Aaron Matthius Eger, Ezequiel Marzinelli, Hartvig Christie, Camilla W. Fagerli, Daisuke Fujita, Seokwoo Hong, Jeong Ha Kim, Lynn Chi Liee, Tristin Anoush McHugh, Gregory N. Nishihara

Kelp forest ecosystems and their associated ecosystem services are declining around the world. In response, marine managers are working to restore and counteract these declines. Kelp restoration first started in the 1700s in Japan and since then has spread across the globe. Restoration efforts, however, have been largely disconnected, with varying methodologies trialled by different actors in different countries. To distil lessons learned over the last 60 years of kelp restoration, we synthesize the results of nearly 200 projects spanning 1957 to 2020, across 16 countries, five languages, and user groups. Our results show that kelp restoration projects have increased in frequency, have employed 12 different main methodologies, and targeted 17 different kelp genera. Of these projects, the majority have been led by academics (62%), have been conducted at sizes of less than 1 hectare (80%) and over time spans of less than 2 years. We show that projects are most successful when they are located near existing healthy kelp forests. Disturbance events such as sea-urchin grazing are identified as regular causes of project failure. Costs for restoration are historically high, averaging hundreds of thousands of dollars per hectare, but we explore avenues to reduce these costs and suggest pathways for scaling-up future restoration efforts. One key suggestion is the creation of a living database which serves as a platform for recording restoration projects, showcasing and/or re-analyzing the data, and providing updated information. In this way, our work establishes the groundwork to provide adaptive and relevant recommendations on best practices for kelp restoration projects today and into the future.