Monitoring California’s Sea Otter Populations

Researching the impacts of human disturbance

Project information
  • Status: In Progress

On the central California coastline in the ecologically precious estuary and salt marshes of Elknorn Slough resides an ‘Ecosystem Superhero’ who needs our help. The Southern sea otter (Enhydra lutris nereis), aka the California sea otter, is a subspecies of otter currently vulnerable to human disturbances. Without intervention not only will this wonderfully unique species be in danger, but we also risk losing their vital services in maintaining ecological balance. Inaction could drastically alter biodiversity in the region.

This project has been co-funded by our members and Not Just Tavel's Climate Heroes programme.

Are sea otters endangered?

Following the fur trade, sea otters are listed as endangered on the IUCN Red List and the southern sea otter subspecies is listed as threatened under the US Endangered Species Act, having undergone rapid population decline in the early 1900s. Today, the Southern sea otter faces daily threats from sharks, toxins, diseases, and human disturbances.

A sea otter resting in the water at Elkhorn Slough, the location of Mossy Earth's rewilding project to monitor their populations.
Southern sea otter's fur is vital for their survival. That's why they spend 8-15% of their day grooming their fur, which traps air and forms a critical layer of insulation between them and the cold California sea water they reside in.

The Intervention

To combat the impacts of human disturbance, this project assesses the impact of increasing marine recreation users to sea otters living in Elkhorn Slough, an estuary just north of Monterey Bay. The data gathered will then be used to inform future management decisions and ensure that otters can safely inhabit the slough.

More about this intervention

Elkhorn Slough is a region along the Central Coast of California that is home to a recovering population of Southern sea otters. The slough also hosts human visitors wishing to kayak, stand-up paddleboard, scuba dive, and participate in other marine recreation activities. As a result of the initial COVID-19 lockdown, marine recreation increased significantly at the start of last year. On the one hand, whilst it’s encouraging to know more people want to engage in nature, many first-time visitors are unaware of specific protocols used to reduce their impact on the surrounding wildlife. Consequently, there is a clear need to understand impacts and take action before irreversible damage is done. Wildlife disturbance is a huge concern of major conservation agencies, yet data correlating increased human disturbances to harmful impacts on wildlife is very rare to come by.

Sea Otter Savvy is an on-the-ground organization on the Central California coast dedicated to fostering awareness and stewardship of this issue amongst the coastal community. The impact of the pandemic on its funding has meant this organization urgently needs support. Therefore, at Mossy Earth, we are supporting them in monitoring, measuring, and assessing the impact of the increased marine recreation on sea otters in the slough. Our members will cover the purchase of a camera trap to monitor visitor numbers and a stipend for a local university student to analyze the data. These data will be used by Sea Otter Savvy to inform stakeholders of best practices for mitigating the effects of crowds to the sea otters and other surrounding wildlife.

Elkhorn Slough on California's coast where we are trying to monitor California's sea otter populations.
Southern sea otters have historically experienced a severe range decline. This project aims to assist a small population of the otters currently located in Elkhorn Slough on California’s coastline.

Learn more about why we are monitoring sea otter populations

Why is this Species Important?

How sea otters help save the planet

As Sea Otter Savvy puts it, sea otters are “ecosystem superheroes.” These furry friends play a critical role in their marine ecosystems as a predator keystone species. This is an ecological term given to species who have a profound impact on their ecosystem by keeping its prey in check, without them trophic cascades can be destabilized and ecosystems suffer. Kelp forests and seagrass beds, maintained by healthy sea otter populations, provide a home for hundreds of fish species, nursery habitats for invertebrate species, and carbon storage that can combat climate change.

When sea otters are removed from a kelp ecosystem

Sea otters are a species that many other creatures in the ecosystem depend on, and their disappearance would have drastic consequences. Marine invertebrates can swarm a habitat and cause imbalance. Sea urchins are one such invertebrate who feed on kelp, without predators controlling their numbers, they can decimate kelp forests.  Another is crab, who feed on sea slugs – a species that graze on eelgrass and keep its blades free of algae and hence, healthy. By consuming large amounts of these invertebrates, sea otters control their populations and protect these important ecosystems.


A sea otter resting in a kelp forest.
Sea otters are essential to the kelp forest ecosystems in which many of them reside.

Sea otters may be fierce predators but they have a number of physiological characteristics that make them particularly vulnerable to disturbance.

Sea Otter Savvy

Why Do They Need Protecting?

During the 18th and 19th centuries, the booming fur trade took a serious toll on sea otter populations, as they were hunted to near extinction. Once residing continuously along the North Pacific Rim, southern sea otters have now reclaimed only about 13% of their historic range. These extant sea otter populations demonstrate low levels of genetic diversity, resulting in relatively weak populations. On top of this, sea otters have unique physiological characteristics that make them particularly sensitive to disturbances. For instance, sea otters spend much of their time resting to conserve energy to forage, and even small disturbances can severely disrupt this critical resting period. This is especially the case for sea otter mothers who need up to double the energy to provide for their young so swimming away from approaching humans exerts extra precious energy. Sea otters also lack the layer of blubber that many marine mammals possess, leaving them with little ability to store their precious energy as fat.  Understanding the threats that various human disturbances pose to these incredible creatures, then, is tremendously important to ensure that current and future populations can survive and thrive.

If you are keen to find out more about marine rewilding or rewilding in general, head over to our kelp restoration trial project page and rewilding blog.

A California sea otter recuperating its energy at Elkhorn Slough.
This project is part of the mission to keep Southern sea otter populations safe and above water.
Heart Image

the team behind the project

Team Member

Gena Bentall- Director, Senior Scientist, Sea Otter Savvy

Team Member

Hannah Kirkland - Mossy Earth

Team Member

Rosalind Searle - Mossy Earth

Sources & further reading

Peer Reviewed Research Section
  1. - Sea Otter SavvyExternal link
  2. Loss of genetic diversity in sea otters (Enhydra lutris) associated with the fur trade of the 18th and 19th centuries - Molecular EcologyExternal linkIcon Peer Review