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By Jan Freiwald, Reef Check Foundation Executive Director
Kelp forests are one of the ocean’s most vibrant ecosystems, providing food, shelter, and protection for countless marine species. But along the West Coast of North America, these undersea forests are in trouble. Between 2013 and 2015, extreme warm-water events devastated canopy kelp species like bull kelp (Nereocystis luetkeana) and giant kelp (Macrocystis pyrifera). The loss was particularly severe in Northern California, where efforts to restore these vital habitats have ramped up in recent years.
Central California’s kelp forests, including those along the iconic Big Sur coastline, have fared better, maintaining healthy kelp longer than other regions. But recent monitoring along the Big Sur coast suggests even these strongholds are now under threat. From 2015 to 2023, data collected by Reef Check’s subtidal surveys at eight long-term monitoring sites showed a sharp increase in purple urchin densities. These herbivores, which feed on kelp, saw their densities increase nearly tenfold at monitored sites, from less than 1 per square meter in 2015 to over 8 per square meter on average by 2023. Some areas reached densities as high as 20 urchins per square meter—a level that can decimate kelp forests. At the same time, at five of the eight monitored sites, giant kelp densities plummeted by more than 75%, dropping from 1.52 stipes per square meter in 2015 to just 0.35 in 2023. Urchin barrens, reefs denuded of kelp and covered in sea urchins, started to form in many places.
Faced with these alarming trends, Reef Check teamed up with local community members and commercial fishers to implement a restoration strategy of early intervention of targeted urchin removal. The idea was to remove urchins from an area that is still close to healthy kelp forest nearby in order to reverse the trend before it becomes widespread. At a site in Big Sur where urchin densities had reached over 26 per square meter, commercial urchin divers removed urchins over two years. By 2024, urchin density had dropped and stayed at just about 4 to 5 urchins per square meter.
The results were immediate and encouraging. By the fall of 2023, canopy kelp, mostly bull kelp, had rebounded dramatically. Densities at this site went from near zero in 2022 to 2.71 per square meter—rivaling those of nearby healthy kelp forests. Even more promising, this recovery persisted through 2024, with kelp densities stabilizing at levels similar to surrounding healthy forests.
This success story shows that early intervention can work, even if urchin populations aren’t reduced to the theoretical level of 2 per square meter, which has been cited in much of the restoration work as needed for the reversal of urchin barrens to kelp forests. The findings suggest that early action can make a big difference, offering hope for the future of California’s kelp forests and the marine life that depends on them.
With continued restoration efforts, it is our expectation to protect some of the kelp forests that are still strong and bring back some of the ones lost to urchin barrens. For now, scientists, divers and supporters are racing against time to protect these essential habitats and the intricate web of life they support.
