Coral restoration: roles of shelter for herbivores and reef state in early recruitment success

Coral recruitment is crucial for reef recovery, yet our knowledge of recruit and juvenile coral demographics remains limited. To experimentally test whether shelter for herbivores and a healthy reefscape enhance colonization by corals, we deployed cubic-meter concrete modules in both low- and high-shelter configurations at the degraded reef off Waikı¯kı¯ Beach and the relatively healthy and fully protected reef at Hanauma Bay, Oʻahu, Hawaiʻi. Settling corals of three dominant genera ( Pocillopora , Montipora , and Porites ) were surveyed quarterly to track individual recruitment, survival, and growth for nearly four years. There was greater colonization of modules by sea urchins, higher visitation by herbivorous fishes, and lower algal overgrowth of recruit and juvenile corals at Hanauma Bay compared to Waikı¯kı¯. Nonetheless, demographic responses by corals were often genus, site, and treatment specific, and failed to strongly corroborate three a priori hypotheses. The hypothesis that herbivores enhance recruit and juvenile coral demographic metrics was partially evident for survival and growth of Montipora , as well as survival of Porites which were positively correlated with herbivorous fish biomass. Additionally, Pocillopora recruitment and survival were negatively correlated with algal overgrowth. The related hypothesis that increased shelter for herbivores indirectly benefits corals was also mostly rejected, except for recruitment of Pocillopora at Hanauma Bay and for survival of Pocillopora and Porites . A third hypothesis that a healthy reefscape benefits corals was corroborated for Pocillopora survival and Montipora recruitment and growth, yet not for Porites . Despite the relatively low number of correlations associated with our three a priori hypotheses, results from this study still suggest that recruit and juvenile corals are most likely to thrive in reef environments with sufficient herbivory and lower nutrients that fertilize algal growth. We conclude that further place-based management of herbivores is needed to replenish herbivore populations, increase the effectiveness of coral restoration efforts, and enhance natural coral recovery on Hawaiian reefs.