Nearshore patterns of coral reef fish larval supply to Barbados, West Indies

Abstract

Although larval supply is essential to the maintenance of many marine populations, relatively little is known about the processes influencing the supply of larvae to nearshore habitats. To measure the temporal and spatial patterns of larval supply to the island of Barbados, West lndies, an array of light traps was deployed at 3 sites along the west coast for 2 spring recruitment seasons (1991-92). Three replicate traps were deployed nightly a t a central site, and 3 more traps were situated at both a northern and a southern site on alternate weeks during a 60 d period in 1991. In 1992, 3 replicate light traps were deployed nightly at each of the 3 sites for 70 d . A total of 82 species from 31 farmlies was collected; collections from each year were dominated by species from several families: Atherinidae, Blennidae. Scaridae, Pomacentridae, and Acanthuridae. Temporal patterns of abundance were similar for the majority of the species collected: total larval abundance and diversity was greatest during the second half of the lunar cycle, peaking on the third quarter moon. Spatial patterns of larval supply were also distinct: the overall supply of larvae was typically lowest in the central reglon of the Island. Supply of individual species was usually higher at one or the other or both ends of the west coast compared to the central site, with only 2 exceptions. Variation in the temporal and spatial pattern of bdal currents may account for patterns In the supply of ichthyoplankton. Temporal patterns of north-south (along shore) tldal flow were similar at all 3 sites: maximum nightly transport to the south occurred durmg maxlmum amphtude t ~ d e s (during the new and full moons), while .maximum nightly transport to the north occurred dunng rmnmum amphtude tides (during the quarter moons). East-west (onshore-offshore) transport due to tidal flows occurred on a much smaller scale and dlffered among sites. At the northern and southern s~ te s , nightly transport by tidal currents was generally eastward, or onshore, while at the central slte transport was generally west, or offshore. At both the southern and central sites in 1992, the temporal pattern of larval supply was significantly correlated to east-west transport by tidal currents integrated over the prevlous night. Onshore transport of larvae by tidal currents may be reduced at the central region of the west coast, resulting in both lower abundances and an overall lower diversity of reef fish larvae there. Patterns in the nightly transport by large-scale, externally forced currents (periods greater than 39 h ) were generally similar across all sites. Transport by these currents in both the north-south and east-west direct~ons was signif~cantly correlated to the mean number of larvae collected by the light traps, suggesting some influence of external forcing such as wind or other oceanographic features. Thus, larval supply to Barbados appears to be Influenced by processes occurring at 2 scales. Behavioral or passive synchronization to the lunar and tidal amplitude cycles results in regular, predictable peaks in the supply of late-stage larvae. Superimposed on this pattern are less predictable, large-scale events which can also greatly influence the nearshore transport of larvae.