Shoaling behavior of coral reef fishes varies between two islands with different predator abundance

Human-induced environmental change has affected ecosystems on a global scale, altering the ecology and evolutionary trajectories of various species. Fishing of marine predators, and any cascading effects on marine ecosystems, is of critical concern. Predators are thought to be an important reason for why fish shoal; thus, reducing predator populations could alter shoaling behavior for prey fish, and impact aspects of their behavior or life-histories. Here, we explore differences in shoaling behavior of coral reef fishes at 2 islands whose predator populations face differing fishing intensities. We compared the tendency to shoal for 3 fish species between 2 Pacific coral reefs: Palmyra Atoll (USA), an unfished reef with high predator abundance, and Moorea (French Polynesia), a fished reef with low predator abundance. We also specifically characterize movement and foraging-associated behaviors of one of these fishes, the convict surgeonfish Acanthurus triostegus, in this same comparative context. Our work suggests that in areas with reduced predator abundance, the tendency of some, but not all, fish species to form shoals is reduced. Decreased predator abundance also appears to have affected movement of shoaling and solitary A. triostegus, with increases in distance travelled and area covered occurring in contexts with low predator abundance. These observations shed some empirical light on how overfishing could affect shoaling behavior. Such insight is specifically valuable in the context of coral reefs, where changes to low-trophic level fish movement and foraging (2 behaviors closely linked with shoaling) could affect the functioning of these vulnerable ecosystems.