Influence of habitat structure on Pomacentrus sulfureus, a western Indian Ocean reef fish

The influence of habitat on the distribution and abundance of Pomacentrus s ulfureus was investigated on coral reefs in Zanzibar. Fish censuses were co nducted using the simultaneous belt-transect method and substrate data were gathered using a point-base method. Densities of juvenile and adult P. sul fureus were examined in relation to habitat composition and structure. The influence of habitat structure on coral reef fishes remains debated and has been obscured by the various methods, scales and levels of detail that hav e been used. In this study, we compared two measures of structural complexi ty. One was a contour measure (rugosity) and the other was the percent cove r of branching structures. Both were applied on the same scale but differed in the level of detail. P. sulfureus was distributed in an uneven pattern around Zanzibar Island and the distribution appeared to reflect local and r egional differences in habitat structure. Multiple regression models identi fied relations between juvenile and adult P. sulfureus abundance and specif ic habitat features. The majority of the variation in juvenile numbers was attributed to percent cover of branching structures, the high-resolution me asure. However, adult abundance was unrelated to this measure. Rugosity, th e low-resolution measure of structural complexity, appeared to influence ne ither adult nor juvenile P. sulfureus. The best predictor of adult abundanc e was substrate diversity (inverse relationship), which, however, did not c ontribute to the proportion of explained variation in juvenile abundance. I n addition, there was a weak positive relationship between adult abundance and the number of hard coral growth forms present. The results indicate tha t P. sulfureus exhibit an ontogenetic shift in habitat use. However, the re gression models also revealed that adults per se have a positive effect on juvenile numbers. We suggest that the limited habitat use of juvenile P. su lfureus is contained within that of conspecific adults, and that juveniles are likely to adopt more general habitat associations as they grow.