Spatial and temporal validation of settlement-marks in the otoliths of tropical reef fishes

The link between the timing of settlement (transition froni the pelagic to the benthic environment) and the formation of the 'settlement-mark' in fish otohths was examined in order to validate the use of this microstructural feature for the back-calculation of settlement patterns and planktonic larval durations.The formation of the settlement-mark was validated to occur at settlement in 2 pomacentrids, Polnacentrus amhojnensis, P nagasakiensis, and less conclusively in 13 other species from 7 families.Within these species, 3 dlstlnct settlement-mark durations were identified: a single-increment mark (P.amhoinensis, P, nagasak~enslsl, a 2-increment mark (l? coelestjs); and a zondl.14-increment mark (Acanthurus sp. 2, probably A. blochi).The structure of settlement-marks for P. amboinensis and P. nagasakjensis were compared among 4 to 5 locations on t h r northern Great Barrler Reel, Australia (maximum of 20 km apart).Otolith increment profiles diffcrcd minimally over spatial scctles of 20 km in these species, with Lizard Island tlsh possessing ~n c r e m r n t profiles diflcrent to those from other locations.This pattern of minimal variation in profiles occurred despitc the fcact that the planktonic larval duration of one species ( P n~lgssaklcwsis) dlffered among locat~ons (range of mean values: 22.4 to 26.3 d ) Further, at Lizard Island, increment profiles of P. ambo~nensis showed stat~stically s~gnificant, although minor, differences among local populations around the ~s l a n d and between successive lunar recruit cohorts.Our evidence suggests that the structure of thc increment transitions associated with settlement are taxon specific, although the optical contrast of settlement-marks (determined by increment width) may show subtle spatial and temporal variation.