Evaluating the effectiveness of restoring longitudinal connectivity for fish migration and dispersal in impacted river systems

Abstract

In this study, the impact of anthropogenic in-stream structures on migration and dispersal of a variety of fish species was investigated. The effectiveness of attempted restoration of longitudinal connectivity in fragmented stream and river systems, through implementation of various fish pass designs, both technical and nature oriented, was evaluated for fish of varying age classes. The outcomes of this research are of importance for river management schemes that aim to restore degraded freshwater systems bound by legislation from the EU Water Framework Directive, which requires good connectivity and habitat quality within freshwater systems.
Fish community composition in relation to effects of in-stream structures and habitat conditions was investigated on three degraded stream networks, so that future opportunities for longitudinal connectivity restoration could be identified. Differences in fish community composition above and below common types of structures were determined, whereby species richness and density per species showed greatest differences for flow manipulating culvert structures, especially pipe culverts.
Obstacle effects on fish dispersal and migration, and the effectiveness of connectivity restoration - obstacle removal and design of various types of fish passes - on those same obstacles were evaluated using a variety of approaches. These included capture-mark-recapture of different life stages of a weak swimmer (European bullhead Cottus perifretum) and of a strong swimmer (brown trout Salmo trutta) tagged with passive integrated transponder (PIT) and/or visible implant elastomer (VIE), displacement studies of juvenile trout and radio telemetry of adult freshwater-resident and sea-going morphotypes of brown trout. Longitudinal connectivity for fish was shown to have improved post-restoration, exemplified by improved fish passage over the multiple structures in both directions, while permeability of unrestored structures remained low.
The efficacy of a technical Larinier super active baffle fish pass, a widespread design in Europe, was assessed for upstream-migrating adult river lamprey. Situated next to a common Crump weir, the pass was designed to facilitate passage for a wide range of fish species. Over two migration seasons, with a variety of flow levels occurring in both, the pass was tested for efficacy before and after modification with studded tiles mounted on the inside wall. By using fixed PIT arrays, poor passage through the fish pass by river lamprey was shown before modification, and improved marginally post-modification. Acoustic telemetry identified direct passage over the weir to be two-fold higher than through the modified fish pass.
In the context of the Water Framework Directive, there is a strong need to develop well-grounded fish passage criteria, not only for fish species with strong swimming capacity, but for a wide range of fish species and life stages. Ideally, all types of fishways should be critically assessed, in situ, to allow river managers to make more informed decisions on restoring fragmented stream systems.