Individual patterns of rhythmic swimming activity in Anguilla anguilla glass eels synchronised to water current reversal

The existence of distinct patterns of activity and swimming behaviour were tested in individual European glass eels by means of Bayesian inference mixture modelling. 36 glass eels were tagged using Visible Implant Elastomer and added to 36 untagged glass eels in February and April. Each group was presented with a change in water current direction every 6,2 h and videotaped during 2 weeks. Two hypotheses were tested: (i) all individuals display a similar pattern of behaviour within a tidal period, glass eels showing both positive and negative rheotaxis in opposite phase (M1 model) and (ii) individuals are distributed in two different groups, some glass eels swimming with and the others against the current (M2 model). Results showed that most glass eels displayed a positive or a negative rheotaxis and only a small number exhibited both behaviours. All swimming behaviours were synchronised to a change in current direction with a period close to the tidal one. Results are discussed in relation to synchronisation and migration behaviour.