Reasonable fragment size and transplant density can effectively improve coral restoration efficiency

Coral restoration technologies are vital for rehabilitating degraded coral reefs, with the gardening approach used as the main global method. However, the success of these efforts depends on restoration-related factors, including coral species selection, fragment size, and transplant density. Here we monitored for over one year period fragments of opportunity from four coral species, Acropora hyacinthus, Acropora microphthalma, Porites cylindrica, and Montipora digitata, that were transplanted on “Framed Reef Modules” located at Wuzhizhou Island, Hainan, China. The fragments were divided into three groups, representing three fragment sizes and two spacing regimens, in four replicates: Group 1 (6 cm size; 16 colonies; spacing 30 cm), Group 2 (3 cm; 32 colonies; spacing 15 cm), Group 3 (6 cm; 32 colonies; spacing 15 cm). Coral survival, growth rates, and physiological parameters were continuously monitored. In A. hyacinthus and P. cylindrica, large fragment size and lower transplant density significantly enhanced growth and survival rates. Under high-density transplant condition, smaller A. microphthalma fragments exhibited higher survival rates. Lower density transplantation significantly improved the survival rate of M. digitata, while its growth rate was not affected by either fragment size or transplant density. Environmental factors, such as seawater temperature, turbidity, and nutrient concentrations, significantly affected coral growth during the 90 to 180 days post-transplantation, when increased environmental stress inhibited coral growth rates. This study adds to our understanding of the selection of fragment size and spacing in direct active transplantation of corals of opportunity.