We studied the swimming behaviour of 60 adult Iberian barbel (Luciobarbus bocagei) of two size-classes (small fish: 15unsteady flow conditions created by three submerged orifice arrangements in a prototype pool-type fishway: i) offset orifices, ii) straight orifices and iii) straight orifices with a deflector bar of 0.5bo located at 0.2 L, where bO is the width of the square orifices and L is the pool length. Water velocity and turbulence (TKE, TI and Reynolds shear stress and eddies) were analysed and related with fish swimming behaviour. The efficiency of each configuration was assessed through the number of successful fish passage attempts and the associated times. The amount of time fish spent in a certain cell of the pool (transit time) was measured and related to hydraulic conditions. The highest rates of passage and the corresponding lowest transit times were found in experiments conducted ·with straight orifices (Kruskal-Wallis ANOV A: P<0.05). Although size-related behavioural responses to turbulence were observed, Reynolds shear stress appeared as the main turbulence descriptor explaining fish transit time for both size classes. The swimming behaviour of larger fish was found to be strongly affected by the eddies created, in particular by those of similar size to fish total length, which were mainly found in straight orifices with a deflector bar arrangement. We believe that the results provide valuable insights on barbel swimming behavioural response to turbulence that could help engineers and biologists develop effective systems for the passage of this species and other of similar biomechanical skills, for which a significant lack of knowledge still exists.