River connectivity reestablished:Effects and implications of six weir removals on brown trout smolt migration

Abstract

Today'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. rather, they provide an opportunity for some fish to move upstream or downstream past the barrier. This is particularly relevant for migratory fish species such as salmonids, which depend on freshwater migrations to complete their lifecycle (Jonsson & Jonsson, 1993;Klemetsen et al., 2003). Better management tools need to be implemented to promote the persistence of these migratory species, such as barrier removal and other types of restoration projects. | Brown troutThe brown trout (Salmo trutta, Salmonidae) is a partially anadromous salmonid species, native to many regions of Europe (Jonsson & Jonsson, 1993). Brown trout spawn in the upper reaches of rivers, where the substrate is typically suitable for spawning and early growth, and predators are typically absent (Armstrong, Kemp, ---------------------------------------------------------This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Passage Performance of Potamodromous Cyprinids over an Experimental Low-Head Ramped Weir: The Effect of Ramp Length and Slope

“…By identifying the importance of aquatic connectivity for good ecological quality in rivers, the European Water Framework Directive (WFD) emphasized the need to re-establish free movements for all fish species and size classes, regulating that member states should assess all instream obstacles, even small weirs, and minimize their barrier effect [4][5][6]. Since then, a few studies on small obstacles (considering assessment protocols, e.g., [7][8][9], or field assessments, e.g., [10][11][12]) and projects, such as the European project AMBER and other operational programs like the EU LIFE programs, have been developed, aiming to enhance the knowledge on permeability of small obstacles and fish passage, recommend strategies for action, and rehabilitate river habitats [3,13,14].…”

Juvenile Salmonid Assemblages at the Mirror Lake Complex in the Lower Columbia River before and after a Culvert Modification

“…, ; Birnie‐Gauvin et al. ), (2) data gaps due to inconsistent site sampling, the timing of our sampling permit, high water levels, or difficulty in accessing sites increased variability in the population measurements, and (3) different gear types or techniques used to sample the MLC sites may have increased uncertainty in comparison among the sites. Additional tests may be needed to examine the catch efficiency of the gear used, both among sites and across conditions within a site.…”

Moving beyond fitting fish into equations: Progressing the fish passage debate in the Anthropocene

“…Despite these efforts, there remains a focus on upstream movements, with less consideration given to getting fish back downstream (although efforts to address downstream movement have risen in recent years; e.g. Arnekleiv, Kraabøl, & Museth, ; Birnie‐Gauvin et al, in press).…”

“…We argue that the first question that should always be asked (perhaps twice) is whether that barrier is necessary at all, and if so whether a fishway will contribute to the maintenance of viable populations upstream and downstream of the structure (Pompeu et al, ). There is strong evidence that removing artificial barriers to migration can be cost‐effective and result in the rapid recovery of freshwater biodiversity and ecosystem processes, as seen for American eel ( Anguilla rostrata ; Hitt, Eyler, & Wofford, ), sea lamprey ( Petromyzon marinus ; Hogg, Coghlan, & Zydlewski, ) and brown trout ( Salmo trutta ; Birnie‐Gauvin, Larsen, Nielsen, & Aarestrup, ; Birnie‐Gauvin et al, in press), as well as other species (O'Connor, Duda, & Grant, ), yet barrier removal remains relatively uncommon, even where structures are redundant. Consequently, despite the growing use of fishways, which are supposedly designed to allow migrating fish to bypass barriers and reach suitable habitat in which to grow and reproduce, these structures remain mere pacifiers of the underlying ecological problems (Bunt et al, , ; Lira et al, ; Noonan et al, ; Roscoe & Hinch, ).…”