First, the Fish Fell From the Sky. Then They Washed Ashore.

Anchovies are a boom-and-bust species: Their populations naturally shrink and expand, and scientists don’t know exactly why. But since a marine heat wave that ended around 2016, the population of anchovies off the California coast has exploded “by orders of magnitude,” Dr. Santora said. It has created a banquet for the birds, sea lions and whales who feast on them, he added.

“These humpbacks are recovering, and they’re very hungry,” Dr. Santora said, adding that he suspected that a group of the whales could have driven the anchovies into shallow water. The whales, he said, cooperatively feed in small groups by splitting the fish into smaller schools, weakening their defenses.

“Five humpback whales can move an anchovy school basically anywhere they want to,” he added, “and just scoop it all up.”

The mass anchovy die-off in Bolinas Lagoon was rare, but not unprecedented. In 2013, anchovies crowded into Santa Cruz harbor, depriving themselves of oxygen. The next year, a mass die-off of the tiny fish fouled an Oregon beach town. Earlier this year, thousands of the fish also washed up dead on a beach in Chile.

Rudi Ferris, a fisherman who has lived in Bolinas for more than five decades, said that he recalled a handful of die-offs in the seaside town, and that only one, in the late 1970s, rivaled the carnage he witnessed last month.

“It stunk horribly for a really long time,” said Mr. Ferris, 71. This time, he added, he watched the scene from afar through his binoculars. A mass of pelicans and gulls were “frantically eating,” he said.