It was a sight that prompted even seasoned wildlife watchers to do a double take: A coyote swimming in San Francisco Bay a quarter of a mile off Angel Island.

“I was surprised because it was so far from land,” said California State Parks environmental scientist Bill Miller, who saw the coyote last month while aboard a boat bound for the island.

At first he thought it was a seal or a sea lion. Then he saw the pointed ears.

The canine’s snout sliced determinedly through the water as it dog-paddled along, before eventually turning around and swimming back to Angel Island.

Staff at Angel Island State Park posted a video of the unusual encounter to Instagram, prompting concerned comments from people who assumed the coyote was in distress. Miller had the same thought at first, but the coyote appeared to be a strong swimmer, he said.

In any event, State Parks has a policy of not interfering with wildlife, he said.

This wasn’t the first time a coyote has attempted to make the mile-long trip across Raccoon Strait between Angel Island and the town of Tiburon in Marin County.

At least one of those voyages was successful. Before 2017, there is no record of coyotes ever existing on the island. Scientists are now studying about 14 coyotes that live there, all of them related, to see how they interact with the island’s once-plentiful mule deer.

The first coyote who swam across to Angel Island was alone, said Casey Dexter-Lee, a State Parks interpreter who lives on the island and has worked there for nearly 25 years. It may have been chasing prey or seeking new territory, but no one knows for sure, she said.

About a year later, a second canine appears to have made the trip, potentially enticed by the first coyote’s calls echoing across the strait, she said.

“We could hear them talking back and forth, especially at night,” she said. “So it’s possible that encouraged the second coyote to swim over.”

Another hypothesis, which is supported by genotype data, is that a lone pregnant female initially swam over and gave birth on the island, said Brett Furnas, an environmental scientist with the California Department of Fish and Wildlife.

Either way, the coyotes found a ready food source in the fawn of mule deer, which themselves have a controversial history on the island. Their numbers were controlled by human hunting for millennia, first by the Coast Miwok people, and then by the U.S. Army, which used the island as a military base, Dexter-Lee said.

When the island became a state park in the late 1950s to early 1960s, the population exploded, leading to concerns about starvation and prompting the state to regularly cull the deer, she said. The population appeared to have stabilized in recent years, until the coyotes came.

Preliminary estimates suggest the deer population has dropped by about half — from roughly 100 to fewer than 50 — since the coyotes’ arrival, Miller said. He is working with Furnas and others at the Department of Fish and Wildlife to study how this predator-prey relationship will play out. The research effort is in its second year of what scientists hope will be five.

Researchers have set up game cameras to capture images of deer and coyotes, and they regularly collect scat from both species. From that, they can learn what the animals are eating and collect DNA that enables them to identify individuals and tease out family relationships.

They’ve learned that the coyotes are all descended from one female. The population is in its third generation and is mostly consuming rats and mice.

The Angel Island mole, a unique subspecies endemic to the island, seems to be just a small part of their diet, which came as a relief.

It’s unclear what will happen to the island’s coyotes in the future. One big question is whether there’s enough food to support what is now a sizable and growing population, Furnas said. On top of that, he said, coyotes tend to want to disperse and establish new territories. And yet the long trip across the bay, with its strong currents, is not an easy one.

Furnas pointed out that the coyote seen swimming in the bay wasn’t trying to reach the island but to get away.

“That’s consistent with dispersal,” he said. “I think some of those coyotes are now saying, ‘Hey, we want our own territory,’ and they’re trying to swim back to Marin.”