Fragmentation of rivers by manmade barriers has impeded the ability of riverine fish to move freely. Barriers can be improved by fishways that can partially mitigate the negative impacts by acting as aquatic corridors. Effective fishways require knowledge about the physiological and spatial demands of fish species, but the existing knowledge largely derived from laboratory settings. Evaluating fishway performance is needed for optimisation of their hydraulic design and positioning. Qualitative methods include trapping, electrofishing, and camera observations to estimate the number of individuals passing (effectiveness). For quantitative assessment, the study of individual fish behaviour can identify fishway sections in need of improvement and estimate associated efficiencies. This can be accomplished by telemetric techniques such as PIT tagging, radio, and hydroacoustic telemetry.

Hydropower dams come with high ecological and social costs, not least concerning longitudinal connectivity in rivers, which causes declines and sometimes local extinctions of fish species. Fishways are widely used to allow fish to pass dams, but their efficiency is highly variable between species and sites. Many species, and at places entire fish communities, remain understudied, likely hindering the implementation of effective remedial measures. Here, we studied fish passage behaviour in a vertical slot fishway in the Po River, Italy. Almost 1000 individual fish of nine species, representing the local fish community, were tagged and released within and downstream of the fishway. The only species passing the fishway at relatively high numbers were potamodromous barbel and Italian chub, and for these species passage success was positively related to fish size. Passage was more likely to occur at night than during the day for barbel, but not for chub. In relation to the dispersal of invasive species, it is noteworthy that a few individuals of wels catfish and common carp passed the fishway.

River fragmentation by dams severely restricts fish migration, yet evaluations of fish passage solutions across species remain limited. We used data from 35 Swedish fishways (20 nature-like and 15 technical) combining optical fish counter observations (2013–2023) with electrofishing records (2008–2023) from adjacent river stretches to assess passage effectiveness on a community level. Nature-like fishways facilitated passage for significantly more fish species than technical fishways and had a higher upstream and downstream passage effectiveness. Specifically, species richness was on average 42 % higher in nature-like fishways for migratory species and 46 % for migratory and stationary species together. On average, 82 % of present migratory species passed upstream in nature-like versus 62 % in technical fishways, while downstream passage effectiveness averaged 81 % versus 55 %. Including stationary species illustrated the same trend, but at reduced effectiveness values. A few species were observed in more nature-like than in technical fishways (Lota lota moving upstream, Leuciscus idus and Rutilus rutilus moving downstream, overall observations of Alburnus alburnus), but data were scarce on the species level. Our results demonstrate that data from fish counter and electrofishing databases can be effectively integrated to assess fishway performance.

Migratory freshwater fishes are highly threatened, yet knowledge of migration timing and its environmental drivers remains limited for many cyprinids. We aimed to define the seasonal and diel timing of vimba (Vimba vimba) movements and their relation to abiotic conditions using fish counter records from four Swedish rivers. Daily counts of upstream and downstream movements were related to river discharge, water temperature, short-term changes in both variables, degree-days, and cyclical predictors: day of the year, day length, and an expected movement season defined by Hidden Markov Models. We used hurdle generalized linear mixed models to test these relationships.

Most movements occurred in May and June during receding discharge and rising water temperature, although the earliest movement wave occurred in late April in River Virån. Frequent movements were expected between May 11 and June 23. Diel patterns differed among rivers, ranging from largely nocturnal in River Virån to mainly diurnal in rivers Trosaån and Svartån, with River Emån intermediate. For upstream movements, the best-supported model included short-term discharge change, water temperature, short-term temperature change, degree-days, and movement season. Higher water temperature increased predicted counts and lowered odds of zero observations, whereas increasing discharge and higher degree-days showed the opposite pattern. Downstream movements showed similar associations, but with greater model uncertainty. These results define a predictable migration window for vimba and suggest that movements occur in response to seasonal state and short-term thermal and hydrological dynamics. The findings can support fishway operation and environmental flow management in Baltic rivers.