Demographics and dynamics of two restored populations of the threatened reef-building coral Acropora cervicornis

Acropora cervicornis is one of the principal reef-building organisms in the Caribbean; it is also considered one of the most threatened coral species. Due to its ecological importance and critical status it is the focus of many restoration and management initiatives. However, studies that quantitatively measure the efficacy or feasibility of these efforts are mostly lacking. In this study, nursery-reared fragments of A. cervicornis were transplanted to two reefs in Puerto Rico as part of a reef rehabilitation program, and their survival, growth, and branch production were measured for a year. We also evaluated the effect of this restoration on the dynamics and viability of the fragment populations by means of a simple model. Survival of outplanted fragments surpassed 60%. Colony growth rate varied between 0.20 ± 0.18 and 0.29 ± 0.21 cm d−1 (mean ± SD) whereas branch production ranged between 7.02 ± 5.72 and 11.86 ± 7.06 (mean ± SD) branches per fragment per year. Survival did not vary considerably with respect to fragment size. In contrast, large fragments (≥25 cm) grew faster and tended to produce more branches than smaller ones. Model simulations indicate that (1) in the absence of recruitment, and without any subsequent human intervention, restored populations will decrease below a quasi-extinction level of 25% of the initial population size after just 3 years and (2) transplanting at least 20 colony fragments per year (12% of initial population) is sufficient to keep the restored populations above the 25% threshold. We conclude that A. cervicornis may be a feasible species for restoration projects given sustained human intervention and that transplanting fragments of at least 25 cm to reefs is an effective restoration protocol that requires minimum effort to maintain a viable restored population of this key reef-building coral.