Influence of tidal eddies and wind on the distribution of presettlement fishes around One Tree Island, Great Barrier Reef

Abstract

Tidally generated eddies that form behind reefs are potentially important physical mechanisms of retention for coral reef fishes during their presettlement phase. The re-circulating properties of eddies may reduce the potential for dispersing fishes to be transported away from settlement habitat and influence patterns of connectivity among adult populations; however, empirical evidence is lacking. We identified the presence of eddies at One Tree Island (OTI) and sampled presettlement fishes in surface waters using a stratified sampling design based on the presence or absence of eddies as predicted from a calibrated hydrodynamic model of the Capricorn-Bunker region. Higher concentrations of presettlement fishes, mostly mullids (goatfishes), were found in the vicinity of the reef in locations where eddies were known to form rather than in locations without eddies, and this was consistent among days and tidal cycles. Locations where eddies were not predicted to form consistently had low concentrations of presettlement fishes. This pattern also occurred under a range of wind strengths and directions. There was evidence for an effect of the windward side of the reef, but areas with eddies maintained high concentrations even when on the leeward side. Higher concentrations were not necessarily found in the eddy itself; rather, they occurred at locations where eddies were predicted to form on the flood or ebb tide. Eddies increase the probability that presettlement fishes will stay near reefs through retention, in some cases their natal reef. Late-stage presettlement fishes are highly mobile and can respond to cues from the reef. Eddies may also increase behavioural interactions among fishes and assist in the detection of reefs that may elicit settlement behaviour.