Abstract
Patterns of distribution and variation in the structure of assemblages of butterflyfishes (Chaetodonitidae) and angelfishes (Pomacanthidae) were studied in the Red Sea and Gulf of Aden. Fish were censused on reef-edge transects at 367 sites and species presence/absence recorded at 371 sites. Data on hard coral cover and variety were collected at most sites, and details of reef structure and zonation recorded at sixty-one. Of seven species endemic to the region, two are restricted to the central and northern Red Sea, while two species occurring in the Gulf of Aden have not colonized the Red Sea. The turbid reef-poor southern Red Sea itself, rather than the narrow Strait of Bab el Mandab, probably acts as a barrier to their further dispersal. Species richness and abundance of butterflyfishes were greatest in the central Red Sea, decreasing both to north and south. This pattern matches similar patterns in reef development, hard coral cover and coral variety. Assemblage structure was spatially consistent over large areas, but a major change in species abundance and assemblage composition occurs between the central and northern Red Sea on the one hand, and the southern Red Sea and the Gulf of Aden on the other. This change appears to be associated with a marked increase in turbidity and a sharp decrease in reef development and coral variety south of 20⚬ N. While increased turbidity and the changed coral community may seem likely to influence assemblage structure through direct effects on post-settlement survivorship, recent reef fish assemblage studies have emphasized the importance of pre-settlement processes. We discuss the possible importance of pre- and post-settlement processes in generating the observed spatial variation of assemblages.