On any given day, dozens of harbor porpoises are swimming right under the Golden Gate Bridge.
Bill Keener knows many of them by sight. In the last two and a half years Keener and a team of observers have photographed more than 200 individual porpoises.
The porpoises do not have names, but they do have code numbers. There's handsome SFB 78 with a rugged scar, the dutiful mother SFB 34 with her jet black calf and SFB 179, whose tail may have been bitten by a shark.
Harbor porpoises are about five feet long, roughly half the size of a dolphin. They lived in the San Francisco Bay for thousands of years. But in World War II, the military strung a net across the Golden Gate Bridge and placed mines around the entrance. They were blocking enemy submarines, but the porpoises were blocked too.
The nets and mines were eventually removed, but as the bay got more polluted, the porpoises did not come back.
That is until 2008, when Keener's colleague first spotted some near Sausalito.
"It was very exciting," Keener said. "And it wasn't just one animal, it was mothers with calves, so we knew there's family groups, and they are coming in to feed."
Keener is an environmental lawyer and former head of the Marine Mammal Center. He and a handful of other experts now have a permit to study porpoises and figure out why they would return to an urban waterway full of boat traffic.
"It could be the bay is cleaner and there are more fish," Keener said. "The bay is in a particularly productive period right now and that's very good and to have naturally animals come back into an area is pretty unusual."
And even more unusual was a photograph Keener took April 30 of a rare white porpoise.
"It appears to be the very first white harbor porpoise ever recorded from the Pacific Ocean," Keener said.
In the last 100 years scientists have only documented six other white harbor porpoises anywhere in the world.
The team is hoping this one will come back. And if it does, it's likely to be visible again from the Golden Gate Bridge.
"It's one of the best places in the world to do porpoise research because unlike going out on a boat where you disturb them by your very presence, they don't know you are on the bridge," Keener said.
Observers note the animals' scars and coloring to figure out who is who in the porpoise world. The porpoise photos reveal a rich social life with lots of interaction.
Researchers believe one reason the animals are thriving is what is happening under water. Along the Northern California coast there are three marine sanctuaries protected by the federal government.
"All of the animals, including the corals on the bottom, the worms in the mud, as well as the porpoises, it all cascades into being more positive for a healthier environment for all the animals," Gulf of Farallones Marine Sanctuary spokesperson Jan Roletto said.
Casual observers can see some of that, including porpoises and a lot more, just by walking out on the Golden Gate Bridge.
"It's a fantastic platform to view wildlife and I think local people don't realize that," Keener said.
Harbor porpoises have also appeared in other areas around the central part of San Francisco Bay. Anyone who sees one, either a porpoise or a dolphin, is asked to report it to the Cetacean Research Group.
Written and produced by Jennifer Olney