Multi-species assessments of fishway performance

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.