At one time, kelp forests — which shelter fish, slow erosion, and sequester carbon — grew along a third of the world’s coastlines. Now, scientists are working to bolster heat-stressed kelp by attacking the urchins that prey on them and transplanting hardier kelp varieties.

In the coastal waters off British Columbia, tribal volunteers from the Haida Nation dive for purple sea urchins amid a dense forest of rippling golden-brown kelp fronds. Sunlight filters through the canopy, creating a mesmerizing dance of light and shadow, as rays and sea lions wend through the kelp maze, sharks glide past, and bright orange garibaldis dash between the swaying fronds.

Kelp forests are biodiversity hotspots teeming with a colorful variety of seaweeds, sponges, crustaceans, and other small ocean animals, many of them found nowhere else.…