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Scientists Uncover the Hidden Life of Otters in Estuaries

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An otter population in Elkhorn Slough is showing scientists an otter lifestyle that has been little seen. Photo: USGS

Sea otters living in Elkhorn Slough are giving this generation of scientists their first look at what life is like for otters in ecosystems other than kelp forests. Photo: Elkhorn Slough National Estuarine Research Reserve

By Melissae Fellet | KUSP News

A group of otters has traded life in a kelp forest for a home in Elkhorn Slough. Although otters have historically lived in estuaries, this is the first opportunity that scientists have had to learn about the details of their daily lives in estuaries. The observations will give them a preview of what might happen as otters expand their range and move into other estuaries.

A sea otter at Elkhorn Slough, near Moss Landing, Calif. U.S. Geological Survey/photo by Robert Scoles/ESNERR/NOAA

A sea otter at Elkhorn Slough, near Moss Landing, Calif. U.S. Geological Survey/photo by Robert Scoles/ESNERR/NOAA

More Than Just a Living

Brent Hughes looks through binoculars at a sea otter worming its way off the marsh before slipping into a tidal channel in Elkhorn Slough. Its movement doesn’t look very natural, he says, but they can move fast, even on land. A graduate student at the University of California in Santa Cruz, Hughes has been studying how the presence of otters benefits the health of seagrass beds in the estuary. When volunteers saw otters spending time in the marshes too, Hughes turned his attention toward their lives in the pickleweed.

“We are noticing and witnessing sea otters not only using the system, but they’re using it really, really well,” he says. “It’s almost like it’s a natural thing for them.”

The sea otter population was practically decimated in the 19th century due to the fur trade in the North Pacific Ocean. However, clusters of otters survived, and it’s thought that a small group of about 50 animals living near Big Sur helped restart the otter population in California. Over the past century, the animals have been expanding their range north and south from there.

Discovering a Different Otter Lifestyle

Over the past couple of decades, the otters reached Monterey Bay and occupied Elkhorn Slough. Today, almost 3000 otters live in the kelp forests between Pigeon Point and Point Conception, and scientists have been surveying them yearly for decades.

Tim Tinker, an ecologist with the United States Geological Survey, says that the surveys revealed a group of male otters hanging out near the mouth of the slough before going to feed in Monterey Bay, but the scientists didn’t pay much attention to the observation. Then the scientists recognized that a second group of otters was living in the estuary full-time.

“They were living this completely different life as salt marsh, eelgrass type otters that didn’t live in kelp beds, that didn’t do all the things that we’re used to seeing sea otters do,” Tinker says.

He recently helped capture and tag 20 of the 100-or-so otters living in the slough. He noticed estuary otters had more fat under their skin than those that live off the coast. And the teeth of the estuary otters lacked purple stains common to otters eating sea urchins in kelp forests.

“Sea otters in estuaries is sort of a whole new chapter in our understanding of sea otter biology,” Tinker says. “We’re starting from scratch.”

Together with his colleagues, he hopes to study the movement, daily activity and feeding habits of the tagged otters for three years. Though these behaviors are new to this generation of scientists, otters have historically lived in estuaries. Elkhorn Slough happens to be the first one they’ve encountered as the population recovers. And compared to the outer coast, the natural reserve can be a comfortable place for otters to live.

“When otters move into these systems, they’re oftentimes welcomed with open arms because there’s an abundance of food, there’s habitat, and there’s protection from predation,” Hughes says.

A New Home Has New Hazards

But living in an estuary could also have drawbacks. The sediments in Elkhorn Slough contain residues of pesticides like DDT. As otters dig up clams in the mud, they re-suspend those contaminants in the water. The clams they eat could also be contaminated. Tinker says it’s too soon to tell if that will affect the health of the otters in the slough, though the information could be useful as the otter population grows.

“As they move north and south and encounter other estuaries, it could be that a fairly significant proportion or chunk of the sea otter population in the future is going to be living in estuaries, not just in the kelp forests along the outer coast,” Tinker says.

San Francisco Bay is one of the largest estuaries in the world, and otters have lived there before. Should they move in there again, the research in Elkhorn Slough will help scientists understand how the otters’ presence could affect ecosystems in the bay.

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