Food intake by the parrotfish Scarus ferrugineus varies seasonally and is determined by temperature, size and territoriality

On coral reefs, feeding by parrotfishes may prevent coral to macroalgal phase shifts by keeping algae in a cropped state. To predict the response of grazers to changing conditions, knowledge of the factors that affect feeding intensity is needed. Therefore, we studied food intake of the parrotfish Scarus ferrugineus on an inshore fringing reef in the southern Red Sea, Eritrea, where seasonality and extreme summer temperatures were expected to influence feeding rates. Bite rates (bites min(-1)), defecation rates (defecations min(-1)) and yield per bite (mass ingested bite(-1)) were estimated for 3 life-phase categories: 20 to 25 cm initial phase (IP), 30 to 35 cm terminal phase (TP) and 30 to 35 cm territorial terminal phase (TTP). The bite rates increased from low levels in the morning to peak values in the afternoon. IP bite rates increased linearly with seawater temperature. For TP, the temperature-bite rate relationship was quadratic, with maximum bite rates at similar to 32 degrees C and lower rates at higher temperatures. Yield per bite in both IP and TP did not differ with time of year. Throughout the year, the total daily bites were highest in IP and lowest in TTP, resulting in similar to 23% lower daily intake in TTP compared to similar-sized TP males. Daily intake of ash-free dry mass of epilithic algal matrix (g AFDM d(-1)) was 7.5 to 9.6 for IP, 12.4 to 17.5 for TP and 8.7 to 13.2 for TTP. Intake and defecation rates and hence gut turnover rates peaked from April to July. The quadratic temperature response of TP suggests that large individuals may be close to their upper thermal limit, implying that temperature rises are likely to negatively affect grazing by large-bodied parrotfishes.