“We’re way slower,” said Blake Tallman, who owns a dive shop in Fort Bragg. Mr. Tallman inherited the store from his father, whose picture hangs on the wall alongside abalone shells the size of basketballs.

Locals worry that rockfish — like sculpin, rock cod and red snapper — may be next. They spawn in the kelp forest. Worldwide, 100 species of rockfish rely on kelp, said Rebecca Johnson, who leads the California Academy of Sciences Citizen Science program.

Saving the kelp

In an attempt to bring back the kelp, Mr. Russo has raised more than $120,000 from grants and individual divers to organize culling events like the one at Albion Cove.

The goal, said Dr. Catton, is to create kelp oases: places where kelp can safely rebound, free from purple urchins. The worry is that if too much time passes there won’t be any kelp to seed future generations.

“There’s a group of people who believe they can change the trajectory here of what is going on in nature,” Dr. Mastrup said. “I am hopeful but skeptical.”

Adding to the challenges, a new blob of warmer waters formed this year in the Northeast Pacific Ocean, though it has not reached the California coast.