Dams and weirs fragment rivers and restrict fish movements between essential habitats. Fishways are widely used to reduce these effects, but their presence alone does not ensure that connectivity has been functionally restored. Performance varies among species, life stages, seasons, and locations, while the conclusions drawn depend on the methods used.

This thesis examines fishway performance through four studies. First, methods for assessing upstream passage were reviewed. Qualitative methods like trapping or fish counters are suited to describing species composition and seasonal patterns, whereas individual-based methods like mark-recapture or telemetry are needed to estimate passage efficiencies and potential delays. Second, an eight-month telemetric field study was conducted to assess the performance of a vertical-slot fishway. Only two of seven species accounted for most passages, and success was size-dependent. Once these fish reached the monitored part of the fishway, most completed passage, highlighting the importance of the stages of attraction to and entry into the fishway, as well as individual motivation. Third, fish-counter observations from 35 fishways in Sweden were combined with electrofishing records from surrounding reaches. More species and higher community-level passage effectiveness were observed in nature-like fishways than in technical fishways. Fourth, analyses of fish-counter data from four of the fishways showed that movements of rarely assessed vimba were concentrated in spring and early summer but varied among rivers and years. The long-term data allowed movements to be related to seasonal state and short-term changes in water temperature and discharge.

The studies show that fishway passage is a multi-stage process that is usually only partly captured. Assessments should match biological and management objectives, distinguish potential factors, and ideally combine methods to evaluate functional connectivity across species, sites, and time.

Dams and weirs fragment rivers and can prevent fish from reaching habitats needed for feeding, refuge, and reproduction. Fishways are built to restore passage, but their presence alone does not guarantee functional connectivity.

This thesis examines how fishway performance can be assessed across species, sites, and time. A review of available methods showed that no single approach captures all relevant aspects of passage. A telemetric field study at a vertical-slot fishway found that passage differed among species and fish sizes and hinted at the importance of attraction, entry, and individual motivation. In Sweden, fish-counter and electrofishing data from 35 fishway locations showed that more species were observed and community-level passage effectiveness was higher in nature-like than in technical fishways. Long-term observations in four of these fishways showed that movements of the otherwise rarely recorded vimba were concentrated in spring and early summer and were related to seasonal state and short-term changes in temperature and discharge.

Together, the studies show that fishway passage is a multi-stage process that is usually only partly observed. Reliable assessment therefore requires clear management objectives and, ideally, combinations of methods.