The Science of Sustainability

Sea Lions, Herring, and Climate Change

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San Francisco's Pier 39 is home to only a few sea lions this summer.

I spent Saturday sightseeing in San Francisco with a friend visiting from out of town, and I thought I’d check in on the sea lions at Pier 39. Just a few years ago, there were about 1600 of them, slithering on and off the wooden docks, basking in the sun, and barking at one another. Then in 2009, most of them swam away, as QUEST blogger Amy Gotliffe explained. The sea lions’ favorite food, herring, was in short supply, so they went to Oregon to feast on anchovies and salmon. Now the herring are making a comeback—will the sea lions return too?

This weekend the floating wooden palates at Pier 39 were mostly bare; there were perhaps a dozen sea lions. We would expect the sea lion numbers to be low this time of year, herring or no herring. In the summer, sea lions travel down south (the Channel Islands, San Diego, Baja) to breed. But there were still fewer sea lions at Pier 39 than in summers past.

Sea lions will eat a lot of different prey items: octopus, squid, small sharks. But their bread and butter is herring, which have been hard to find in recent years. The herring fishery is the only fishery still in operation in San Francisco Bay, and it closed during the 2009/2010 season (December through April), because there were so few fish. This year, the herring fishery opened again, but with a lower quota than in the past, to allow the fish to recover.

There are a few hypotheses about why the herring numbers dipped so low in 2009. First, herring lay their eggs in the brackish waters of the estuaries around San Francisco Bay. Each female fish can lay up to 50,000 eggs, which are a prized commodity in Japan. However, the years leading up to 2009 were drought years, so the estuaries were saltier than usual. That may have affected the herrings’ spawning success. Second, the 2007 Cosco Busan oil spill may have affected herring health. Researchers found oil-soaked embryos, which were deformed. Third, herring declines may be the result of climate change. As surface waters get warmer, there is less mixing with cold, nutrient-rich water from the bottom of the ocean. There are also big patches of the ocean that have very little oxygen. These hypoxic zones are deadly to their inhabitants, and are affecting many marine species.

However, the herring appear to be making a comeback, possibly because the past few years have been wet and the estuaries are sufficiently fresh, or because the spilled oil has been flushed from the Bay. Time will tell whether the sea lions follow their food and return to Pier 39. I hope they come back—along with the twists and turns of Lombard Street and the Golden Gate Bridge, the Embarcadero’s sea lions are one of my favorite San Francisco treasures to show off to visiting friends.

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Category: Biology, Climate, Environment

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Jennifer Skene

About the Author ()

Jennifer Skene develops curriculum on climate change and ocean sciences at the Lawrence Hall of Science and teaches biology and science communication at Mills College and the University of California Berkeley. She has a degree in biology from Brown University and a Ph.D. in Integrative Biology from UC Berkeley. She started working with QUEST in 2008 as an intern. She has written for the Berkeley Science Review and the UC Museum of Paleontology’s Understanding Evolution and Understanding Science websites.