Schooling reduces energy consumption in swimming male European eels, Anguilla anguilla L.

At all swimming speeds the oxygen consumption was significantly lower in group-wise vs. individually swimming males. The cost of transport at the optimal swimming speed of group-wise swimming males was significantly lower than that of the individually swimming males (21.3 ± 3.2 vs. 32.0 ± 0.6 mg O2 kg− 1 h− 1, respectively). The optimal swimming speeds, however, were not significantly different (0.57 ± 0.02 and 0.52 ± 0.04 m s− 1 respectively). At speeds of 0.50 m s− 1 and above, tail beat frequency was lower in males swimming in groups than in males swimming individually (2.6 ± 0.1 and 3.8 ± 0.1, respectively). In contrast to the 'diamond' shape pattern of many group-wise swimming fish, eels tend to swim in a synchronized fashion parallel to each other, possibly reducing swimming costs by Karman gaiting. Although not an evidence, the present study provides support for the assumption of group migration in European eels.