Sea otters helping hold up California’s kelp forest

MONTEREY — Within the last century, the recovery of southern sea otters, the unofficial mascot of Central California, has contributed to the health of California’s underwater forests according to a new research paper from the Monterey Bay Aquarium.

Researchers found that off the Central Coast, kelp forests are more resilient to climate change and have increased where southern sea otters are present, compared to the decreases Northern and Southern California without sea otters.

“This particular study really demonstrates the value of using historical data to better understand important ecological relationships between top predators, other environmental factors, and changes in our coastal ecosystems, like kelp forests,” said Teri Nicholson, a senior research biologist for the Monterey Bay Aquarium Sea Otter Program and the author of the research paper. “It also helps us and others to develop more informed and effective conservation strategies moving forward.”

The historical data used ship surveys of kelp from 1910-1912 and compared it to aerial surveys of kelp in a similar three-year period of 2014-2016 from the California Department of Fish and Wildlife.

Aquarium researchers found that in Northern and Southern California, within the last century, the kelp forest has declined drastically by 63 and 52 percent, respectively. Meanwhile, in Central California, with the presence of otters, kelp forests have increased by 56 percent.

“The advantage of using the historical data is they’re able to then look at these very broad scale changes, like increases in human density, increases in temperature over a century scale, and then the recovery of sea otters,” said Dr. Tim Tinker, adjunct professor at UC Santa Cruz.

“So when you take this sort of big step back and look at how California has changed over the last 100-120 years, you see the areas that have increased in kelp abundance, mostly explained by where sea otters have recovered.”

One of the disadvantages of using historical data for analysis is the lack of finer details, said Tinker

“You lose a lot of the details of the things that affect kelp abundance month to month or year to year,” said Tinker.

Historical records have shown that the presence of sea otters does have a positive relationship in helping kelp forests come back.

As a result of the fur trade, sea otters were thought to be extinct in the early 1900s. This left many invertebrates like urchins, abalone, and crabs unchecked.

“The decline of otters here meant the growth of the urchin population and the decline of kelp,” said Steve Palumbi, professor and marine biologist at Hopkins Marine Station.

Luckily in 1938, a small raft of around 50 sea otters were found in Big Sur. Over the following years, the population increased and began to spread along the Central Coast.

“When the otters began to come back in the 1960s in Monterey Bay, they ate through all the urchins, and then the kelp forest began to come back,” said Palumbi.

When unchecked, urchins can mow down kelp beds creating “urchin barrens.”

Within the last decade, with detailed research, kelp forests have declined dramatically in Northern and Southern California after major ecological events, but other factors buffer the decline in kelp and help forests rebound.

Kelp forest ecosystems also have other urchin predators than just otters and they can also play a beneficial role.

“One of the things that is now thought pretty well to be true up and down the coast is that there are other predators besides sea otters that are playing a strong role,” said Palumbi. “Sunstars in the north and in the south, there’s the spiny lobster, California sheephead, and wolf eels.”

However, in 2013, the sunflower star started to die out due to a disease called Sea Star Wasting Syndrome. The West Coast was also hit with a marine heat wave deemed “the Blob” that peaked in 2014 and 2015.

With the lack of predators, urchins started to feed on kelp beds, and during an extreme heat wave, the kelp in Northern California was almost completely decimated.

In Southern California, the lack of rocky bottoms where kelp thrives, increased human population, and coastal development over the last century could also be factors in the decline of kelp.

Kelp requires a hard substrate to latch upon. In soft sediment, like sand and mud, kelp forests are almost non-existent.

Extreme heatwaves also have a more significant effect on kelp in the southern waters than in Central and Northern California.

Currently, where kelp is present in Southern California, there are still urchin predators and fisheries that keep the spiny creatures away from kelp.

“Kelp is affected by more than just otters and sea urchins. Kelp is affected by runoff and sedimentation,” said Palumbi. “Over the course of that whole century, a bunch of other things have changed.”

Southern sea otters are mainly found off the Central Coast.

Where there is a sea otter population, otters can affect the whole ecosystem, and the researchers say they are the primary driver of kelp changes.

Sea otters don’t have a thick blubber layer to stay warm in California’s cold water compared to other marine mammals, like seals and sea lions. Instead, they have thick fur with one million hairs per square inch on certain parts of their body and an extremely high metabolism.

That metabolism and a colossal appetite make them a keystone species with strong interactions in the ecosystem.