Temperature effects on herbivory for an Indo-Pacific parrotfish in Panama: implications for coral-algal competition

An assumption that a positive relationship exists between temperature and herbivorous fish grazing rates in coral reef environments has been used to explain seasonal and latitudinal trends in herbivore pressure. However, this assumption has not been systematically quantified over short-term (hours-days) changes in temperature, avoiding the confounding influences that can occur on seasonal or latitudinal scales. This study measured grazing activities of the pan Indo-Pacific parrotfish Scarus ghobban over short-term changes in temperature in upwelling and non-upwelling environments on the Pacific coast of Panama. Individual juvenile fish were followed over naturally varying temperatures to determine their bite rates (bites min(-1)), the foray frequency (forays min(-1)) and bites per foray. In the upwelling environment, there was a significant positive correlation between temperature and bite rate (R-Partial = 0.63, P < 0.0001) and there was a marked fourfold change in bite rates over the range of temperatures encountered in the study (21.2-29.4 degrees C). Bites per foray were also positively correlated to temperature (R-partial = 0.27, P < 0.0001), and tide height (R-partial = 0.26, P < 0.001), whilst foray frequency was positively correlated to temperature (R-partial = 0.63, P < 0.0001), but negatively to tide height (R-partial = -0.31, P < 0.0001). The effect of temperature on grazing rates may explain differences in herbivore pressure across different thermal environments and may be a factor contributing to algal biomass increases in low-temperature coral reef environments.