Time: July 2023- July 2025
Project leaders: VHL (Alwin Hylkema), SCF (Kai Wulf), STENAPA (Erik Boman), WUR (Dolfi Debrot)
Many Caribbean reefs have shifted from coral-dominated to algal-dominated ecosystems. The high algae cover reduces coral recruitment, making the reef unable to recover from other disturbances and resulting in flatter reefs with lower biodiversity. One of the reasons for the proliferation of algae is a mass die-off of the herbivorous sea urchin Diadema antillarum in the early 1980s. Natural recovery of Diadema populations is slow to non-existent, making active restoration of this important grazer a top priority in Caribbean coral reef management, especially since Diadema densities were reduced by another mass mortality event in 2022. The marine park organizations of Saba and St. Eustatius want to restore Diadema populations by restocking cultured individuals. However, important knowledge gaps need to be addressed before large numbers of Diadema can be restocked on the reef. Current culture methods can only produce a limited number of competent larvae. In addition, only 8% of the settlers survive and after restocking, survival on the reef is low as well. In the RAAK PRO Diadema II project, the bottlenecks in Diadema culture will be addressed by comparing larval survival across multiple culture methods and investigating the relation between larval size and post-settlement survival. Growing-out juveniles at sea is likely to help prepare them for life in the wild, while restocking at an optimal size might also increase survival. Finally, a thorough restocking site selection based on high shelter availability and settlement rates will increase the long-term Diadema densities. The acquired knowledge and developed practices will be verified in a larger scale restocking experiment involving at least 5000 Diadema urchins. By restoring Diadema populations through restocking, macroalgae will be more intensively removed and corals will have a chance to settle and to survive, increasing the ability of the reef to cope with other stressors.