Understanding the impact of barriers to onward migration; a novel approach using translocated fish.

River catchments worldwide are heavily fragmented by anthropogenic barriers, reducing their longitudinal connectivity and contributing to the decline of migratory fish populations. Direct impacts of individual barriers on migratory fish are well-established, but barrier impacts on onward migration are poorly understood, despite their relevance to evidence-based, catchment-scale, management of threatened species. This study investigated the upstream spawning migration of 352 acoustic tagged river lamprey (Lampetra fluviatilis), translocated upstream of two key barriers (R2: n = 60 & 59; R3: n = 59 & 52) compared to a control group (R1: n = 61 & 59), across two contrasting (dry and wet, n = 180 and 172) years in the River Yorkshire Ouse, England, to reveal the impact of barriers on the onward migration of upstream migrating fish. Release further upstream increased the degree of catchment penetration, with median distance upstream of R1 56.1% and 68.6% greater for lamprey released at R2 and R3 respectively. Median delays at the two downstream-most main river barriers by the control group were 23.8 and 5.4 days (2018/19) and 9.3 and 11.4 days (2019/20). However, impacts of delay were only observed on the time to reach spawning habitat, time to reach final assumed spawning location and speed of movement in one upper catchment tributary during 2019/20 whilst they were only observed on time to reach spawning habitat during 2018/19 and on assumed spawning location distance during 2019/20 in the other. Ultimately, limited impacts of delay at barriers on onward fish migration post-passage were observed but median catchment penetration was increased with consecutive release upstream. This study demonstrated the importance of a true understanding of barrier impacts to inform catchment-wide planning, evidence vital for management worldwide. Although the findings of this study do support the use of trap and transport as a measure to remediate barrier impacts on migration, fish passage engineering improvements or barrier removal, at structures shown to be the most inhibiting to fish migration should be considered the best and most sustainable option to improve barrier passage.

Overcoming the dichotomy of implementing societal flood risk management while conserving instream fish habitat - A long-term study from a highly modified urban river.

Flood Risk Management (FRM) is often essential to reduce the risk of flooding to properties and infrastructure in urban landscapes, but typically degrades the habitats required by many aquatic animals for foraging, refuge and reproduction. This conflict between flood risk management and biodiversity is driven by conflicting directives, such as the EU Floods and Water Framework Directives, and has led to a requirement for synergistic solutions for FRM that integrate river restoration actions. Unfortunately, ecological monitoring and appraisal of combined FRM and river restoration works is inadequate. This paper uses a case study from the River Don in Northern England to evaluate the effects of the FRM and subsequent river restoration works on instream habitat and the associated fish assemblage over an 8-year period. Flood risk management created a homogeneous channel but did not negatively affect fish species composition or densities, specifically brown trout. Densities of adult brown trout were comparable pre and post-FRM, while densities of juvenile bullhead and brown trout increased dramatically post FRM. River restoration works created a heterogeneous channel but did not significantly improve species composition or brown trout density. Species composition post-river restoration works returned to that similar to pre-FRM over a short-term period, but with improved numbers of juvenile bullhead. Although habitat complexity increased after river restoration works, long-term changes in species composition and densities were marginal, probably because the river reset habitat complexity within the time framework of the study.

Dispersal and survival of stocked cyprinids in a small English river: comparison with wild fishes using a multi-method approach.

Mark-recapture and fixed-station passive integrated transponder (PIT) telemetry were used to compare movements, distribution and survival of stocked juvenile chub Leuciscus cephalus and roach Rutilus rutilus with those of wild conspecifics. Daily activity of wild fish activity was affected by a combination of river flow and temperature, whereas stocked fishes were not influenced by environmental factors. PIT telemetry recorded exploratory movements of stocked L. cephalus immediately after stocking, a substantial number of stocked fish moved both downstream and upstream during periods of elevated flow, and proportionally more stocked fish moved during the first 6 weeks after release than later on. Proportionally more stocked fish than wild fish moved through PIT antennae, stocked L. cephalus moved greater distances than wild L. cephalus and were more widely distributed than wild fish. Minimum estimates of survival after 5 months were 50.5% for stocked R. rutilus and 28.0% for stocked L. cephalus. Ultimately, stocked cyprinids appeared to be able to cope with elevated flows and most remained in the river section local to the stocking location.