Survival and health assessment of out-planted coral cookies at Mnemba Island, Zanzibar

Coral reef restoration through nursery propagation and out-planting has become a widely used strategy to counteract the ongoing decline of coral reef ecosystems worldwide. This study assessed the survival, health status and site-specific performance of coral fragments attached to concrete substrates, referred to as “coral cookies” at Mnemba Island, Zanzibar. A total of 288 cookies belonging to five coral genera (Porites, Pachyseris, Montipora, Turbinaria and Goniastrea) were distributed across three out-planting sites, differing in depth and environmental conditions, and one nursery control; with 72 cookies allocated to each site. This present study aims to identify which combinations of coral taxa and environmental conditions are most suitable for future restoration efforts comparing performance across sites and genera. The results are expected to provide practical guidance for the NGO Under The Wave (UTW) in selecting optimal out-planting sites for different coral genera, ultimately contributing to more effective and targeted coral reef restoration strategies in Zanzibar. Monitoring was conducted by SCUBA diving over a three-months period, from May to August 2026. Survival and health were assessed weekly using a semi-quantitative bleaching and algal cover scoring system, together with several other biotic and abiotic parameters used to characterize each site. Photographic monitoring of tagged colonies was performed to estimate growth. After approximately three months, 86.6% of the out-planted cookies remained alive and present, compared with 95.8% in the nursery. No overall differences in operational survival were detected among the three out-planting sites or among genera, however, Goniastrea showed a marked descriptive decline at Site 3, where only 50% of its cookies remained alive and present. Algal coverage differed among both coral genera and site-depth settings; Turbinaria was the genera having the highest frequence and Site 1 was the site with more algal coverage recordings. Bleaching severity was more strongly associated with genus than with site, with Goniastrea showing the highest frequency of bleaching. No overall temporal change in living-tissue area or differences in growth trajectories among sites or genera were detected and recorded predator abundance was not associated with the subsequent operational loss of cookies. These findings indicate that coral-cookie out-planting can achieve high short-term persistence under local field conditions, while also showing that restoration performance cannot be evaluated through survival alone. Physical stability, algal colonisation, bleaching and tissue condition provide complementary information, and their responses may depend on specific combinations of coral genus and receiving environment. Longer monitoring and replicated sites within comparable depth ranges will be essential to determine whether these early patterns persist in order to provide stronger guidance for coral restoration in Zanzibar.