of prolific adults into an area to preserve the species. 

Before being hunted to near extinction in the 1800s, the geographic range of southern sea otters used to extend from as far south as Baja, Mexico through the entire state of California and into Oregon with nearly 300,000 individuals. Now concentrated solely in central California, the population has grown to nearly 3,000. A conservation estimate found the entire California coastline can sustain a maximum southern sea otter population of 16,000 individuals. This subspecies currently occupies only 13% of its historic range and has made no significant expansion from the central coast since 1998. 

Despite legislative protection and conservation efforts, the southern sea otter population has maintained a low level of growth for another thirty years. Studies have found that southern sea otter populations are unable to expand their territory due to threats including lack of food, diseases, and oil spills contaminating their fur, which causes hypothermia. However, the two largest threats to spatial restraint against the southern sea otter populations are their natural predator, the great white shark, and the isolation of kelp cover, found only in patches along the California coast. 

Sea otters are restricted to areas with benthic prey in depths they can withstand while hunting. They prefer to stay within kelp forests close to the shoreline and avoid deeper areas further offshore, where great white sharks hang out along the edges of the kelp canopy cover. Fatalities documented from shark bites have evidence of taking a single bite to kill the animal, but they do not eat them as a meal, making it a non-consumptive kill. Kelp forests and canopy cover are found in patches along the coastline, occupying warm waters in the shallow shore. Because ocean depth increases significantly with distance from shore, the otters’ range expansion is limited to travelling north and south along the coastline. Since the southern sea otter population has not been successful with this range expansion due to predation and gaps of open ocean between kelp cover, reintroduction techniques are being considered by conservation organizations as the next big effort in saving this species.

If southern sea otters benefited Elkhorn Slough, an estuary with protected status and abundance of prey, reintroduction of sea otters in other estuaries could prove a beneficial conservation effort, supporting population growth and pulling this species off the endangered list. Reintroducing a species to an ecosystem requires enormous amounts of effort. Arguably, the most important factor to determine the success of reintroduction is determining the habitat suitability of a release site. Several locations have been examined as possible sites optimal for relocation, but San Francisco Bay is the largest estuary on the Pacific coast. This area provides habitat for over five hundred species, from marine mammals to birds, fishes, and invertebrates. An ecosystem that was historically home to thousands of southern sea otters, the San Francisco Bay could potentially carry 1.5 times the current population, over 6,600 individuals. 

Although this seems like an obvious choice for conservation efforts, a profound number of challenges face this urbanized estuary. For example, anthropogenic activities in the San Francisco Bay would have a much greater effect on the species than at Elkhorn Slough. These increasing threats mainly include vessel traffic, such as commercial fishing, commercial shipping, and recreational vessels, as well as pathogen disease and oil spills, as a result of intense urbanization. The San Francisco Bay is one of the busiest ports in the world, with major cities on each side, refineries, and shipping ports, creating heavy vessel traffic. These anthropogenic threats needed to be measured to find out if the San Francisco Bay can support a reintroduction plan of the southern sea otters and ecologists from the Sonoma State University were determined to find out.

In 2020, a significant study by the ecologists at Sonoma State University developed a risk assessment to determine the measurable hazards posed against a potential sea otter population within San Francisco Bay. This important evaluation regards the possible threats to the species, how those threats are distributed throughout the area, and how they might impede recovery of a species as an effort for conservation. The results of this report would pinpoint the exact locations of ideal release and development of the southern sea otters in San Francisco Bay. Dividing the study site into North Bay, Central Bay, and South Bay, each section was tested for which stressors occupy the potential habitat location and for how long. Vessel traffic proved to be the most frequent stress exposure to the area. However, contaminants and oil spills would also occur but have less contiguity overlap. Results from this risk assessment showed that Central Bay had the highest risk of threats to the species, while North Bay had the lowest, as demonstrated in Figure 1.

A study such as this is imperative in an adaptive management framework, informing decision-makers on plausible reintroduction locations to obtain the most success out of a population management plan, and finding suitable habitats in the study area. This information is also equally important to share with the community, affecting management actions. Community involvement would implement change to the Bay area to make it a more suitable environment for southern sea otters. Actions such as promoting education programs to ensure motorists participate in proper safe recreational vessel activities and limiting recreational vessel traffic would reduce disturbances in a vulnerable ecosystem. Informing the public is a difficult first step, but community participation is vital when anthropogenic factors are involved in conservation efforts. To fully participate in conservation, a community must recognize the importance of ecosystem health, understand the anthropogenic effects on the ecosystem, and monitor legislative involvement. With all of this in mind, a reintroduction plan for the southern sea otters could prove beneficial to the species but would take an enormous, coordinated effort between the community, ports and fisheries, marine scientists, researchers, and rescuers to make it happen. 

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