Exploring the suitability of corals of opportunity for direct transplantation onto Mauritian sediment-impacted reefs

Coral reefs decline in Mauritius, driven by rising temperatures, sedimentation, and physical disturbance, has rendered passive conservation insufficient. This 27-month study assessed the survival and growth of naturally dislodged but viable coral fragments (corals of opportunity; COOs) left at a sediment-impacted site in Pointe aux Feuilles, as compared to COOs collected and immediately attached in situ to elevated metal frames. COOs of five species were used namely: Acropora selago, Acropora muricata, Acropora Cytherea, Pocillopora damicornis and Millepora alicornis. A controlled field experiment was conducted in which growth was quantified from standardized photographs using ImageJ, while survival and bleaching patterns were statistically analyzed to determine species-specific responses under natural and nursery conditions. Substrate-elevated modules significantly enhanced coral performance: Pocillopora damicornis and Millepora alicornis showed the highest growth (55.8 cm² and 36.2 cm²), while three branching Acropora species exhibited lower survival and greater bleaching. Final bleaching rates were lowest in P. damicornis (0 %) and highest in A. muricata (46.7 %) on nursery modules, while in situ COOs suffered bleaching rates up to 66.7 % and over 25 % dislodgement. Sediment accumulation was consistently higher on natural reefs (Welch's t = 18.9, p < 0.001), highlighting site exposure as a key stressor. These results show that COOs, when stabilized in elevated, higher-flow environments, support reef recovery without harming donor colonies or donor reefs. Often overlooked, COOs offer a sustainable restoration option when identified early and maintained under favorable conditions.