With the winter cold bearing down, it’s a nice time to imagine experiencing this phenomenon. To dive with the sardines in the Philippines, all you have to do is book a trip with one of many local dive shops. A boat will take you just a few yards from the beach. You dive in, swim down a few feet and look up at the fish clouds. For the best view, head out early. But if you can’t make it soon, don’t worry: They are there every day, unlike the more famous sardines that migrate by the billions to the coastal waters of South Africa only in the summer.

The sardines’ schooling behavior is sometimes called a bait ball, and it confuses most predators and very likely helps the sardines to survive. But it doesn’t always work out for them. In 2010, Simon Oliver, a conservation biologist now at the University of Chester in Britain, got lucky and captured video of the thresher shark, which has a unique workaround to counter the bait ball: a really long tail. The sharks slap at the ball with sickle-shaped tails specially adapted for the task, at speeds averaging 30 miles per hour, stunning some of the fish. The sardines drop from the ball, and the sharks swim over to gobble them up.

Why are the sardines there? That remains somewhat of a mystery. Until recently, when the sardine industry started worrying about the dwindling numbers of the fish offshore, no one had really thought to study them, let alone the smaller groups found closer to shore. As a result, the science on the populations of these fish near Cebu’s beaches is lacking.

“They were nonexistent a decade ago, and now they’re fairly regular,” said Dr. Oliver, who encountered the sardines a few years ago while studying thresher sharks just outside Moalboal on Cebu Island. But he’s not ready to call them residents.

He thinks the sardines probably emerged as anything does in nature that finds a niche. Perhaps a few eggs floated into the waters around Cebu and found conditions favorable enough to persist: few predators, plenty of plankton and ideal water temperatures. The sardines appear to hop from one location to the next, like between Pescador Island and the shores of Moalboal. If this setup ever changes, they might go away for good.