Downstream passage behavior of potamodromous fishes at the fish protection and guidance system Flexible Fish Fence

Using an outdoor experimental channel, this study evaluates downstream passage behavior of potamodromous fish species at an innovative fish protection and guidance system, the Flexible Fish Fence (FFF). Under variation of the spacing between the cables, the exposition angle and the approach velocity, the fish protection and the guidance efficiency are investigated. Experiments were carried out using wild born and PIT-tagged chubs (Squalius chephalus), graylings (Thymallus thymallus), brown trouts (Salmo trutta) and rainbow trouts (Oncorhynchus mykiss) with body lengths between 100 and 200 mm. The results show that fish protection efficiency is predominately dependent on the cable spacing and varies strongly between fish species. While all fish (100%) are prevented from passing through the FFF at 10 mm spacing, it is far less effective at 20 mm spacing (83% of chubs, 59% of trouts and 26% of graylings). The effects of the exposition angle, the approach velocity, and the fish length are identified using multinomial logistic regression models: fish protection efficiency increases significantly with increasing fish length and increasing approach velocity (particularly for the highest angle) for chubs and trouts. Increasing flow velocity induces cable vibrations presumably triggering avoidance behavior. Unfavourable hydraulic conditions at the bypass entrance (approach velocity of 0.65 m s-1, velocity changes or turbulences) negatively affect the bypass efficiency for all fish species, pointing out that the design of bypasses for downstream migration is of major relevance.