LARVAL SUPPLY OF CORAL-REEF FISH POPULATIONS - MAGNITUDE AND SYNCHRONY OF REPLENISHMENT TO LIZARD ISLAND, GREAT-BARRIER-REEF

Larval supply is crucial for the persistence of coral reef fish populations, yet little direct information exists about any aspect of this life history phase. Using light traps, this study directly measured larval supply of 27 coral reef fish taxa to Lizard Island (northern Great Barrier Reef) over 2 successive recruitment seasons. Descriptions of the magnitude and synchrony of replenishment to 3 different habitats (front-reef, back-reef and lagoon) within a single reef are the first of their kind and reveal several important facets of the larval supply process. Larval supply during the first season was much stronger into the front-reef side than into the back-reef habitat. Synchrony of daily replenishment between these 2 habitats was characteristically weak during this year. This result was not related in any simple way to low levels of absolute abundance in the back-reef habitat. In the following year, this dominance of the front-reef habitat was diminished. In many cases, this decline could be explained by a rise in the strength of the replenishment signal in the back-reef habitat. Correlated with this, there was a general increase in the strength of synchrony in larval supply between the 2 habitats. Again this trend could not be simply explained by any increase in absolute abundance in the back-reef habitat. It appears that there was a change in the nature of the larval supply process between these habitats over the 2 years. Although temporally variable, replenishment into either habitat showed consistent patterns across many taxa indicating that larval supply to a single reef is systematic rather than 'stochastic'. We suggest that investigation of the relationship between larvae and the local physical conditions surrounding each reef may reveal the reasons for much of this complexity in replenishment. Larval supply into the lagoon was highly variable among taxa. The greater association of the lagoon with the front-reef habitat was revealed by the cross-correlation coefficients of many taxa. In some cases this could be explained by a progressive depletion of larvae as they crossed the front-reef crest and entered the lagoon. However, larval abundance of 4 taxa in the lagoon was so low (seasonal sum < 10 for Pomacentrus coelestus, Lethrinidae, P. lepidogenys and Neopomacentrus spp.) that active behavioural choice of settling larvae seems to be involved.