Caging enhances post-settlement survival of juveniles of the scleractinian coral Acropora tenuis

Low cost, simple approaches leading to enhanced numbers of viable, mature corals on reefs are prerequisite to active reef rehabilitation at even modest spatial scales. Mass culture of coral larvae to settlement, utilising improved knowledge of major coral spawning events, promises to be relatively straightforward, but very significant mortality in the early post-settlement period remains a major hurdle. This study was conducted to examine the effect of herbivore exclusion on the survival of 6 week old coral spat of Acropora tenuis (Dana, 1846) reared ex situ at a site in north-western Philippines. Coral spat were placed on the reef approximately 6 weeks after settlement in three treatments, caged, open-sided cage and no cage at two depths (4 m and 9 m). Mean survival of coral spat was significantly higher at the deep sites compared to the shallow sites. Among treatments, survival was significantly lower in the uncaged treatment (4.7% ± 2.6% and 10.5% ± 4.5%, mean ± SE in shallow and deep respectively) compared to the open-sided cage (18.6% ± 5.0% and 22.5% ± 7.1%) and fully caged treatment (17.0% ± 4.5% and 33.0% ± 6.0%) after 3 months. The results indicate that removal of coral spat by grazers may have reduced survival in the uncaged treatment, although the fact that survivorship was not significantly reduced in the open-sided cage treatments suggests that the presence of the cage also had some effect on survival. It is possible that the open-sided cage prevented access by larger fish that may have actively removed coral spat or that shading provided by the cage enhanced spat survival. Further research is needed to see if survivorship in cages decreases at a later stage due to overgrowth by other biota and whether survivorship is enhanced if spat are settled on more complex surfaces that provide refuge from grazers. This study demonstrates that using cages to exclude herbivores and corallivores and/or to provide shading may be beneficial to survival during the early stages when rearing corals in situ for reef rehabilitation.