Coral transplantation as an aid to reef rehabilitation: evaluation of a case study in the Maldive Islands

Abstract

As part of a study of reef rehabilitation, whole coral colonies (primarily Acropora, Pocillopora, Porites, Favia and Favites) were transplanted and cemented in place onto three approximately 20 m² areas of Armorflex concrete mats on a 0.8–1.5 m deep reef-flat in the Maldives which had been severely degraded by coral mining. Growth, in situ mortality, and losses from mats due to wave action of a total of 530 transplants were monitored over 28 months. Natural recruitment of corals to both the transplanted Armorflex areas and concrete mats without transplants was also studied. Overall survivorship of corals 28 months after transplantation was 51%. Most losses of transplants due to wave action occurred during the first 7 months when 25% were lost, with only a further 5% of colonies being lost subsequently. Within 16 months most colonies had accreted naturally to the concrete mats. Thirty-two percent of transplants which remained attached died with Acropora hyacinthus and Pocillopora verrucosa having the highest mortality rates (approx. 50% mortality over two years) and Porites lobata and P. lutea the lowest (2.8 and 8.1% mortality respectively over two years). Growth rates were very variable with a quarter to a third of transplants showing negative growth during each inter-survey period. Acropora hyacinthus, A. cytherea and A. divaricata transplants had the highest growth rates (colony mean linear radial extension 4.15–5.81 cm y⁻¹), followed by Pocillopora verrucosa (mean 2.51 cm y⁻¹). Faviids and poritids had lowest growth rates. Favia and Favites showed the poorest response to transplantation whilst Acropora divaricata, which combined a high growth rate with relatively low mortality, appeared particularly amenable to transplantation. Natural recruitment did not differ significantly between concrete mats with and without transplanted corals. ‘Visible’ recruits were first recorded 10 months after emplacement of the mats and were predominantly Acropora and Pocillopora. On near vertical surfaces their density was almost 18 m⁻². Recruits grew fast producing many 20–30 cm diameter colonies on the mats within 3.5 years. Growth and survival of transplants are compared with results of transplantation studies in other locations. We conclude: (1) species transplanted should be selected with care as certain species are significantly more amenable than others to transplantation, (2) the choice of whether fragments or whole colonies are transplanted may profoundly influence survival, (3) considerable loss of transplants is likely from higher energy sites whatever method of attachment, (4) transplantation should, in general, be undertaken only if recovery following natural recruitment is unlikely.