CORAL RESTORATION: ROLES OF SHELTER FOR HERBIVORES AND REEF HEALTH IN EARLY RECRUITMENT SUCCESS

Abstract

Survival and growth of new coral recruits is crucial for reef restoration, yet few details are known about coral demography during the first several years post-settlement. We experimentally tested how reef health and shelter availability for fishes and sea urchins affect juvenile coral demographic rates on coral-restoration platforms. On the island of O‘ahu, Hawai‘i, we deployed cubic-meter concrete modules in both low- and high-shelter configurations at the relatively degraded reef off Waikīkī Beach and the relatively healthy reef at Hanauma Bay. Naturally settling corals, especially of the genera Pocillopora, Montipora, and Porites, were mapped and sized quarterly to track individual colony recruitment, survival, and growth for nearly the first four years post-deployment. We predicted that demographic metrics would be enhanced on high-shelter modules at both sites by providing refugia for herbivores (Shelter Hypothesis), and on all modules in the relatively healthy reefscape at Hanauma Bay (Reefscape Hypothesis). Across experimental modules, overgrowth of coral by benthic algae was negatively correlated with herbivore biomass, which was greater on high-shelter modules, and in turn, total coral cover often benefited from reduced algal abundance. However, the shelter effect was evident only for recruitment of the genus Pocillopora, as well as for survival of Porites. We hypothesize that this weak trend was due to very low recruitment of herbivores, except for a single pulse of sea urchins at Hanauma Bay. Significant patterns for Montipora and Pocillopora were more consistent with the benefits of a healthy reefscape. Compared to Waikīkī, corals at Hanauma Bay experienced higher recruitment (Montipora only), survival (Pocillopora only), and growth (Montipora only), with none of the predicted patterns occurring for Porites. We conclude that determinants of coral recruitment success in this Hawaiian system are idiosyncratic at small scales, due to fine-scale variation in larval settlement and environment, as well as differences in biology and life history among genera. Nonetheless, on average, herbivores benefitted corals by reducing competition with benthic algae, and the context of a healthy reefscape did enhance two common corals. We recommend that reef managers implement policies fostering herbivory, especially on coral restoration structures in degraded habitat and where patches of healthy reef occur nearby, to facilitate coral recruitment, survival, and growth.