Validation of a fish larvae dispersal model with otolith data in the Western Indian Ocean and implications for marine spatial planning in data-poor regions

Abstract

The pelagic larval period is probably the least understood life stage of reef fish, yet the processes of larval dispersal and settlement exert a strong influence on the persistence of reef fish populations. A thorough understanding of these processes is essential to determining whether distant populations are connected and how to adapt management plans to patterns in connectivity. Managers may erroneously assume that local populations are isolated when they are actually replenished by distant reefs beyond their jurisdiction. Researchers increasingly rely on numerical hydrodynamic models that simulate the spatiotemporal dispersal of larvae by ocean currents to elucidate these connections and guide marine spatial planners, yet relatively little work has been done to validate these models with empirical data. In this study, we tested a dispersal simulation model against in situ observations of young post-larval fish to investigate a whether larvae settling at La Réunion (in the western Indian Ocean) might have originated at Mauritius, 200 km distant. First, we collected post larval specimens of honeycomb grouper (Epinephelus merra) shortly after an episodic mass settlement that occurred in 2002 at La Réunion. Using sclerochronology, we established the age of the fish from their otoliths. Finally, we simulated dispersal of larvae from La Réunion, Mauritius, and other reefs in the region by ocean currents using a 2D Eulerian advection-diffusion model driven by current velocities derived from satellite remote sensing. The simulation suggested that larvae spawned at La Réunion were carried away from the island while larvae spawned at Mauritius were carried to La Réunion. The otolith-derived ages of the fish were compatible with this hypothesis, when we accounted for the time required for larvae to drift from Mauritius to La Réunion. The combined results suggest a dispersal connection from Mauritius to La Réunion. To best maintain populations of adult reef fish at La Réunion, managers should protect stocks spawning at Mauritius. Although more study is needed to characterize patterns of regional connectivity and account for seasonal and inter-annual variations in these patterns, the example presented here demonstrates the possibility of distant connections in the western Indian Ocean. We urge managers in the region to look beyond their own jurisdictions, view their jurisdictions as part of a connected network, and undertake a collaborative approach to protecting the network as a whole.