Fig. 1.

Characteristic swimming patterns for increasing fish group size at two different swimming speeds. (A–D) $U=0.77BL.s^{-1}$ (see also Movies S1 and S2). The school pattern is spread downstream with characteristic angles and distances to nearest neighbors. (E–H) $U=3.91BL.s^{-1}$ (see also Movies S3 and S4). As more effort is required to hold a high swimming regime, the fish reorganize in a compact in-line formation. In this configuration, fish within the group are synchronized with their nearest neighbors, corresponding to collaborative efficient swimming modes.