In a single cove north of San Francisco, one diver hand-removed 7,700 kilograms of sea urchins from the seabed within one season. Then bull kelp grew back. This equation, laborious and direct, forms the core of a $9.5 million campaign to rescue the kelp forests of Northern California's Greater Farallones National Marine Sanctuary.
2013: A Pathogen Kills Billions of Sea Stars
The crisis origin lies roughly a decade back. In 2013, a wasting disease swept North America's west coast, now known as sea-star-wasting disease. The pathogen killed an estimated six billion sea stars. The sunflower sea star (Pycnopodia helianthoides) bore the worst. Nearly 95 percent of its global population died within years. In California, sunflower sea stars are functionally extinct across much of their former range, NOAA says.
That alone would be consequence enough. Simultaneously, a multi-year marine heatwave struck the west coast starting in 2014, combined with an El Niño event. Both factors weakened kelp before recovery could occur and created ideal conditions for purple sea urchins (Strongylocentrotus purpuratus): their natural enemies, the sea stars, had vanished. Their population exploded to as much as 60 times historical average. More than 90 percent of Northern California's kelp forests disappeared. In the Greater Farallones refuge, up to 80 percent vanished.
What remained, marine biologists called urchin barrens: barren rock covered by millions of hungry sea urchins consuming every kelp seedling instantly.
Urchins Without Predators, Fisheries Without Catch
Kelp forest collapse is not purely an ecological matter. Kelp forests serve as nursery and shelter for numerous marine animals, including rockfish species, salmon, and cod. When the forest goes, the fishing grounds vanish with it.
The economic damage is measurable. Commercial red sea urchin fisheries lost $3 million in annual revenue from population collapse. Abalone fishery, historically one of Northern California's most important coastal fisheries, has been fully closed since the collapse. Its pre-collapse value was $44 million annually.
What Divers Are Doing Now
Since 2024, the Greater Farallones Association has partnered with commercial divers to literally hand-harvest sea urchins from the ocean. At a single site, Timber Cove north of San Francisco, one diver removed roughly 7,700 kilograms of urchins within one season, capturing 90 percent of urchins at that location. In the resulting gaps, kelp was planted three ways: on concrete blocks with cultivated seedlings, in mesh bags with kelp spores, and on anchored ropes with young kelp plants.
Under favorable conditions, kelp grows up to 25 centimeters daily. That is the pace at which this underwater forest can theoretically return.
$9.5 million in funding is so far secured, including $4.9 million from the US Bipartisan Infrastructure Law via NOAA. The goal: restore roughly 11 hectares of kelp forest across five priority coves along Sonoma County's coast. An additional $7 million is sought for the next five years.
Comparison: Tasmania Shows What Is Possible
A similar collapse was already successfully reversed on the Pacific's far side. Tasmania experienced a 95 percent decline in its giant kelp forests (Macrocystis pyrifera) through ocean warming in the same decade as Northern California. Starting in 2024, a $3.5 million project led by the Institute for Marine and Antarctic Studies at the University of Tasmania began. Divers remove sea urchins, lobster are relocated to the coast as a biological control agent to long-term regulate urchin pressure without sustained human work. The result: at multiple sites, giant kelp has grown to the water surface for the first time in a decade.
The parallels between Tasmania and Northern California are plain. A natural predator vanishes, an herbivore explodes, a forest disappears. The restoration method is identical: first remove pressure from the system, then bring plants back.
Three Requirements for Lasting Return
Complete recovery of Greater Farallones' kelp forests is not guaranteed. Three conditions must converge.
First, sea urchin pressure must remain constantly low. A single season without an active dive team suffices to tip the balance again. The project requires funding across years, not seasons. That $7 million currently remains unfunded for the next five years represents the most critical vulnerability in the entire restoration plan.
Second, water temperature must not rise permanently. Kelp requires cold, nutrient-rich water. If climate change makes marine heatwaves more frequent and intense, restoration work must cluster in periods of natural cooling. That demands precise monitoring.
Third, the sunflower sea star must return as a natural predator. Without it, sea urchin populations remain controlled only by human intervention long-term. Multiple institutions, including NOAA and the California Ocean Protection Council, launched sunflower sea star reintroduction programs in 2024. How long before it functions again as an ecosystem factor remains uncertain.
Until then, the diver with the net remains the pivot between collapse and recovery.