Warmer waters have more bottlenose dolphins turning up in SF Bay

Kaimi, a bottlenose dolphin who has taken up residence in S.F. Bay, is a favorite of a canoe club’s members who often visit the marine mammal as she hangs out around a buoy off the old Alameda Naval Air Station. less Kaimi, a bottlenose dolphin who has taken up residence in S.F. Bay, is a favorite of a canoe club’s members who often visit the marine mammal as she hangs out around a buoy off the old Alameda Naval Air ... more Photo: Bill Keener, Golden Gate Cetacean Research Photo: Bill Keener, Golden Gate Cetacean Research Image 1 of / 4 Caption Close Warmer waters have more bottlenose dolphins turning up in SF Bay 1 / 4 Back to Gallery

Bottlenose dolphins are moving north from their warm-water haunts in the ocean waters off Southern California, and seaside observers are spotting more and more of them as far north as Mendocino.

Some seem to be taking up temporary residence inside San Francisco Bay, while others appear to be commuters from distant waters, marine biologists say.

Whether their movements mark another signal of a changing climate is still unknown, but the phenomenon is more than a curiosity, and the scientists are tracking the marine mammals closely.

And while the aggressive animals don’t normally attack their smaller harbor porpoise cousins, cases of aquatic murder that naturalists call “porpicides” are on the rise.

Scientists studying the lives and movements of the dolphin species, called Tursiops truncatus, recalled the startling change in their range over the past decade or so during a recent research meeting at San Francisco State University’s Romberg Center in Tiburon.

Once seen only rarely north of Santa Barbara, more than 400 bottlenose dolphins have recently been spotted north of Pescadero Point in San Mateo County, and nearly 100 of them have been identified as regular residents in local waters, said marine biologist William Keener of Golden Gate Cetacean Research.

Tracking individual dolphins as they move up and down the coast, the naturalists have identified many by the unique patterns — bite-marked notches on their dorsal fins that come from playful fighting or attacks by hungry sharks.

Keener’s colleagues spotted the first dolphin in the bay more than 15 years ago, and “now they come and go in small groups of six or 10 on an irregular schedule,” he said. The researchers have now counted a total of 91 animals swimming in and around the bay from Ocean Beach to the Marin coast and are keeping detailed records on more than 70 of them to chart their still-mysterious migration patterns, he said.

Those dolphins have been seen by professional and amateur observers as far away as Monterey Bay, Santa Monica and even Ensenada on the coast of Baja California, more than 600 miles away, Keener said.

Dolphin researchers give each animal they spot a number for their records, and sometimes a name. A dolphin named Smootch, for example, is a regular in the bay but has been spotted by researchers at least 21 times traveling up and down the coast between San Francisco and Ensenada.

Seven dolphins were spotted as far north as Point Arena in Mendocino County in April, and Keener said he saw one of them, a male named Vibe, swimming beneath the Golden Gate Bridge in February. Vibe swam back from Mendocino in June and was spotted again off Ocean Beach, “where he participated in an attack on a couple of harbor porpoises,” Keener said.

One local dolphin who stays in town is Kaimi, a bottlenose who hangs out around a green harbor buoy off the former Alameda Naval Air Station, where members of the O Kalani Outrigger Canoe Center paddle out from nearby Encinal Boat Ramp to visit the dolphin frequently — from a distance.

“You can hear Kaimi breathing regularly, and it makes a pretty sound over the water,” said Jaz Zaitlin, a canoe club racer. “It’s often there, along with a big brown sea lion, and many of us can watch it when we’re on the water. The name means Seeker.”

But Keener, the biologist, is concerned: “Dolphins are very social animals, and they move up and down the coast in groups, but Kaimi’s all alone, so now we want to keep an eye on her to make sure she’s really healthy.”

Cases of porpicide, first detected in 2011, when six dolphins were identified, have been increasing steadily, according to Keener and Padraig Duigman, chief pathologist at the Marine Mammal Center in Sausalito. There have been 35 cases since then, Keener said.

“It’s basically teenage males doing what they do best — like bully others, or (they have) too much energy to burn off, and too much time on their hands,” Duignan said.

It’s still too soon to tell whether the changing climate is influencing the northward movement of the dolphins, Keener said. It’s likely that sea surface temperatures have been higher during the recent El Niño. If it becomes permanent, he said, “then we might predict the dolphins could become more established farther north, as temperature and food allow.”

David Perlman is The San Francisco Chronicle’s science editor. Email: dperlman@sfchronicle.com