FISH ASSEMBLAGES ON ARTIFICIAL AND NATURAL REEFS IN THE FLOWER-GARDEN-BANKS-NATIONAL-MARINE-SANCTUARY, USA

Visual censusing was used to characterize fish assemblages on artifici al and natural reefs located within the boundaries of the Flower Garde n Banks National Marine Sanctuary (FGBNMS) in the northwestern Gulf of Mexico. Emphasis was placed on determining spatial and temporal patte rns in habitat utilization by fishes on an offshore artificial reef(Mo bil Platform HI-A389A). Overall, 43 species were observed during diurn al surveys in the upper 24 m of the artificial reef Midwater pelagic f ishes (i.e., carangids and scombrids) accounted for over 50% of all ta xa enumerated on the artificial reef; however, these taxa were transie nt members of the assemblage and were observed infrequently. Labrids, pomacentrids, and serranids were the dominant reef-dependent taxa. Dis tinct trends in vertical, diel, and seasonal abundances were observed for juvenile and adult fishes. Of the three designated depth zones (up per 1.5-9.0, middle 9.0-16.5; lower 16.5-24.0 m), abundance and specie s diversity were lowest in the upper zone. Nocturnal counts were chara cterized by a marked reduction or complete absence of most species, du e in part to twilight cover-seeking and movement activities. Seasonal variation in community composition and species abundance (May versus S eptember) was primarily due to recruitment of juveniles (0-age fishes) to the artificial reef in late summer. Increases in total fish abunda nce (all taxa combined) coincided with both increasing habitat rugosit y and degree of fouling. Species richness on natural coral reefs in th e FGBNMS was higher than on the artificial reef. Unlike the artificial reef, fish assemblages on the natural reefs were dominated by a singl e family (Pomacentridae) which accounted for over 50% of all individua ls observed.