Intra-school positional preference and reduced tail beat frequency in trailing positions in schooling roach under experimental conditions

Using three different swimming velocities and a school size of eight roach Rutilus rutilus, individual intra‐school position and tail beat frequency were examined in a flume tank. Tail beat frequency was determined in defined leading and trailing positions. Individual roach showed consistent intra‐school positional preferences which implied a sustained positional pattern where certain individuals took up front positions whereas other individuals swam in the rear part of the school. The positional preferences could not be attributed to inter‐individual differences of the eight roach in terms of total length, mass or condition factor. At the tested swimming velocities of 2, 3 and 4 LT s−1, roach in trailing positions swam with tail beat frequencies reduced by 7·3, 11·9 and 11·6%, respectively, compared to roach in leading positions. These results suggested that roach situated in trailing positions experienced energetic savings due to hydrodynamic interactions at a wide range of swimming velocities. This may be important during migrations or when a school of roach is holding its position against the current in a lotic habitat. The observed sustained positional pattern combined with a hydrodynamic advantage in trailing positions would indicate that these energetic savings might not be evenly shared among schoolmates of roach. A positive correlation between swimming velocity and stride length was found. The present study, however, does not support any conclusions concerning these findings.