Some like it dirty: Less frequent nursery cleaning can reduce reef restoration costs with limited negative effects on coral performance

Samenvatting

Coral gardening is a reef restoration technique in which corals are first grown in nurseries and then outplanted onto degraded reefs. However, coral gardening does not yet achieve restoration at ecologically-relevant scales due to associated high costs. Coral nurseries are often manually cleaned to remove biofouling and improve coral performance, although putative benefits of this costly activity remain unconfirmed. We quantified the benefits and costs of various cleaning frequencies to identify the most cost-effective coral nursery approach at a study site with low herbivorous fish biomass. During this one-year study, nurseries were either cleaned weekly, monthly, quarter-yearly or never. Nurseries contained four coral species in three fragment sizes to examine species- and size-specific effects. Coral production (combined coral growth and fraction live coral tissue) and associated costs were quantified. No significant differences in coral production were found across cleaning frequencies and this result was consistent among coral species and fragment sizes. Therefore, no cleaning was clearly identified as the most cost-effective option. Costs could be further reduced by selecting fast-growing corals (e.g. Acropora) and stocking nurseries with large fragments, as these contributed most to coral production. The resulting minimum production cost is then US$0.26 per fragment including dive, wage and material costs for the building, deployment and filling of nurseries and sourcing of corals. For this study location and potentially many others with a similar or higher fish biomass, less frequent cleaning can substantially reduce reef restoration costs without having negative impacts on coral nursery production.