Cobble substrate in a surface bypass reduces bypass acceptance by common roach Rutilus rutilus

Historically, ecological engineered solutions for fish passage across anthropogenic barriers in rivers has mainly focused on facilitating upstream long-migrating diadromous fish, such as salmonids. More recently, have shifted their focus to a more holistic perspective, allowing species with different swimming capacity, both and downstream. This experiment investigated whether the addition of cobble passageway surface bypass could facilitate downstream movement cyprinid common roach Rutilus rutilus . Surface bypasses were constructed large experimental flumes released into monitored using PIT-telemetry through 11-h night-trials. Behavior was scored four continuously-recording video cameras at construction. There negative effect substrate-treatment rate bypass. The majority No substrate treatment had successfully passed within 4 h, while lesser proportion Substrate done so (additional latter later trials). Fish exposed showed avoidance-like behaviors ramp section tended return back often than no-substrate control trials. When reaching passageway, substrate-exposed expressed no that be indicative reduced success, compared controls. We conclude not hindered by presence substrate, but delayed due avoidance behavior when present. Based these results, cannot recommended measure performance bypasses. • Bypass acceptance tested river flume experiment. Cobble passage. With added display behaviors.