Stopping invaders: Moving towards a selective vertical slot fishway to prevent the passage of non-native cyprinids

Invasive fish species are a major driver of freshwater ecosystem degradation across the globe. This urgent problem is particularly tough to manage in dammed rivers, where the reestablishment of longitudinal connectivity for native fish is achieved through the placement of fish passage devices, which can open a new corridor for the dispersal of these taxa to previously inaccessible habitats. In an attempt to solve this dilemma and prevent their dispersal, an experimental study was conducted in a full-scale Vertical Slot Fishway (VSF) to assess the passage performance of the common carp (Cyprinus carpio), an invasive non-native cyprinid species widespread in the Iberian Peninsula. With this objective, two configurations were designed and tested, where the main hydraulic parameters that govern fishway operation (discharge, flow velocity, turbulence and slope) were adjusted to exceed design guidelines set for cyprinid species. Common carp passage trials were conducted in configuration VSFh1 and VSFh2 (N = 8 in each configuration), varying in water depth - 0.55 m and 0.80 m, respectively, and both were set up with a high slope (15,2 %), head drop (Δh = 0.28 m) and volumetric dissipation power higher than literature recommendations (Pv > 150 Wm^-3). Fish movements were assessed in terms of motivation, transit time and ascent analysis using a time-to-event approach. The hydrodynamic scenarios experienced by fish during the trials were investigated with a computational fluid dynamic (CFD) model. Common carp passage results were compared with the performance of a native cyprinid species, namely the Iberian barbel (Luciobarbus bocagei), and pointed to selective fishway configurations, which hindered invasive fish passage movements, but favored the native species. In both configurations, common carp revealed a lower motivation with a significantly lower probability of performing passage attempts compared to the Iberian barbel. Regarding the ascent movements, none of the common carp tested managed to pass VSFh1 while in VSFh2 only one individual managed to ascend (of 3 that attempted - 33 %). Comparatively, the Iberian barbel managed to ascend both configurations, with VSFh1 showing a higher number (17) of these movements (of 17 that attempted to pass - 100 %). Overall, these promising results point to a selective passage under the tested configurations, specifically configuration VSFh1 that can assist managers in reestablishing river connectivity while deterring the spread of non-native invasive fish. Nonetheless, further studies and field validation are required to reinforce the present findings.