Effects of fish grazing and damselfish territoriality

An experiment was performed on Britomart Reef (central region, Great Barrier Reef) to determine the effects of fish grazing and territoriality by the damselfish Hemiglyphidodon plagiometopon on benthic algal community structure. This pomacentrid excludes herbivorous fish from its territory, creating undergrazed patches with characteristic 'algal lawns' within coral reef lagoons. Pieces of natural coral substratum were placed at a depth of 10 m under 4 sets of treatments: within cages protected from fish grazing, within damselfish territories, within shaded damselfish territories, and outside temtories exposed to natural levels of fish grazing. A reduction in grazing by herbivorous fish, caused by caging or by the territorial behaviour of H, plagiometopon, resulted in marked changes in algal community structure. After 3 to 4 mo, the response of algal abundance and community structure to the treatments was highly variable. After 11 to 12 mo, however, decreased fish grazing was found to cause a significant increase in algal biomass and percent-cover. Algal species diversity (H',, and number of species) was relatively high wlthin damselfish territories, while diversity within cages and on substrate fully exposed to fish grazing was relat~vely low. Thus, species diversity was maximized at intermediate levels of accessibility to fish grazing. Algal species composition also shifted with accessibility to fish grazing. The algal community within cages was dominated by rhodophytes, particularly Polysiphonia subtilissima. Damselfish territones were dominated by a mixture of rhodophytes and cyanophytes, including Centroceras clavulatum, Gracilarjopsis rhodotricha, and Lyngbya aestuarii. Species composition within shaded territories was intermediate between these 2 treatments, exhibiting affinities to both. The naturally grazed algal community outside the damselfish territory was dominated by filamentous cyanophytes such as L. aestuarii and Symploca hydnoidesand by the encrusting chlorophyte Pseudendoclonjum sp. The interstices of the coral substratum were found to be riddled with S. hydnoides, L. convervoides, Oscillatoria aff. amphibia, and 0. aff nigroviridis. Cyanophytes clearly dominated substratum exposed to natural levels of fish grazing. It is suggested that rates of nitrogen fixation on natural coral reef substratum are positively related to levels of fish grazing.