# Passive retention of simulated larvae on coral reefs

## TL;DR

This study explores how larvae from coral reefs are retained locally, finding that reef area and water depth strongly influence retention times.

## Contribution

The study introduces empirical relationships linking reef area and water depth to larval retention times.

## Key findings

- Retention time varies widely between reefs and is strongly explained by reef area (r2 = 0.90).
- Tidal eddies and proximity to other reefs enhance larval retention.
- Well-mixed larvae disperse more slowly and follow more circuitous routes than surface-floating larvae.

## Abstract

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 (Rt) varies widely between reefs, with most of the variation observed in this study (r2 = 0.90) explained by reef area (A) represented by the empirical power law relationship Rt = 10.34 A0.65, or alternatively by a combination of reef area and mean water depth (h¯) using the linear relationship Rt = 1.23(A) – 6.38(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.