Interactions amongst herbivorous fishes on a coral reef: influence of spatial variation

Abstract

Abundances of the surgeon fish Acanthurus lineatus (Linnaeus) within a single reef-system were estimated using a hierarchical sampling design during 1982. Additional sampling was carried out during 1983 and 1984 at a restricted number of sites. This species is aggressive toward other herbivorous fishes and is strongly site-attached. On the study reef (Lizard Island; 14°40′S; 145°28′E), A. lineatus was restricted to reef-crest sites below mean low water, mainly on reefs not directly exposed to prevailing winds. Within this reef zone and depth-defined stratum, A. lineatus was distributed heterogenously with high abundance, (approximately 14 fish per 300 m2) at a single sample-locality covering 600 m of reef crest. Abundances of herbivorous species (members of the families Acanthuridae and Scaridae) at other localities did not correlate with abundance patterns of A. lineatus. Subsampling within study localities revealed considerable heterogeneity in the abundance patterns of herbivorous fishes, especially within the area of high A. lineatus density. A detailed behavioural study of interactions among herbivorous fishes at two adjacent sites within the locality of high A. lineatus abundances revealed a complex pattern of site-general and sitespecific features. A. lineatus excluded smaller scarids from its feeding territories at one site, but not at another. Scarids attaining large size (>350 mm standard length) were present at one site and consistently fed within A. lineatus territories; large scarids were rare at the second site, even though the distances involved were small. In addition, the small surgeon fish A. nigrofuscus, a consistent target for A. lineatus aggression, was rare at one site but moderately common at the other. Finally, the abundant surgeon fish Ctenochaetus striatus was present at high densities at both sites and fed within A. lineatus territories. This species was not attacked by A. lineatus nor did it attack other herbivorous fishes within the vicinity. Small but consistent differences in reef structure were detected at each site. Local-scale heterogeneity in these interactions makes it difficult to develop generalizations concerning the role of territorial herbivores such as A. lineatus within reef systems. We hypothesize that very local differences in the within-habitat component of acanthurid and scarid abundances and distributions may reflect site-associated variability in recruitment patterns, post-recruitment mortality or behaviour that is independent of A. lineatus activities. Replicated removal experiments which include explicit tests for local site-effects and better descriptions of recruitment in larger herbivorous fishes are required before these interpretations can be evaluated.