UC Santa Barbara

There is abundant evidence that herbivorous fishes play an important role in mediating the competitive interactions between corals and algae and that with the reduction or absence of these grazers there may be regime shifts from domination by reef-building corals to non-accreting turfs and fleshy algae. This topic is of particular interest because of the perceived, documented, and projected declines in coral abundances across the tropics and because of the myriad of ecosystem services that healthy coral reefs provide to coastal tropical communities. Parrotfishes in particular appear to be important for promoting coral recruitment, because they scrape away bare patches on the substrate creating suitable settlement habitat for coral larvae. Because of this unique function and its potential to influence long-term coral reef resilience, there have been many appeals from the scientific community for the conservation of parrotfishes.

In terrestrial systems the interactions between the spatial patterns of grazing and the spatial patterns of primary producers have been well explored, but we know less about the fine-scale spatial behaviors of individual coral reef herbivores and how these behaviors influence reef benthic dynamics. There is evidence from spatially explicit models of herbivore foraging behavior that the spatial patterns of feeding by individual herbivores may have a significant impact on the long-term outcomes of coral-algal competition. However, there are multiple drivers that have been identified as influential to the spatial behaviors of parrotfishes and there is no current consensus as to what the primary driver is. In the first chapter of this thesis, we explore the relative influences of competition, predation, and resource abundance on the short-term space use patterns and feeding rates of Chlorurus sordidus, an abundant and widespread Pacific parrotfish. We characterized the foraging behaviors of these fish across sites that vary strongly in the presence of piscivorous predators and herbivorous competitors. We found that feeding rates are mainly influenced by direct interference competition and chronic predation risk and that short-term space use is mainly influenced by exploitative competition from the herbivore community. We found little evidence that acute predator presence has any influence on the short-term diurnal foraging behaviors of these fish, even where predators are large, diverse, and abundant. This provides insight into how foraging behaviors may be affected with changing herbivore and piscivore populations, for instance within a Marine Protected Area.

In the second chapter of this thesis we evaluated the movement patterns of Chlorurus microrhinos, a large-bodied Pacific parrotfish, across multiple spatial and temporal scales to determine what biological and environmental driver influence space use patterns at each scale. We found evidence for the influences of reproduction and oceanographic conditions, habitat features, predation, resource abundance and competition, however these drivers operated at varying spatial and temporal scales to influence the movement behaviors of these parrotfish. Large-scale movement was driven by oceanographic conditions that influenced the spatial and temporal patterns of reproductive events, as well as night sheltering behavior that was likely related to habitat selection to mediate predation risk. Fine scale movement patterns appeared to be strongly related to resource abundance and competition. We found that measures of movement and the emergent patterns between movement and these drivers depend heavily on the scale of study.