Resilience potential of an Indian Ocean reef: an assessment through coral recruitment pattern and survivability of juvenile corals to recurrent stress events

Coral reefs are degraded by the synergistic action of climate and anthropogenic stressors. Coral cover in the Palk Bay reef at the northern Indian Ocean largely declined in the past decade due to frequent bleaching events, tsunami and increased fishing activities. In this study, we carried out a comparative assessment to assess the differences in the recovery and resilience of three spatially distant reefs viz. Vedhalai, Mandapam and Pamban along Palk Bay affected by moderate, severe and low fishing pressure respectively. The assessment was based on the juvenile coral recruitment pattern and its survivability combined with availability of hard substratum, live coral cover and herbivore reef fish stock. The Vedhalai reef has the highest coral cover (14.6 ± 6.3%), and ≥90% of the live corals in Vedhalai and Mandapam were affected by turf algal overgrowth. The density of herbivore reef fish was low in Vedhalai and Mandapam reefs compared to the Pamban reef with relatively few grazing species. The juvenile coral diversity and density were high in the Pamban reef and low in Vedhalai and Mandapam reefs despite high hard substratum cover. In total, 22 species of juvenile corals of 10 genera were recorded in Palk Bay. Comparison of the species diversity of juvenile corals with adult ones suggested that the Pamban reef is connected with other distant reefs whereas Vedhalai and Mandapam reefs were self-seeded. There was no statistically significant difference in the survivability of juvenile corals between the study sites, and in total, ≥90% of the juvenile corals survived the high sedimentation stress triggered by the northeast monsoon and bleaching stress that occurred recurrently. Our results indicated that the human activities indirectly affected the juvenile coral recruitment by degrading the live coral cover and contributed to the spatial variation in the recovery and resilience of the Palk Bay reef. Low species diversity of the juvenile corals will increase the vulnerability of the Palk Bay reef to species-specific endemic threats.