Passive retention of simulated larvae on coral reefs.

The extent to which local coral populations are self-sustaining through local recruitment has important implications for managing coral reef systems. However, a lack of understanding has led to overly simplistic representation of this phenomenon in coral reef population models. In this study, we simulate the dispersal of artificial larvae from 24 selected individual reefs across the Great Barrier Reef, Australia, over a spawning period in December 2016, to identify key physical factors influencing their retention. We found the dispersal pattern of larvae differed depending on whether they are well mixed throughout the water column and transported by depth-averaged velocity or floating near the surface, with well-mixed populations following more circuitous routes and dispersing more slowly. Retention time (R_t ) varies widely between reefs, with most of the variation observed in this study (r ² = 0.90) explained by reef area (A) represented by the empirical power law relationship R_t = 10.34 A^0.65, or alternatively by a combination of reef area and mean water depth ( $\overline{h}$ ) using the linear relationship R_t = 1.23(A) - 6.38( $\overline{h}$ ). The formation of tidal eddies and being situated among closely aggregated reefs are shown to be important factors for larval retention. Simple retention relationships like these have the potential to be incorporated into larval connectivity modelling and reef meta-community modelling where reef area and water depth are known. Further research is needed to determine how different oceanographic conditions and interannual variability will affect these relationships.