Does size matter? Trade-offs in swimming speed and schooling behavior among shiner perch (Cymatogaster aggregata) conspecifics of dissimilar size

Abstract

In animals, group motion can be highly organized in oriented and polarized structures. In fish, such movement is referred to as schooling behavior, and while a large body of literature has focused on characterizing this behavior in relation to group size, the implications of having individuals of differences in body sizes on group cohesion and coordination are unexplored. This study focused on this knowledge gap and investigated characteristics pertaining to swimming behavior in surfperch Cymatogaster aggregata among pairs of similar and dissimilar size (i.e. groups of two big individuals, two small individuals, or one big and one small individual). We analyzed the swimming speed of both individuals, nearest neighbor distance (NND), and characterized the proportion of time spent swimming actively as a group for 9 pairs of fish. Results showed that individuals in big-big groups interact with one another almost half of the time (proportion of time spent swimming together was 48%), a high proportion relative to small-small or big-small groups (12% and 7%, respectively). Swimming speed distributions indicated that the big-big groups were the most active. The average swimming speeds were 135.5 mm/s for big-big groups (equivalent to 1.37 BL/s), 82 mm/s for small-small groups (equivalent to 1.6BL/s), and 116.3 mm/s for groups of dissimilar size (equivalent to 1,05 BL/s for the big fish and 2.32 BL/s for the small fish). Although the observed patterns will require confirmation by completing the analysis with other pairs, and by quantifying other variables linked to locomotion and trajectory, these preliminary results show that while individuals of different size classes tend to adopt group swimming for shorter time periods than when paired with individuals of a same size class, they are also able to adjust their respective swimming speeds during such periods of interaction to maintain group swimming. The existence of such tradeoffs could explain the capacity of smaller individuals to remain in shoals and schools of mixed size classes.