2 - Mission impossible: unlocking the secrets of coral reef fish dispersal

Summary

Our knowledge of the distance and directions that reef fish larvae disperse each generation has been transformed over the last 15 years. New discoveries from a diversity of approaches, including chemical tagging of larvae, genetic parentage analysis, population-level genetic analyses, otolith chemical signatures, and biophysical modeling, have all indicated a higher proportion of larvae settle closer to home than previously thought. The relative dispersal frequencies appear to decline with distance from source, and while average dispersal distances may be small, dispersal kernels are likely to have a long tail. Levels of self-recruitment increase with isolation or entrapment, while hydrodynamic forcing will stretch the dispersal tail in large oceanic systems. Species that differ in pelagic larval durations (PLDs) have broadly overlapping dispersal kernels, although species with longer PLDs and pelagic eggs can disperse further. The emerging picture suggests reef fish populations have an innate resilience to local disturbances and that local management can achieve both biodiversity conservation and fisheries objectives. Parentage analysis appears to be the most promising approach for describing full dispersal kernels, validating other genetic approaches, and biophysical and spatially explicit demographic models. Emphasis must be placed on identifying and protecting key subpopulations for meta population persistence in the face of increasing human pressures.

The task of establishing where coral reef fish larvae go in that brief period between spawning and settlement to the reef was once considered impossible. Yet for all organisms, the link between generations is fundamental to understanding how populations persist, how species adapt to their environment, and how they are affected by human actions. Prior to the 1990s and in the first book on the ecology of coral reef fishes [2230], reef fish populations were assumed to be “open”, connected by larval dispersal over long distances driven by oceanic flows. At the time, the evidence seemed convincing. It was known that the pelagic larval duration (PLD) could last for weeks [1443, 2571] and in that time currents could transport larvae across oceans [1411].