Successful large-scale and long-term kelp forest restoration by culling sea urchins with quicklime and supported by crab predation

Abstract In 2013, a large scale (70 hectare) kelp forest restoration experiment was conducted in a sea urchin barren that had been stable for 45 years. We used 200 tons of quicklime to eradicate the sea urchin population. Kelp recovered within a year in the lime-treated sites. Quickliming did not significantly affect the abundance of non-target species. Mobile kelp fauna colonized the restored kelps, but at a slower rate than kelp and other macroalgae, probably due to slower dispersal abilities. The monitoring period (2012–2021) encompassed the expansion of invasive red king crabs, which emerged as a novel predator of sea urchins. This led to a gradual decline in sea urchin populations and facilitated kelp recovery also in the control sites. This delayed recovery does not invalid the conclusions of the short-term efficiency of the quicklime treatment, but it makes it difficult to conclude about the long-term effects. The study shows that quickliming can initiate kelp recovery within large urchin barrens. We postulate that the recovery of kelps outside the limed areas was due to crab predation, and that successful kelp restoration in areas with recruitment of sea urchins depends on a persistent top-down control of the urchins.