Abstract
Abstract River barrier removal is used increasingly as a conservation tool to restore lotic habitat and river connectivity, but evidence of its efficacy is incomplete. This study used a before–after methodology to determine the effects of removing a tidal‐limit barrier on the fishes, macroinvertebrates, and habitats of an English coastal stream. Following barrier removal, habitat diversity increased immediately upstream and remained similar downstream. Mobilized silt altered the substrate composition immediately downstream, but this was temporary as silt was flushed out the following winter. Changes to macroinvertebrate communities occurred upstream and downstream of the former barrier but these were transient. A dramatic and sustained increase in fish density occurred immediately upstream of the barrier after its removal, but effects downstream were minor. The fish community upstream changed, largely due to rapid recruitment and dispersal of endangered European eel (Anguilla anguilla). Eel density in the formerly impounded zone increased from 0.5 per 100 m2 before barrier removal to 32.5 per 100 m2 5 months after removal. By 17 months after barrier removal there was no difference in eel density across the six sections sampled. Although resident stream fishes such as bullhead (Cottus gobio species complex, protected under the European Habitats Directive) were abundant in middle and upper‐stream sections, brown trout (Salmo trutta, a listed species for biodiversity conservation in England and Wales) density remained low during the study and recruitment was poor. This suggests that although colonization access for anadromous trout was available, habitat upstream may have been unsuitable for reproduction, indicating that wider catchment management is required to complement the restoration of connectivity. These findings suggest that tidal barrier removal is an effective method of restoring lotic habitats and connectivity, and can be beneficial for resident and migratory fishes including those of conservation importance (e.g. European eel) in coastal streams.
Highlights
Instream obstacles such as dams, weirs, and culverts fragment rivers by interrupting longitudinal connectivity and altering habitat (Nilsson et al, 2005; Sun, Galib & Lucas, 2020), having major effects on the biodiversity and functioning of river ecosystems (Bunn & Arthington, 2002; Pringle, 2003; Galib et al, 2018)
Eel densities before barrier removal differed significantly among sampling sections (LMM, F5,60 = 29.54, P < 0.001); eel abundance was higher in Section 1 than other upstream sections in both seasons (Figure 6, Table S7; Linear mixedeffects models (LMMs) pairwise post hoc, P < 0.001 in all cases)
This study suggests that for small tidal barriers in temperate climates barrier removal is an appropriate method by which to restore aquatic habitat and increase the abundance of both resident and migratory fish species, especially benefitting eels
Summary
Instream obstacles such as dams, weirs, and culverts fragment rivers by interrupting longitudinal connectivity and altering habitat (Nilsson et al, 2005; Sun, Galib & Lucas, 2020), having major effects on the biodiversity and functioning of river ecosystems (Bunn & Arthington, 2002; Pringle, 2003; Galib et al, 2018). Tide flaps and management of sluices can be used to support the passage of eels in tidal reaches (Environment Agency, 2011; Wright, Wright & Kemp, 2015; Guiot et al, 2020) Unlike these mitigation measures, barrier removal reinstates hydrological connectivity, more natural habitat, sediment transport, and free movement of aquatic biota (Roni, Hanson & Beechie, 2008; Kemp & O'Hanley, 2010; O'Hanley, 2011). The changes in aquatic habitat, fish abundance, and fish and benthic invertebrate communities were measured in response to the removal of a tidal weir in a small stream of the River Tees, north-east England. A bristle elver pass was installed on the left side of the pool fish pass in September 2018
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