Conservation & Science

Sea otter insights: monitoring apex predators in Elkhorn Slough

Michelle Staedler stands atop a hill above Elkhorn Slough. It’s low tide – low enough to see the green eelgrass just under the surface of the water. Michelle peers through a spotting scope with a directional radio antenna attached. Static hums on the radio until it’s broken by a quiet blip…blip…blip coming from a radio tag inside the abdomen of a sea otter. Michelle records the time and notes that the otter she’s been watching for the last fifteen minutes, 501, has come up with a clam that she shares with her pup, floating by her side.

Michelle Staedler with the  Sea Otter Program at Monterey Bay Aquarium, monitors the behavior of sea otters in Elkhorn Slough. Photo by Cynthia McKelvey
Michelle Staedler with the Sea Otter Program at Monterey Bay Aquarium, monitors the behavior of sea otters in Elkhorn Slough. Photo by Cynthia McKelvey

Michelle is recording foraging data on Otter 501, perhaps the most famous sea otter in the  history of the Monterey Bay Aquarium’s otter surrogacy program. Rescued as a pup by the Aquarium and raised in captivity, she was successfully released in 2011 into Elkhorn Slough, a major estuary system in Moss Landing that feeds into Monterey Bay. Otter 501 has gone on to raise several pups of her own in the slough, where many of her species have come to flourish.

Michelle and her collaborators at the U.S. Geological Survey and the Elkhorn Slough National Estuarine Research Reserve have been counting otters in the slough since September 2013 as part of a population monitoring project. She has conducted several research projects on sea otter behavior since she began working with the Aquarium nearly 30 years ago. Her work focuses on sea otter mothers, their pups and how they feed. Foraging data gathered in the slough has proven particularly useful to ecological research.

Crabs are a favorite prey item for sea otters in Elkhorn Slough. By eating the crabs, otters help restore the health of the slough ecosystem. (Photo by Ron Eby)
Crabs are a favorite prey item for sea otters in Elkhorn Slough. By eating the crabs, otters help restore the health of the slough ecosystem. (Photo by Ron Eby)

Sea otters are a keystone species – central to the overall health of ecosystems of which they’re a part. Like other top ocean predators, their presence helps maintain a diverse community of animals and plants. The web can unravel if otter numbers dwindle. That’s exactly what happened when they were hunted to near-extinction by fur traders in the 18th and 19th centuries.

In kelp forests, otters eat sea urchins and other grazing animals, keeping them from devouring the kelp. This allows the productive ecosystem to thrive.

Using several data sets, including Michelle’s extensive foraging data, researchers at the University of California, Santa Cruz discovered that otters play an equally important role in the slough.

The sea slug Phyllaplysia taylori, also known as the "eelgrass sea hare," feeds on algae growing on the leaves of eelgrass. (Photo by Brent Hughes)
The sea slug Phyllaplysia taylori, also known as the “eelgrass sea hare,” feeds on algae growing on the leaves of eelgrass. (Photo by Brent Hughes)

As the story goes, before the otter population bounced back in the mid-1980s, eelgrass beds in the slough were being smothered by algae that grew unchecked on the leaves, absorbing the sunlight eelgrass needed for photosynthesis. In a healthy ecosystem, snails, slugs and other invertebrates would eat the film of algae, cleaning the grass and allowing it to get the sunlight it needs. But those grazers were being devoured by crabs, which had few natural predators in the slough – that is until sea otters turned up and began gobbling up the crabs.

Brent Hughes, who led the UC Santa Cruz research team, said he couldn’t have cinched his conclusion without the help of Michelle’s data.

“In the slough, we have unprecedented coverage of what a top predator is doing in terms of the ecology of a system, the behavior, exploitation of resources and habitat use,” Brent said. “It’s pretty much unprecedented in the marine ecology world, and that’s because of all the work that Michelle and [Brent’s collaborator] Tim Tinker have been doing.”

It’s important that she and her colleagues continue to monitor the otters, as well.

“We’re looking at the population of otters here,” Michelle explained. “How many are here, what areas of the slough they use, how they take advantage of micro habitats.”

For example, she said, a pioneer population of about 20 sea otters used to live around the jetty system in Moss Landing Harbor, at the mouth of the slough. Over time, their numbers ballooned to over 100 animals, and she’s observed them moving farther and farther up into the slough. Because of the significant restorative impact the otters can have on the ecosystem, it’s critical for researchers keep an eye on them to see what happens if their numbers continue to grow, Brent said.

– by Cynthia McKelvey

Reference:

Hughes B.B., Eby R., Van Dyke E., Tinker, M.T., Marks, C.I., Johnson, K.S., Wasson K. (2013). “Recovery of a top predator mediates negative eutrophic effects on seagrass.” Proceedings of the National Academy of Sciences. 110(38).   15313–15318, doi: 10.1073/pnas.1302805110

Sea otter raft photo © Jane Vargas-Smith

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