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A giant sea cow once roamed California’s coast. Its disappearance is linked to major transformation

San Francisco Chronicle 12/1/2022 By Tara Duggan

An enormous marine mammal once roamed the California coast, floating along the ocean’s surface and grazing only on plants. It weighed twice as much as a hippo and had skin so rough and scaly that it was compared to tree bark.

The extinct sea beast, called a Steller’s sea cow, helped the vital kelp forest thrive off the Pacific Coast, a new report from the California Academy of Sciences says.

Before disappearing in the 1700s, when Europeans hunted it to extinction, the sea cow inhabited the West Coast for millions of years, growing as large as 25 feet long and weighing 4 tons. The mega-herbivore recently captured the attention of Cal Academy scientists looking into the causes and possible solutions of kelp forest decline in California and throughout the Pacific.

The vast underwater forests play a crucial role in the marine ecosystem, protecting the coast from storms, absorbing carbon and creating habitat for fish and marine mammals.

The researchers say the sea cow helped the kelp forest grow more robust, even when eating its weight in seaweed, because it grazed kelp near the surface of the water. That allowed more light to penetrate, encouraging more growth of both kelp and other types of algae, say the authors of the study, which was published last week in Frontiers in Ecology and Evolution.

“If you spend time in a kelp forest today, at least a healthy one, one of the impressions we take away is it’s dark, almost cathedral like, and the light is filtering down. The kelp absorbs most of the incoming sunlight right at the surface,” said Peter Roopnarine, co-author of the study and a curator of geology at the California Academy of Sciences. “If the sea cow was grazing the canopy, there would have been more light penetration.”

Knowledge of the Steller’s sea cow is based on fossils found from Baja California to Northern Alaska, skeletons collected when they were hunted and historic observations made in Alaska, especially by scientist Georg Wilhelm Steller, who gave it its name. The animal may have had a peak population of 200,000, according to a study published this year.

Like its closest living relative, the dugong, a marine mammal in the Western Pacific and Indian oceans that is in the same family as manatees, the beast had a long, rotund body, short front flippers and a whalelike tail. Its skin was “more the bark of an old oak tree, than the skin of an animal,” Steller reportedly observed. It also had a downturned snout for feeding on kelp while floating on the surface, Roopnarine said.

“They were incredibly buoyant to the point where they could not submerge,” Roopnarine said. “Whatever they ate would have had to be on the surface.”

Most of the focus on the disappearance of the kelp forest, which shrank by 95% from 2014 to 2019 in California’s Sonoma-Mendocino coast, has been on purple sea urchins that have taken over the habitat because of a loss of predators. Those predators used to be sunflower sea stars, until they were struck by disease starting in 2013, and sea otters, which were hunted to extinction on the North Coast over a century ago. Kelp forests also were hurt by a marine heat wave of 2014-16 that was linked to climate change.

But looking at a historical perspective before those human-caused changes is important to understanding the larger picture of kelp forests, Roopnarine said.

“As we try to understand our ecological systems today because we’re trying to preserve these systems, we have to think about how what we’re looking at has been transformed by climate change, habitat destruction and other impacts,” he said.

The study’s authors used mathematical models to find out what the impact of the sea cow was on the kelp forests. Overall, Roopnarine said, the mega-herbivore made those forests more resilient to marine heat waves and other changes in the environment. The sea cows’ disappearance may be a missing link in understanding why the kelp forest is declining, they said.

Tara Duggan (she/her) is a San Francisco Chronicle staff writer. Email: tduggan@sfchronicle.com Twitter: @taraduggan

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