Fine-scale spatial patterns of parrotfish herbivory are shaped by resource availability

Herbivory by fishes and sea urchins is a powerful mechanism on coral reefs that mitigates coral−algal competition by physically removing algae and creating bare space. Spatially constrained grazing by herbivores, particularly parrotfishes, may foster coral recruitment by creating a spatially continuous refuge of bare substrate for settlement and survival. However, frequent bouts of concentrated feeding potentially remove newly settled corals. Understanding the frequency and intensity of parrotfish foraging behavior at appropriate scales (months and meters) is essential for connecting herbivory with benthic processes. We documented the foraging behavior of a large, mobile coral reef herbivore (the steephead parrotfish Chlorurus microrhinos) on Palmyra Atoll using various techniques (observational follows, fish surveys, and bite scar tracking) across 2 habitat states (one dominated by algal turfs, one dominated by live coral and crustose coralline algae) that are representative of reefs throughout the Central Pacific. Large differences in the abundance of a preferred resource were associated with major differences in feeding behavior. Where resources were abundant and regrew quickly, foraging areas were small and spatially focused, resulting in dense patches of bite scars (>100 bites m−2). Feeding behavior also showed a temporal periodicity, with individuals defending and regrazing the same area after regrowth occurred. In areas where resources were less abundant and recovered more slowly, parrotfish movements and foraging areas were significantly larger and bites were distributed sparsely across food patches. The large variability we observed within a single atoll suggests that characterizing species-specific foraging patterns from small-scale studies may not be appropriate.