Southern sea otter numbers have declined off the coast of California since their most recent high in 2016. According to data released by the California Department of Fish and Wildlife (CDFW) and U.S. Geological Survey (USGS), the three-year average population index this year dropped to 2,962, which is 166 sea otters fewer than the 2018 survey.
Southern sea otters are a protected species under the federal Endangered Species Act and Marine Mammal Protection Act and listed as a fully protected species under California law. Sea otters play a critical role in the nearshore marine ecosystem, serve as indicators of ocean health, and keep important elements of coastal ecosystems, like kelp forests and seagrass beds, in balance.
This year’s surveys found that the population of sea otters was largest in the central part of the species’ range, which is between Seaside and Cayucos, and that the five-year trend for this portion of the range remains positive. The specific areas where the population trends are most negative, from Pigeon Point to north Monterey Bay and most areas south of Cayucos, coincide with the areas known for high shark bite mortality.
“One factor likely contributing to the positive trend in the central range is the recent increase in sea otter prey availability: sea urchins and mussels,” said Mike Harris, senior environmental scientist with CDFW.
Last year’s numbers were significant in that they marked the third consecutive year the three-year average of the population index had exceeded 3,090 – which is a condition of the U.S. Fish and Wildlife Service’s (USFWS) Southern Sea Otter Recovery Plan to consider the species for delisting under the Endangered Species Act.
The U.S. Fish and Wildlife Service is currently reviewing the status of the southern sea otter, gathering the best available scientific and commercial data regarding the species, including population trends, distribution, demographics, genetics, habitat conditions, threats and conservation measures.
“By thoroughly reviewing the best available data, we’ll have a better understanding of all factors influencing the sustainability of the southern sea otters in the wild,” said Lilian Carswell, southern sea otter recovery coordinator for the USFWS. “These remarkable marine mammals continue to encounter hurdles, like shark bite mortality, that limit their ability to expand into areas where they historically thrived.”
Scientists from the CDFW, USGS, and Monterey Bay Aquarium have conducted a range-wide census of southern sea otter populations annually since the 1980s. Researchers compute the annual population index and evaluate population trends, providing the USFWS and other resource agencies with insight into southern sea otter abundance and distribution.
In addition to the sea otter population along the mainland California coast, which stretches from about Point Año Nuevo in San Mateo County to Gaviota State Beach in Santa Barbara County, USGS and partners also survey the sea otters at San Nicolas Island in the Southern California Bight. This population, established by introducing sea otters back into the area in the late 1980s, struggled at low numbers through the 1990s. However, over the last decade, the population has grown rapidly at an average rate of about 10 percent per year.
The five-year average trend in the range-wide index, including both the mainland range and the San Nicolas Island populations, was flat at 0.12 percent growth per year. The northern range was not calculated because of limitations in survey resources required to survey areas north of the range from the previous year (2018); however, the southern range expanded slightly by 0.5 kilometer, where a negligible trend in sea otter counts (0.55 percent growth per year) corresponded to the lack of meaningful population range expansion at the southern periphery.
“The total raw count of sea otters along the mainland coast was lower this year even though the survey was conducted under more favorable viewing conditions this year than the previous two years,” said Brian Hatfield, USGS wildlife biologist.
The sea otter survey and stranding programs are one part of a larger research program investigating sea otters and their role as predators in coastal ecosystems. USGS researchers monitor changes in the kelp forest ecosystems that provide sea otters with suitable resting and feeding habitat and collaborates with CDFW, and the Monterey Bay Aquarium, The Marine Mammal Center, and others in implementing a sea otter stranding network. These efforts inform and support effective management of sea otter populations, helping to guide them toward recovery.
The annual population index is calculated from visual surveys conducted via telescope observations from shore and via low-flying aircraft along the California coastline by researchers, students and volunteers from CDFW’s Office of Spill Prevention and Response, Monterey Bay Aquarium, USGS and USFWS.
Sea otters were presumed extinct in California after the fur trade years. Although a small group of sea otters near Point Sur was known to locals and state employees when the state of California granted them “fully protected” status in 1913, they were rediscovered by the public in the late 1930s, when up to 150 animals were documented near Bixby Creek north of Big Sur.
Sea otters are considered a keystone species of rocky subtidal ecosystems because they prey on sea urchins, which if left unchecked can decimate kelp beds, eliminating the habitat for the many species and systems that rely on them.
Sea otters help to maintain the biodiversity of Pacific nearshore ecosystems, which support diverse wildlife species and provide economic support for coastal communities.
Sea otters can give scientists clues on the state of nearshore ecosystems, since they feed and live near the coast and often are the first predators exposed to pollutants and pathogens washed down from coastlands.