Spatial preferences of Iberian barbel in a vertical slot fishway under variable hydrodynamic scenarios

Abstract

Fish require access to multiple habitats to persist. Therefore, obstacles blocking a fish’s volitional ability to move between habitats can lead to its decline. A common way of addressing this problem is the construction of fishways. The design criteria of fishways are based on steady, uniform flows with the assumption that the water levels remain the same across the fishway. However once constructed, fishways are fundamentally dynamic systems exhibiting different hydrodynamic scenarios. These scenarios are classified as uniform and non-uniform water level profiles and are driven by changes in the up- and downstream water levels caused by operational and hydrological variability.In this paper, we demonstrate the need to consider a wider range of fishway hydrodynamic scenarios for an integral assessment of fishways. To do this, the spatial distribution of Iberian barbel (Luciobarbus bocagei) in a full-scale model of a vertical slot fishway under three different hydrodynamic scenarios is investigated. Video tracking was used to analyze the fish’s spatial distribution, and an acoustic Doppler velocimeter and lateral line probe were applied to characterize the local hydrodynamic conditions of each scenario. Spatial differences were tested using Syrjala's test and the relation with hydrodynamic variables analyzed by means of zero-inflated Poisson models. This study revealed three significant findings: 1) Fish made broader use of the fishway pool in scenarios with lower water drops, which are highly correlated with regions of overall lower turbulence and velocity magnitude. 2) The relative importance of physical variables varied between scenarios. Specifically, the protection offered by walls was found to be a determining factor in the spatial usage. 3) It is shown that a vertical slot fishway designed with the standard dimensions for fish passage under the assumption of uniform flow conditions may result in an adverse hydrodynamic environment for fish swimming under non-uniform conditions.