SETTLEMENT VS. ENVIRONMENTAL DYNAMICS IN A PELAGIC‐SPAWNING REEF FISH AT CARIBBEAN PANAMA

How are the dynamics of the settlement of pelagic larvae of marine shore organisms into benthic habitats affected by larval production and mortality dynamics, and by environmental (wind and tidal) control of settler delivery? We examined the dynamics of reproduction of the pelagic-spawning fish Thalassoma bifasciatum from that perspective at San Blas, Caribbean Panama. During April–June 1993, ∼75% of females spawned each day, with mean output per female per day at about one-third the potential maximum. Output peaked several days before the new and full moon, when tides were high during the afternoon spawning period. Daily fluctuations in output were weakly correlated with light (+), swell (−), and onshore wind (−) on preceding days. Settler production (determined from otolith back calculations of settlers' fertilization dates) peaked during the wet season (May–November), when winds were light and variable, spawning-period tidal heights were moderate to low, and tidal flows were off-reef at night. These conditions correspond to when most settlement occurs. Settler production was minimal during the dry season (January–March), when onshore trade winds were strong and consistent, and tidal conditions were the reverse of those in the wet season. Settlement usually occurred in one variably timed pulse per lunar cycle, around the new moon. Variation in monthly settlement strength over 11 yr (1986–1996) was unrelated to onshore wind stress during either settler arrival or settler production. The daily dynamics of settlement were unrelated to the concurrent dynamics of wind or tides, or wind dynamics up to a week before settlement. Settler production was much less continuous than spawning. Wet season settler-production dynamics were unrelated to tidal dynamics, but (weakly) negatively related to onshore wind stress. Although the dynamics of settler production and settlement were similar, bouts of settler production were about twice as long as bouts of settlement. The average lunar patterns of settler production and observed spawning were similar (weakly bimodal). Settlers produced during different lunar phases differed in the length and variability of their pelagic larval durations (PLDs). Fish with different PLDs all tended to settle around the new moon. Weakly semilunar spawning may reflect effects of weak tidal influences on egg survivorship, or of environmental constraints on adults' spawning capacity that also limit spawning to well below the potential maximum. Dry season trade winds may produce the seasonal low of settlement by depressing spawning or forcing the onshore transport of eggs and larvae. If settlement is transport-dependent, weak variable winds may have little effect on currents during the wet season, or local geography may limit such effects; tidal dynamics may not affect wet-season settlement dynamics because small (<0.6 m) tides produce weak flows. Alternatively, settlement may not be dependent on transport mechanisms. Short-term settlement dynamics result from both the dynamics of larval survival (as most larvae have near-mean PLDs) and larvae delaying or advancing settlement to settle around the new moon. Larvae spawned at different lunar phases have different patterns of variation in their PLDs but achieve similar relative settlement success. Larvae with extended PLDs may not suffer low settlement success because the inevitable age-dependent decline in survival has smaller effects on relative settlement success than does large monthly variation in the survivorship of fish spawned during different lunar phases.