Rapid response of fish and aquatic habitat to removal of a tidal barrier

Abstract

1. 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.2. 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. 3.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 m 2 before barrier removal to 32.5 per 100 m 2 5 months after removal. By 17 months after barrier removal there was no difference in eel density across the six sections sampled. 4. 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.5. 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.

Short-Term Effects of Low-Head Barrier Removals on Fish Communities and Habitats

“…This is one of the first studies to report on the effects of removal of small barriers across multiple catchments, albeit over a short time scale and in the immediate upstream locality of the former barrier. Previously reported studies have predominantly focused on the effects of single barriers (Birnie-Gauvin et al, 2017a;Poulos and Chernoff, 2017;Sun et al, 2021) or multiple barriers within the same catchment (Birnie-Gauvin et al, 2018Sun et al, 2022). Results of this study show, as hypothesized, that removal of barriers can have a positive impact on lotic habitat and fish density at some sites.…”

“…Results of this study show, as hypothesized, that removal of barriers can have a positive impact on lotic habitat and fish density at some sites. This adds to the growing evidence that barrier removal can be an effective measure for stream conservation (Burroughs et al, 2010;Hitt et al, 2012;Birnie-Gauvin et al, 2018Sun et al, 2021Sun et al, , 2022. The strongest positive effect was recorded at some of the Danish sites, which showed large rapid increases in salmonid density following barrier removal.…”

“…Studies on fish communities following the removal of small structures have largely shown ecologically positive responses (Burroughs et al, 2010;Birnie-Gauvin et al, 2017a;Poulos and Chernoff, 2017;Birnie-Gauvin et al, 2018Sun et al, 2021). Thus far, however, river barrier removal studies have focused on single sites or single catchments.…”

Back to Ecology: Reference Conditions as a Basis for Assessment, Restoration and Sustainable Management of Large Rivers

“…Parameters related to barriers have also been shown to be key factors for benthic invertebrate community composition and ecological status [19], together with fish composition in large rivers [54]. Fish communities may show a rapid response to dam removal [55]. However, although restoration is needed as river degradation is ongoing in Europe, it is evident that large rivers are affected by multiple stressors [4,35,56,57].…”

Slippery customers for conservation: Distribution and decline of anguillid eels in South Africa

“…Where possible, river barriers that no longer have a use should be removed, as this permits rapid recolonization by eels (Hitt, Eyler & Wofford, 2012; Sun, Galib & Lucas, 2021). In this regard, the Kruger National Park in South Africa is taking the lead by removing all of the artificially constructed river barriers present in the park (Freitag‐Ronaldson & Venter, 2008; Pollard, Du Toit & Biggs, 2011) for the benefit of all biota present.…”