Abstract

Nassau Grouper (Epinephelus striatus) populations have declined throughout the Caribbean largely due to overexploitation of fish spawning aggregations (FSAs). The FSA off the west end of Little Cayman Island is one of the largest remaining of the species and has dramatically increased in size since protections took effect in 2003. However, it remains unclear how this increased reproductive capacity will support population recovery and productivity via recruitment. Understanding the mechanisms underlying recruitment is important to linking changes in adult numbers to future, long-term population status. Here, we use novel in situ plankton imaging to investigate the fine-scale dispersal of Nassau Grouper eggs, as well as the ocean conditions and plankton community of the waters they were spawned into. We successfully followed one cohort in February 2016 (4 hours) and three cohorts in February 2017 (16, 36, and 6 hours), mapping the density of eggs at fine spatial scale (10s of meters horizontal, 1s of meters vertical). We observed eggs from cohort #2 in 2017 hatching into yolk-sac larvae. Ocean conditions at the FSA were anomalously calm in 2017, and all five drifters released with cohort #2 stayed within 2 km of Little Cayman Island and grounded on the reef. Finally, we develop and evaluate the ability of a physical advection-diffusion model to calculate expected egg concentrations in time and space. Our work to develop mechanistic understanding of how eggs survive to become spawning adults will allow for appropriate management to help protect this species.

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