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

Abstract In 2013 we initiated a large-scale (70 hectare) kelp forest restoration experiment in a sea urchin induced kelp barren that had been stable for 45 years. We used 200 tons of quicklime for sea urchin culling. The immediate high sea urchin mortality was followed by recovery of kelp species within a year. Quickliming did not significantly affect the abundance of nontarget species on the barrens. Whereas the kelps and other macroalgae recovered within a year after the quicklime treatment, the mobile kelp fauna community did not recover at the same rate, probably due to slower dispersal abilities and predation from increasing fish and crab populations. The monitoring period (2013-2021) encompassed the expansion of invasive red king crabs as a new urchin predator, leading to reduced sea urchin populations and kelp recovery also in the control areas from two years after the experimental culling. Even in the presence of high sea urchin recruitment, the restored kelp forest was sustained, and the area of recovered kelp forest increased in the following years, likely due to increased predation on the urchins. The study shows that quickliming can initiate kelp recovery within large urchin barrens, but it cannot conclude that the lime-initiated kelp forest would have persisted without predation by king crabs. 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 predator control, alternatively repeated culling or harvesting of the urchins.