Sea otters can help conserve vital kelp forests

Historical analysis shows that the marine mammals likely kept some of California’s kelp forests growing over the past 100 years.
A sea otter resting in a kelp bed in Monterey Bay, California.
A sea otter resting in a kelp bed in Monterey Bay, California. François Gohier/VW Pics/Universal Images Group via Getty Images

Sea otters are more than just photogenic fluff balls known for floating on their backs with buoyant fur and eating about 25 percent of their body weight in one day. They are also potentially strengthening the health of California’s kelp forests. A study published January 18 in the journal PLOS Climate found that the sea otter population is significant in regions where kelp forest canopy has increased over more than a century. The study suggests that sea otter populations kept the kelp forests more resilient. This research reinforces why conservation and recovery of the threatened southern sea otter is important and highlights some solutions to restoring California’s famed kelp forests.   

[Related: These otters learn how to snag snacks by watching their friends.]

Why this big seaweed is a big deal

Kelp is a large, brown algae that lives in relatively shallow waters close to shore. They grow in large groups that resemble forests on land. These kelp forests are important because they contain a greater variety and higher diversity of plants and animals than most of almost any other community in the world’s oceans. Fish, sea birds, seals, small marine invertebrates, and more use the thick blades of kelp as a safe shelter for their young and even take shelter in them during storms. They also provide a barrier against coastal erosion for the mainland. In the Western Pacific Ocean, Kelp forests predominantly grow from Alaska and Canada south to the waters of Baja California, Mexico. 

According to the National Oceanic and Atmospheric Administration (NOAA), they also provide humans with a habitat for commercially important fishery species including kelp bass and black rockfish. 

Some of the primary threats to kelp forests include runoff from chemicals and sewage on land, rising ocean temperatures, overharvesting, and invasive species. 

More sea otters, more kelp

The study looked at changes to the kelp canopy from 1910 to 2016. During this time period, the central coast of California was the only region to see a significant increase in kelp forest canopy. This is also the only region in the state that had a population of southern sea otters. Coveted for their warm fur, the mammals were nearly hunted to the extinction during the 19th Century. 

Over the 100-year period, the southern sea otter’s favorable impact on kelp forests along the central coast almost completely compensated for the kelp losses along both southern and northern California.

“Our study showed that kelp forests are more extensive and resilient to climate change where sea otters have reoccupied the California coastline during the last century. Where sea otters are absent, kelp forests have declined dramatically. In fact, we found sea otter population density as the strongest predictor of change in kelp canopy coverage across this hundred-year span,” study co-author and Monterey Bay Aquarium Sea Otter Program research biologist Teri Nicholson said in a statement

The study also noted similar trends in the Channel Islands, with kelp canopy increasing where sea otters were present and a similar assessment in Alaska also showed that sea otters help maintain kelp forests. The otters are likely an important carnivore in degraded ecosystems where there were previous predator-prey imbalances. They can eat sea urchins and other organisms that could overpopulate the ecosystem and hurt the kelp.

[Related: Why seaweed farming could be the next big thing in sustainability.]

The team analyzed historical surveys of kelp forests dating back to the early 1900s to generate estimates of kelp canopy extent, biomass, and carbon storage. Their method also corrected for year-to-year variation and differences in surveying methods. Looking at these records allowed the team to study California’s kelp forest trends over a longer time period. These records went back more than 60 years before aerial and satellite imagery was available for studying kelp canopies. They took these historical estimates and compared them to contemporary datasets and used computer models to assess what has been driving changes over the last century.

“The use of historical maps provided an important opportunity to help us examine long-term kelp forest trends,” Monterey Bay Aquarium Sea Otter Program Manager Jess Fujii said in a statement. “This broader view is important for understanding trends related to climate change, and developing effective science-based conservation strategies.”

The study found that statewide, there was only a six percent decline in kelp canopy from 1910 to 2016. However, the regional changes were more sizable. In northern regions, it decreased by 63 percent. The southern region saw a 52 percent decrease in kelp canopy. The kelp canopy increased nearly everywhere throughout the central coast which gained an estimated 56 percent of kelp forest.

The computer modeling showed that sea otter population density was the strongest predictor in changes to kelp forest coverage. It also revealed that rising marine temperatures due to human-caused climate change were other factors in kelp loss. 

“Today, extreme heat in the ocean is intense and persistent. Beginning a decade ago, this threat now affects more than half the ocean’s surface,” study co-author and Duke University research scientist Kyle Van Houtan said in a statement. “This is a major problem for kelp forests as chronic temperature stress undermines kelp growth and health. Ecosystems are complex, and to give them their best chance at surviving these extreme changes, they need all their component parts. Sea otters, of course, are hugely influential for Pacific kelp forests. Historical studies like this are a crucial demonstration of this dynamic over the long term.”