New study shows sea otters helped prevent widespread decline of California’s kelp forests over the past century

Monterey Bay Aquarium researchers strengthen the link between sea otters and the long-term health of California’s kelp forests in a new study released today. The article, published in the journal Climate PLOSfinds that sea otter population growth over the past century has improved the resilience of kelp forests in the state.

This discovery reinforces the importance of conservation and recovery of the endangered southern sea otter and highlights a potential nature-based solution for restoring kelp forests along the California coast, and may -be beyond.

The study found dramatic changes in the regional kelp canopy over a 100-year period, from 1910 to 2016. During this period, there was a significant increase in kelp forest canopy along the coast Central, the only region in California where southern sea otters have survived after being hunted. nearly extinct because of their fur by the 1800s. On a century scale, the species’ favorable impact on kelp forests along the central coast has nearly offset kelp losses along the northern and in Southern California, leading to a slight overall statewide decline during this period.

“Our study showed that kelp forests are more extensive and more resilient to climate change where sea otters have reoccupied California’s coastline over the past century. Where sea otters are absent, kelp forests have declined dramatically. In fact, we found that sea otter population density was also high. “This is the strongest predictor of change in kelp cover over this hundred-year period. years,” said lead author Teri Nicholson, senior research biologist in the Monterey Bay Aquarium’s sea otter program.

Aquarium scientists used historical studies of kelp forests dating back to the early 1900s to make detailed estimates of canopy extent, biomass and carbon storage, while correcting for annual variations and differences in survey methods.

This allowed scientists to examine trends in California’s kelp forests over a longer period of time, going back more than 60 years before available data from modern surveys based on aerial or satellite images. The team then compared the corrected and conservative historical estimates to contemporary datasets and used a machine learning framework to assess key drivers of change over the past century.

“Using historical maps provided an important opportunity to help us examine long-term trends in kelp forests,” said Jess Fujii, manager of the Monterey Bay Aquarium’s Sea Otter program. “This broader view is important for understanding climate change trends and developing effective science-based conservation strategies.”

Statewide, data showed only a 6% decline in kelp cover between 1910 and 2016. Regional changes, however, were more significant. Kelp cover decreased in the northern and southern regions by 63 and 52%, respectively. In contrast, it increased almost everywhere on the central coast, gaining about 56% of kelp forest cover.

Although the modeling showed that sea otter population density was the best indicator of change in kelp cover, it also identified other factors, including extreme marine heat due to climate change.

“Today, extreme heat in the ocean is intense and persistent. Over the past decade, this threat now affects more than half of the ocean’s surface,” said Kyle Van Houtan, a research scientist at the University Duke and lead author of the study.

“This is a major problem for kelp forests because chronic heat stress compromises the growth and health of kelp. Ecosystems are complex, and to give them the best chance of surviving these extreme changes, they need all of their components. Sea otters, of course, are extremely influential to Pacific kelp forests. Historical studies like this are a crucial demonstration of these long-term dynamics.

Healthy kelp forest ecosystems provide many benefits, including serving as breeding grounds for fisheries, reducing coastal erosion caused by storms, and contributing to carbon storage. The study indicates that returning otters to areas of their historic range could help recover kelp forests and restore their benefits in more places along the California coast.