Predator-prey interactions in two schooling fishes, Caranx ignobilis and Stolephorus purpureus

Interactions between the jack, Caranx ignobilis, a facultative schooling species, and the Hawaiian anchovy, Stolephorus purpureus, an obligate schooler were studied within an enclosure in the field in Hawaii. Single predators were the most successful at capturing isolated (individual) prey, and relatively unsuccessful at capturing individuals in schools. Grouped (schooled) predators were the most successful at capturing schooled prey. The leading, or first, predator was the most successful member of a group or school at capturing isolated or schooled prey. Following predators tended to make it possible to catch more prey earlier in the experiments. Larger predator groups were able to break up schools of prey quickly, resulting in increased numbers of prey becoming isolated. These prey were captured before they could reform or join other schools. As the initial size of the prey school increased, the per cent of individuals captured declined. Schooling in prey reduces the time a visually orienting predator has to align himself with an individual prey. Schooling in predators may have co-evolved as an adaptation, making it possible for predators to break up and isolate schooled prey. Larger prey schools may have co-evolved to satiate or swamp the feeding capacity of a finite number of schooled predators and decrease the probability that a specific given individual would be captured.