The non-linear relationship between grazing function and size of two parrotfish species in the Red Sea : Implications for coral reef resilience

Abstract. Parrotfish, a herbivorous reef fish, is considered to play an important role within coral reef ecosystems, enhancing coral reef resilience by keeping algal growth in check, allowing slower-growing coral recruits to settle. The functional performance of parrotfish species has been indicated to be dependent on body size. Albeit increasing size selective fishing pressure maintains, leading to immense effects on the dynamics of coral reefs, with consequences often much larger than changes in biomass and abundance of the targeted stock itself. The relative importance of the relationship between function (grazing) and body-size was tested in two parrotfish species (Scarus niger and Chlorurus sordidus) in the Red Sea. A non-linear relationship was found between functional performance and body size, with function found to increase notably as fishes reach a size of 15-20 cm At a regional level, parrotfish function was found to be low. These findings support the idea of a generalized ecological pattern of functional performance in parrotfishes. Demographic skewness within parrotfish species populations could easily be incorporated in routine monitoring schemes, thus act as a comparatively cheap and easily monitored indicator for assessing coral reef resilience, especially useful for managers in areas of limited budgets. Other alternative stable states than macroalgae, are speculated to be the case in the Red Sea.