MARINE LIFE: Bottlenose dolphins, which until recent years mostly swam, mated and hunted—by echolocation—in Southern California waters, have been a regular presence in the Bay Area since 2010. A large portion of the local population is known to also frequent Monterey Bay.  Dale Trockel

“It’s sort of common knowledge in Dillon Beach that they’re out there now,” said Tom Thornley, a longtime resident of the tiny village. He was talking about bottlenose dolphins.

The bottlenose dolphin, a roughly six to 12-foot cetacean that can live for 50 years, usually reside in southern California water. But Mr. Thornley said people in Dillon Beach who have lived there for years, even decades, are seeing pods of them off the beach for the first time ever. 

“I mean, it’s pretty fascinating. It’s so unusual. They’re so playful, Jesus. It seems like everyone’s kind of enjoying it. But we’re all just curious,” he said.

The story of how bottlenose dolphins ended up in the Bay Area is one of a strange ecological quirk, according to Bill Keener, the director of Golden Gate Cetacean Research.

There is archaeological evidence that the dolphins once inhabited Northern California; their bones have been found in centuries-old American Indian middens in the Bay Area. 

But in recent history, there have been no signs of them.

Instead, dolphins lived off the coast of Southern California, never venturing further north than Point Conception—until the early 1980s. Then an El Niño event lured the dolphins to temporarily warmer waters in Monterey Bay. But when the El Niño ended, the dolphins stayed. Slowly, they made their way further north, Mr. Keener said.

“They’ve got fairly complex brains, so once they consider how to navigate an area and find food, there’s no real reason why they had to go back, even though the waters are cooler,” he said.

(Some porpoises might not be so happy about the change. Although dolphins are known for their playful demeanor, they have some aggressive tendencies. Between 2007 and 2009, 44 harbor porpoise strandings—many in Monterey Bay—were linked to trauma inflicted by bottlenose dolphins, according to a 2011 study on “porpicide” published in the journal Marine Mammal Science.)

Some dolphins stick to a particular area while others use broad swaths of the coast to live, hunt and breed. Researchers saw one dolphin near Ensenada, in Mexico, in 2000—and then, over a decade later, in Bodega Bay.

Today a few hundred of the 500 to 600 that live along the state’s coast are regularly spotted in Monterey Bay.

Mr. Keener’s nonprofit started observing dolphins on a regular basis around the Bay Area in 2010. At the time they instigated a rigorous photo identification program to get a firm count of the creatures. Between 2010 and 2013, the group identified 41 dolphins in the area. (The nicks and notches in their dorsal fins, the result of social interactions or fights, are a kind of fingerprint researchers use to tell individuals apart.)

Now they have tallied almost 80 adults, and another 20 calves and juveniles. That’s a fairly significant chunk of the total state population. 

Because they move around, there is overlap in regional counts. For instance, 90 percent of dolphins seen around the Bay Area also spend time in Monterey Bay. “That’s really the interesting story: they’re using a huge chunk of California for their habitat. On the East Coast, they might remain in a bay their whole lives,” Mr. Keener said. 

Karl Menard, the aquatic resources manager at the Bodega Bay Marine Lab, hypothesized that they might be hanging out around Dillon Beach because of an unusually abundant food source. He said high ocean temperatures in the past year have sparked a boom (“literally by the tons”) in the local squid population, a food source for dolphins. He said they have also seen unusual amounts of other sealife in Bodega Bay in the past year: white sea bass, thresher sharks, mola mola.

Whether ocean temperatures play a role in where the dolphins might venture is uncertain, Mr. Keener said in an email. “It is hard to say why they are ranging farther north along our coast,” he wrote. But, he said, sea temperature does affect fish and squid populations, and the dolphins did move north during the 1980s El Niño.

To some extent, the growing number of dolphins documented could arise from greater awareness. But Mr. Keener’s group also believes the local population is growing.

“We have more people watching… But we also assume that, yes, it could be there are more animals using this area. Some go to Monterey Bay and bring back a few new ones. Then they’re exposed and they learn to navigate here, so it’s possible that it’s growing.”

And the number of spottings isn’t the only change his group is documenting. Initially, people saw the dolphins just in the summer and fall, but that’s no longer the case. “The first few years, [they were] absent during the winter. The past winter and this winter, they were here all year long in smallish numbers, six or eight or 10 moving between Marin and San Francisco, and the San Mateo coast.” 

And sightings of calves mean they could be giving birth here, too.

How far north could they go? Mr. Keener doesn’t know. So far, Bodega Bay is about far as they have ventured; they have been spotted “very occasionally” there since 2012, he said. 

That seems to square with reports from Dillon Beach resident Dale Trockel, who moved from Davis about four years ago. He surfs most days of the week, though not always at Dillon Beach, and he’s seen the creatures occasionally over the past couple years. (He went surfing with one of his daughters, a 6-year-old, in November. One swam near them. “She was pretty excited,” he said.)

At some point it could get too cold, but he noted that the animals can handle fairly cool temperatures on the East Coast and Great Britain.

Mr. Keener said researchers at his organization are working on a research paper to document their new Northern California home. 

Linda Schultz has lived in Dillon Beach for three years, and before that in Tomales for a decade and a half. Ms. Schultz, who used to work with a humane society and has volunteered with the nonprofit Save the Whales, walks on the beach almost every day but recently saw dolphins at Dillon for the first time.

Just this month, she was walking along the beach and saw a pod of about six. “It was very beautiful, very graceful,” she said. To her, the dolphins are a reminder of the need to be better stewards of the planet.

She watched them for about a half an hour as she stood with her legs in the water. People gathered around. “When all those people showed up when I was standing in the water, there was a reverence around the crowd… I see the effect a lot that the larger animals in the ocean have on us. It’s a very reverent place and no one was talking. They were just watching them. We don’t have moments like that a lot in our culture,” she said.


To report sightings of dolphins or other cetaceans, visit The organization is particularly interested in dolphins spotted north of Half Moon Bay, harbor porpoises north of the Richmond-San Rafael Bridge and all minke whales.