You shall (not) pass! Steep low-head ramps as potential selective barriers for fish movements

Abstract

River fragmentation by instream barriers obstructs the movements of organisms, reducing the amount of available habitat for native fishes. However, instream barriers can also block the spread of non-native invasive ones. This study aimed to assess the selectivity of a small instream barrier, a steep (120%) low-head ramp, for successful movements of a native (the Iberian barbel, Luciobarbus bocagei), a non-native translocated (the Pyrenean gudgeon Gobio lozanoi) and a non-native invasive (the bleak, Alburnus alburnus) fish species, representing a harmfulness gradient to river ecosystems. Two variables were recorded: i) the number of attempts (AT) to negotiate the ramp and ii) the number of successful negotiations (S), which were used to calculate passage efficiency PE(%). Overall, AT was significantly higher for the barbel relative to the gudgeon and the bleak. PE(%) was also far superior for the barbel than for the non-native fishes: accordingly, thirty-four ramp successful negotiations were reported for the former – resulting in an overall PE (%) of 26.6 - whereas no gudgeon or bleak individuals were able to negotiate it (PE(%) = 0). The results of this study show that selectivity at small barriers can be achieved if properly designed, being useful in river management where non-native invasive species are a concern.