STEVE INSKEEP, HOST:
On the other coast, sea life is making a comeback. For the first time since the Second World War, Harbor porpoises are showing up in growing numbers in San Francisco. Researchers are trying to understand why they've returned to the bay. Lauren Sommer reports from member station KQED.
LAUREN SOMMER, BYLINE: The walkway across the Golden Gate Bridge is almost always packed with people taking photos. But Bill Keener isn't here for snapshots of the stunning views.
BILL KEENER: There's a porpoise right here, coming very, very close.
SOMMER: Keener is with Golden Gate Cetacean Research, a non-profit group. He's aiming his massive telephoto lens at a dark shape in the water 200 feet below.
KEENER: Here's a mother and calf right here coming straight at us.
SOMMER: Harbor porpoises have dark gray backs and they're about five feet long - smaller than most of their dolphin relatives.
KEENER: Look at that, that one's on its side. The porpoise turned on its side; it's spinning and it's feeding.
SOMMER: They're feeding in the middle of a busy shipping lane. Seeing this behavior is huge for Keener. Harbor porpoises are notoriously shy in the open ocean. But the fact that they're here at all is what's most remarkable.
KEENER: All right, I'm going to try to get some photos of this one.
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SOMMER: Keener and his colleagues have indentified 250 porpoises with their photos by looking for unique scars on the animals. When they first started working on the bridge, the patrol officers took notice.
KEENER: We're starting down at the water for hours. They start getting worried about us.
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KEENER: But they know us now; they know what we're doing.
SOMMER: Of course, the big question is why harbor porpoises disappeared in the first place. Keener says the bay has always been porpoise habitat. Sightings were common until the 1930s.
KEENER: We don't really have any reports from around World War II. And there was a lot of things going on during World War II that could have caused that.
SOMMER: San Francisco Bay became a wartime port. It was a major ship-building center.
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: Into fog-shrouded San Francisco Bay, where thousands lined the hills and piers, sailed 14 warships beneath crowd-jammed Golden Gate Bridge.
SOMMER: The Navy strung a seven-mile-long net underwater across the opening of the bay to keep out Japanese submarines. Hundreds of mines were planted in the waters outside the Golden Gate. But there's a bigger change that may have driven them away.
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KEENER: Clearing up a little, Jon.
JONATHAN STERN: It is clearing up.
SOMMER: I'm heading toward the Golden Gate Bridge on a 22-foot boat with Keener and Jonathan Stern, a whale researcher at San Francisco State University. The bay we're gliding over today is a far cry from the bay of the 1950s and '60s. As the region boomed, so did water pollution.
KEENER: I remember coming across the Bay Bridge when I was very young and it would just smell. It would stink.
SOMMER: After the Clean Water Act was passed in 1972, the bay's water quality began to improve. But it took time for the food web to come back. And, Stern says, maybe the porpoises had to rediscover the bay.
STERN: Because over 60 years we're talking about a number of generations of porpoises. So it's quite likely that San Francisco Bay as a habitat was sort out of the institutional memory.
SOMMER: The boat slows down as we pass under the bridge.
KEENER: There's porpoises between us and the south tower at 200 yards.
SOMMER: Keener and Stern have a special permit to approach the porpoises.
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SOMMER: We wait, listening for them to surface.
KEENER: I just heard one here. Here's a cow-calf pair close to the boat and we'll hear this puff.
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KEENER: The old time sailors used to call them puffing pigs. That's the exhalation.
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KEENER: Wow, that was five or six feet from the boat.
SOMMER: The porpoises seem calm around boats in the bay, which Stern says will let researchers study their life cycle and social structure.
STERN: It's one of those very few good news environmental stories. And it's like in our backyard. It gives one hope.
SOMMER: It also gives researchers a chance to study how porpoises will react to the America's Cup race, which comes to the Bay Area in two years.
For NPR News, I'm Lauren Sommer on San Francisco Bay. Transcript provided by NPR, Copyright National Public Radio.