Abstract
AbstractToday's river systems have been extensively modified, requiring us to rethink how we approach the management of these important ecosystems. We evaluated the effects of removing 6 weirs in River Villestrup (Jutland, Denmark) on the smolt run of brown trout (Salmo trutta) over the course of 12 years. During 5 of these years, we evaluated the number, size, and timing of smolts during their downstream migration. We found an increase in smolt output following the weir removals, along with a decrease in average length and indications of an earlier peak migration. Our results suggest that barrier removal has led to an increase in spawning success by adults, fry survival, recruitment, and smolt migration success. Weir removal is therefore a viable management approach to restore connectivity in freshwater streams and rivers, which promotes the passage of smolts as they migrate to marine environments.
Highlights
We found an increase in smolt output following the weir removals, along with a decrease in average length and indications of an earlier peak migration
Our results suggest that barrier removal has led to an increase in spawning success by adults, fry survival, recruitment, and smolt migration success
The size of the smolt run increased following the removal of weirs, with the largest class in 2015, followed by 2016 (Table 2; Figure 2)
Summary
The diversity, abundance, and sustainability of aquatic species have long been threatened by the human‐induced fragmentation of rivers (Khan & Colbo, 2008; Saunders, Hobbs, & Margules, 1991). Efforts to mitigate the impacts of barriers, such as fishpasses, have seen limited success (Bunt, Castro‐Santos, & Haro, 2012) and are usually costly (Gibson, Haedrich, & Wernerheim, 2005) Such approaches do not repair the damage done to the ecosystems as a whole (Birnie‐Gauvin, Aarestrup, Riis, Jepsen, & Koed, 2017); rather, they provide an opportunity for some fish to move upstream or downstream past the barrier. This is relevant for migratory fish species such as salmonids, which depend on freshwater migrations to complete their lifecycle (Jonsson & Jonsson, 1993; Klemetsen et al, 2003). This study investigated the effectiveness of this restoration approach with regard to the smolt run over the course of 12 years (five study years)
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