Passage performance of three small-sized fish species in a Vertical Slot Fishway with and without overhead cover

Hydropower dams block fish movement, cause river fragmentation, and constitute a threat to biodiversity worldwide. Vertical Slot Fishways (VSF) are a widespread solution to mitigate these effects by enabling fish passage at obstacles. In the past, fish passage research has focused mainly on strong swimming salmonids, often resulting in knowledge gaps and low passage success for other fish species, especially small-sized ones. In addition, although hydrodynamics is fundamental to assess the functionality of a fishway, other environmental factors can impact passage success. Light can affect several behaviors in fish and often plays a role in fish migration and movement. At fishways, light can both promote and hinder passage, with highly species-specific effects remaining underexplored for many species. Here we study the passage performance in an alternatingly open and covered VSF of three small-sized fish species: brook barbel (Barbus caninus), European bullhead (Cottus gobio), and Italian riffle dace (Telestes muticellus). Passage success was very high for brook barbel (100 %) and Italian riffle dace (95.2 %) while fewer European bullhead (46.4 %) successfully passed the fishway. Fish showed a preference for passage without overhead cover: both brook barbel and Italian riffle dace displayed lower overall passage rates under covered compared to open conditions, and entry rates were lower with cover for all three species.