Using archaeology to uncover sea otters’ historical habitats

When a hungry sea otter uses a rock to crack open a tasty mussel, it’s doing something unique among marine mammals: using a stone tool. Researchers are now revealing how this behavior makes it possible to study sea otters’ past through the lens of archaeology.

A sea otter cracks open a mussel shell using a shoreline “anvil rock”.

Sea otters use large stationary stones like anvils, to smash open mussel shells. Their hammering leaves distinctive marks on the rock. An ideal place to document this behavior is just a short drive up the coast from the Aquarium, near Elkhorn Slough.

It’s an area that Monterey Bay Aquarium researchers like Jessica Fujii have monitored for years. Southern sea otters are a threatened species, and many of the orphaned pups reared by the Aquarium’s one-of-a-kind surrogacy program end up joining the wild population in Moss Landing Harbor. Years of data on sea otter foraging gathered by Aquarium researchers showed otters like to dine near the Bennett Slough culverts at the north end of the harbor.

“We have observations spanning about ten years in this location, both on wild animals that have no tags, and also sea otters that were raised at the Aquarium and successfully transitioned to the wild,” says Jessica, who has also monitored wild sea otters (while avoiding brown bears) in Alaska.

Using science to save ocean wildlife

The Monterey Bay Aquarium is a science-driven organization, and rigorous science underpins all of our public policy, research and education programs. Much of our research centers on marine life that visitors can also see in our exhibits – from sea otters to sharks and tunas, even our giant kelp forest. Here’s some of what we’ve learned over the past 30-plus years that is contributing to conservation of key ocean species and ecosystems.

A sea otter works to crack a mussel shell open on a rock off the coast of Moss Landing, California. Photo by Jessica Fujii

Sea otters crack open tool-use secrets

Revolutionary female scientist Jane Goodall was the first person to discover that chimps use tools and live within complex social systems. Our team of female researchers are walking in Jane’s footsteps with their recent studies on use of tools by another mammal: the sea otter. When observing sea otters along the Monterey Peninsula, sometimes we can hear a “crack, crack, crack!” above the roar of the tide. That sound comes from sea otters using rocks and other tools to open prey items, such as crabs or bivalves, as they float on their backs. Sea otters are avid tool users, but until recently not much was known about how sea otters choose their tools, what aspects of their environments influence tool use, or whether they teach tool use to other otters. The Aquarium’s decades of research into sea otter behavior provided years of observations of sea otter foraging and tool-use behavior, including sea otter pups pounding empty fists against their chests. Could such activity be instinctual? Research Biologist Jessica Fujii has devoted much of her young career to studying the frequency and types of tools used and whether tool use can be coded in sea otter genes. Jessica is looking ahead to see how sea otters learn, teach, and eventually master tool use in the wild.

A sea otter rests in an eelgrass bed in Elkhorn Slough National
Estuarine Research Reserve. Sea otters contribute to the recovery of eelgrass and ecosystem health in this vital wetland on Monterey Bay. Photo by Ron Eby.

Sea otter surrogacy helps restore Elkhorn Slough

With 15 years of experience rescuing, rehabilitating, and then releasing surrogate-reared sea otters into Elkhorn Slough, an estuary near Moss Landing, California, the sea otter research team at the Aquarium began to wonder how and if their work was affecting the otter population there. Does releasing a few animals into the slough each year really make any difference? After crunching some serious numbers from the surrogacy program and the U.S. Geological Survey’s (USGS) annual sea otter census, the researchers discovered that it did. Nearly 60 percent of the 140 or so sea otters living in Elkhorn Slough today are there as a result of the Aquarium’s surrogacy program. While we’d known that sea otters served as ecosystem engineers for the giant kelp forests in Monterey Bay, we have now documented that sea otters in Elkhorn Slough are restoring the health and biodiversity of the estuary. This gives us further insights into how sea otters may contribute to coastal ecosystem resilience.

Sea otters are handy with tools

What makes people different from other animals? Scientists used to think the ability to make and use tools was a distinguishing characteristic. That changed in the 1960s, when Jane Goodall observed chimpanzees using sticks to fish termites out of mounds. Now, crows, dolphins and sea otters make the short list of creatures that use tools.

Sea otters dive in shallow coastal waters to collect hard-shelled prey like sea urchins, mussels, abalones, clams and snails. Some shells, like the calcium carbonate armor that protects snails, are harder to crack than others—so otters sometimes use rocks as anvils to break them open.