Joy Hollenback straps a small hammer to her wrist. She slips on blue fins. Then she dives toward the seafloor. There, she spots her target: hungry, kelp-eating purple urchins. Within seconds, she smashes 20 to smithereens.

Ms. Hollenback is part of a crew of volunteers. They crush purple urchins. They hope to save northern California’s kelp forests.

About 10 years ago, a mysterious disease sickened sunflower sea stars. It made their arms fall off. They turned into piles of goo. Out of every 10 sea stars, nine died.

The star fish is the main purple urchin predator. The disease killed more than five billion sea stars. Then the urchin population exploded. The spiny creatures destroyed most of California’s bull kelp forests between 2014 and 2020. 

Urchins aren’t bad. God made them with a purpose. They help keep coral clean. But when one species in an ecosystem disappears, that can throw other species out of balance.

Too many urchins devoured kelp. That left seascapes almost empty. Without kelp, sea creatures like red abalone sea snails starve.

Efforts to remove urchins seem to be helping. Healthy patches of kelp and schools of fish returned this summer to Caspar Cove, California.

Nearby at Albion Bay, commercial divers removed many of the urchins in 2021. Scientists put kelp grown in a lab on underwater lines. The kelp grew well.

Josh Russo, a former abalone fisher, helped start the urchin crushing. The first group was mostly local divers armed with sledgehammers. But they struggled to swing them underwater. So they switched to small hammers and icepicks.

Some people fear urchin crushing could spread urchin eggs. That would make the problem worse. Mr. Russo says he has seen no evidence of that.

Scientists say nothing can replace natural predators like the sunflower sea star. Biologists are breeding them in captivity. They hope to reintroduce them to the wild.

At least four sunflower star fish were spotted off the Mendocino coast in California this year. No one had seen any there for years. That’s good news for kelp and the creatures that need it!