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

The influence of habitat on the distribution and abundance of Pomacentrus sulfureus was investigated on coral reefs in Zanzibar. Fish censuses were conducted using the simultaneous belt-transect method and substrate data were gathered using a point-base method. Densities of juvenile and adult P. sulfureus 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 have been used. In this study, we compared two measures of structural complexity. One was a contour measure (rugosity) and the other was the percent cover 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 regional differences in habitat structure. Multiple regression models identified relations between juvenile and adult P. sulfureus abundance and specific habitat features. The majority of the variation in juvenile numbers was attributed to percent cover of branching structures, the high-resolution measure. However, adult abundance was unrelated to this measure. Rugosity, the low-resolution measure of structural complexity, appeared to influence neither adult nor juvenile P. sulfureus. The best predictor of adult abundance was substrate diversity (inverse relationship), which, however, did not contribute to the proportion of explained variation in juvenile abundance. In addition, there was a weak positive relationship between adult abundance and the number of hard coral growth forms present. The results indicate that P. sulfureus exhibit an ontogenetic shift in habitat use. However, the regression models also revealed that adults per se have a positive effect on juvenile numbers. We suggest that the limited habitat use of juvenile P. sulfureus is contained within that of conspecific adults, and that juveniles are likely to adopt more general habitat associations as they grow.