Feeding habits of range-shifting herbivores: tropical surgeonfishes in a temperate environment

A widely documented impact of ocean warming is the poleward shift in species' distributions. This includes the global movement of tropical fishes into temperate rocky reefs. The ecological impacts of such range extensions are, however, largely unknown. We compared the feeding habits of herbivorous tropical surgeonfishes (Acanthuridae) to that of warm-temperate surgeonfishes near Sydney, Australia. The abundance of tropical surgeonfishes peaked during warmer months before they became locally extinct in winter. Comparisons of bite rates in the field between tropical (Acanthurus triostegus, Acanthurus dussumieri) and warm-temperate (Prionurus microlepidotus, Prionurus maculatus) surgeonfishes showed a significant effect of schooling, with both groups feeding most intensely in monospecific schools. In aquarium feeding trials, tropical surgeonfishes consumed more algae than their warm-temperate counterparts at both high and low temperatures (25 and 20 degrees C), and had higher bite rates at 25 degrees C than at 20 degrees C. A. dussumieri also had significantly higher consumption rates on brown algal recruits at warmer temperatures. We further compared gut indices and jaw-lever ratios among the four focal species, and found no consistent pattern between tropical and warm-temperate fishes. This study suggests that the continued intrusion of tropical surgeonfishes in temperate reefs will result in increased herbivory, as a result of both higher herbivore abundance and higher consumption rates per capita by tropical species.