Abstract
Barrier removal and fish pass construction are increasingly used as tools to restore river connectivity and improve habitat quality, but the effectiveness of subcatchment-scale connectivity restoration on recovery of fish communities is poorly understood. We used a before-after-downstream-upstream methodology to determine the effects of subcatchment-scale connectivity restoration on fishes in a fragmented tributary of the River Wear, Northeast England, between 2013 and 2019. Following restoration (three barriers removed, five barriers fitted with fish passes, two barriers unaltered), riffle habitat increased, fine sediment decreased, and most fish species benefitted. Total fish abundance, comprising seven native species, increased 3 years after the restoration and remained elevated to the end of the study. Mean brown trout (Salmo trutta) density increased from 20.9 ± 6.3 to 33.8 ± 16.8 per 100m2 from 2013 to 2019, with Young-of-Year trout increasing from 10.6 ± 4.6 to 19.8 ± 11.8 per 100m2. Connectivity restoration reduced the mean age of trout, suggesting a change to an increased migratory component of the population. Density of bullhead (Cottus perifretum), a species with poor dispersal ability, increased from 4.6 ± 2.7 to 32.6 ± 17.9 per 100m2 over 2013 to 2019. Stone loach (Barbatula barbatula), also a less mobile species but tolerant to fine sediment, decreased in abundance where barriers were removed. Atlantic salmon (Salmo salar) were absent over the study timescale, despite being common in the Wear, and despite suitable habitat and water quality in the restored subcatchment, suggesting a hysteresis effect. Our findings indicate that, where good water quality exists, restoring river connectivity and hydromorphology at a subcatchment scale is beneficial for most native resident and migratory fishes. However, the ecological benefits of connectivity restoration, especially in rivers with many barriers, may take several years to develop. We encourage well-controlled long-term studies reporting the outcomes of large-scale connectivity restoration.
Highlights
Relative to their area, freshwater ecosystems are disproportionately important for biodiversity (Dudgeon et al, 2006; Reid et al, 2019) and ecosystem service provision (Auerbach et al, 2014; Costanza et al, 1997)
Sandy substrate coverage was lower where barriers were removed than where fish passes were installed (LMM, F1,21 = 6.51, P = 0.019)
The proportion of riffle habitat in the barrier-removed sites was significantly higher compared with fish pass sites after the restoration (LMM, F1,22 = 5.46, P = 0.029)
Summary
Freshwater ecosystems are disproportionately important for biodiversity (Dudgeon et al, 2006; Reid et al, 2019) and ecosystem service provision (Auerbach et al, 2014; Costanza et al, 1997). Rivers have been altered for millennia by humans (Gregory, 2006), but the rate of river ecosystem degradation has increased during the Anthropocene (Meybeck, 2003; Reid et al, 2019). Longitudinal river connectivity in Europe is still affected by the large number of redundant river infrastructures (Belletti et al, 2020), including in Great Britain (Jones et al, 2019), which impact fish migration and dispersal, inhibit sediment transport and alter upstream habitat and biotic components upon which fish depend (CarpenterBundhoo et al, 2020; Mueller et al, 2011; Sun et al, 2021). The restoration of more naturally functioning river ecosystems in postindustrial and other heavily modified rivers typically requires a variety of further actions, including reinstatement of hydrological, geomorphic and biological connectivity and a return to more natural instream and riparian habitats (Bernhardt et al, 2005; Feld et al, 2011; Wohl et al, 2015)
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