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

During migration, swimming in schools provides fish with a number of behavioural and ecological advantages, including increased food supply and reduced predation risk. Previous work shows that carangiform and tunniform swimming result in energetic advantages for individuals using a diamond swimming formation. However, little is known about the potential energetic advantage associated with schooling for anguilliform fish, such as European eel. European eels migrate over a long distance to their spawning area in the Sargasso Sea and may experience energetic advantages when swimming in schools. In this study the effect of group-wise swimming on the cost of transport was tested. In addition, the swimming pattern of eels swimming in groups was studied. Male silver eels were individually subjected to an increased velocity test (0.4-0.9 m s(-1)) with increments of 0.1 m s(-1) every 2 h. Groups comprising of seven males were swum following the same protocol. Video recordings were obtained to analyse tail beat frequency at the various water speeds. 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 +/- 32 vs. 32.0 +/- 0.6 mg O-2 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 gaffing. Although not an evidence, the present study provides support for the assumption of group migration in European eels. (C) 2013 Elsevier B.V. All rights reserved.