Direct estimate of bioerosion by two parrotfish species, Chlorurus gibbus and C. sordidus, on the Great Barrier Reef, Australia

A description of the daily feeding patterns is provided for the two most abundant bioeroding parrotfish species on the Great Barrier Reef: Chlorurus gibbus and C. sordidus. This study was based at Lizard Island in the northern section of the GBR. Dawn to dusk observations were used to estimate feeding rates and the length of the feeding day. The effects of location and season on these values were also examined. These data are used to estimate erosion rates by the two species. A direct estimate of erosion rates by the two species is provided based on estimated feeding rates and bite sizes. In both species, feeding was characterized by an abrupt initiation and termination with relatively consistent feeding rates throughout the day. Both species spend 83.6 to 91.0% of daylight hours feeding. The proportion of daylight hours spent feeding increases in winter, although the actual length of time spent feeding decreases. Overall, the two species are similar in their general feeding patterns but differ in their seasonal response to time of day effects. C. gibbus is conservative with limited site, time of day or seasonal changes in feeding rates. In contrast, C. sordidus displayed marked seasonal changes in daily feeding rates and diel feeding patterns, with higher feeding rates in the afternoon and in the summer. Seasonally adjusted estimates of annual erosion rates per individual of the two species were 1017.7186.3 kg yr-1 (0.410.07 m3 yr-1) for C. gibbus and 23.63.4 kg yr-1 (9.7 10-31.3 10-3 m2yr-1) for C. sordidus. The likely impact of location and season effects on erosion rates appear to be limited.