The impact of abiotic drivers on the dispersal of reef fish eggs near Bonaire

Abstract

Coral reef ecosystems are declining due to climate change and other anthropogenic factors. Studies have shown that the presence and integration of reef fish are essential for the health and resilience of coral reefs. Consequently, RoffaReefs made them an integral part of their program to safeguard and improve the health of coral reef ecosystems. With this goal, they developed a floating breeding system designed to increase the survival rate of reef fish. Defining an optimal location for this breeding system requires knowledge of spawning locations and egg dispersal patterns in nearshore waters. However, spawning conditions are species specific and not known beforehand. Furthermore, current dispersal studies often opt for global or regional models, which do not resolve local scale abiotic drivers. The objective of this research is to identify possible spawning grounds near Bachelor's Beach based on the sampling data and varying abiotic drivers in nearshore conditions.

The sampled reef fish eggs are pelagic, exhibiting slight positive buoyancy, which keeps them within the surface layers. Given the focus on abiotic dispersal mechanisms, the eggs are modelled as passive Lagrangian particles that remain near the surface throughout the simulation. The literature review identifies tidal, wind-driven and wave-induced currents to be the key abiotic drivers near Bachelor's Beach. Through Delft3D-FLOW and -WAVE a hydrodynamic model is developed to simulate different scenarios, entailing the conditions of each abiotic driver and possible combinations. The obtained horizontal flow velocities and accompanying dispersal patterns indicate that wind-induced currents are the primary abiotic driver of dispersal near Bachelor's Beach, under both uniform and time-varying conditions. The impact of wave-induced currents only become significant under the presence of wind, whereas tidal forcing seems to play a limited role.

Using Delft3D-PART, dispersal patterns are simulated based on the hydrodynamic output. Under time-varying conditions, a consistent north to north west dispersal pattern was observed, moving eggs away from Bachelor's Beach. These results suggest that spawning grounds that cause eggs to pass Bachelor's Beach should be located within close proximity of the sampling area. It is recommended to RoffaReefs to position their breeding system in southern curve of Klein Bonaire, as many eggs tend to pass this location. Future research should prioritise local hydrodynamic measurements for model calibration and validation. To add to this a more detailed assessment of the spawning release conditions are advised to move from simplified to more biologically realistic scenarios.