Effects of group size on schooling behavior in two cyprinid fish species

Group living is a commonly observed phenomenon in fish species in nature while the benefits and costs of group living may vary among species with different habitats. To test whether the properties of schools and individual response to group size vary between fish species, I used juveniles of 2 closely related group-living cyprinids as an experimental model: Chinese bream Parabramis pekinensis, which prefer still water and display high levels of aggression, and qingbo Spinibarbus sinensis, which prefer flowing water and are relatively unaggressive. I videoed both species in groups of 2, 4 and 8 individuals at 25 degrees C. Their motion and the spatial structure of the schools were then analyzed, mostly using an automated tracking program. In general, qingbo showed higher spontaneous swimming speeds than Chinese bream, but both species showed increased swimming speed with increased group size, possibly due to increased boldness and decreased vigilance when in larger groups. The nearest neighbor distance decreased with the increase in group size in both species. The synchronization of swimming speeds and the separation swimming angle between individuals decreased with increasing group size in Chinese bream but remained the same in qingbo. The higher synchronization of swimming between qingbo and the relatively random distribution of group members within the schools compared to those of Chinese bream schools suggest that qingbo are more adapted to group living than Chinese bream, which is possibly due to differences in their preferred habitats and/or the structure of their social hierarchy.