The UK lies towards the southernmost distribution limit of the circumpolar Arctic charr (Salvelinus alpinus), and native populations of high national conservation value occur in all four component countries. However, given the temperature requirements of this species and the already relatively mild UK climate, there is strong reason to expect a significant negative impact of climate change on local populations. Data were assembled from repeated Arctic charr population assessments using combinations of hydroacoustics, gill netting, entrapment records and fisheries catches at five Scottish populations, five English populations and one Welsh population. These data were then used to test the hypotheses that there has been a widespread decline of Arctic charr in the UK and that it can be attributed at least in part to climate change. Ten of the 11 studied populations exhibited significant 1990-onwards declines in abundance, while only the most northerly population showed a significant increase. Overall, there was a significant positive relationship between the observed population decline ranking and a vulnerability to climate change ranking based on water body latitude, altitude and mean depth. These observations support the hypothesis that this species has suffered a recent and widespread decline in the UK, and although additional factors are undoubtedly also involved in some specific cases, climate change is a significant factor.

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A method was developed to quantify the number and biomass of European eels Anguilla anguilla escaping to the ocean for breeding. The non-intrusive method, involving a fixed-position, high-frequency multi-beam sonar, permitted constant surveillance of A. anguilla movements throughout their 5 month escapement season (July to December). During this period, >1000 individuals were monitored escaping to the Atlantic Ocean from their freshwater habitat in the River Huntspill study site (Somerset, U.K.). The total length of each fish was measured using the sonar software. These measurements were then converted to an estimate of mass using a length:mass regression relationship derived from historical fyke-net data from this site, comprising c. 500 A. anguilla length:mass measurements collected over a 10 year period. The net biomass of escapement from the study site was equivalent to c. 6 kg ha⁻¹ year⁻¹, lower than the present European target which would require at least 7 kg ha⁻¹ year⁻¹ from this habitat. These findings demonstrate the capabilities of this monitoring technique and its usefulness both as a tool to assess the compliance with conservation targets and as a tool to evaluate the success of conservation measures for elusive aquatic species such as A. anguilla.

Hydroacoustic methods are routinely used for fish population assessment and monitoring in lakes and reservoirs around the world and are particularly embedded in European and North American lake management. However, the comparability of hydroacoustic results can be difficult to assess due to the large number of variables (sound frequency, sound pulse duration, system manufacturer, analysis method, etc.) that can influence acoustic estimates. This study investigates the effect of variations in analysis method and hydroacoustic system on commonly produced outputs. Volume backscattering strength (Sv), mean target strength (TS), fish abundance, and biomass were estimated using two analysis methods (echo integration and track‐counting) using data recorded from a large, deep lake (Windermere, UK) by four vertically oriented split‐beam systems operating at three sound frequencies (120 kHz, 200 kHz, and 400 kHz). The two analysis methods used produced similar estimates of fish abundance and fish biomass. We therefore concluded that echo integration and track‐counting can be robustly used and outputs justifiably compared under the conditions of the Windermere surveys. In addition, we compared mean Sv, mean TS, fish abundance, and biomass estimates across the four hydroacoustic systems. The only significant result was for abundance estimates, however, the system made a small contribution to the overall variability. All four systems provided similar results for the other metrics, demonstrating that independent hydroacoustic surveys operating within the European Standard can make a major contribution to the assessment and monitoring of fish populations in deep lakes and reservoirs.

The exploitation of riverine systems for renewable energy has resulted in large numbers of small-scale hydropower schemes on low-head weirs. Although considered a clean and ‘green’ energy source in terms of emissions, hydropower can affect upstream migrating species by diverting flow away from viable routes over the impoundment and attract fish towards the turbine outfall. In an attempt to reduce this negative effect, hydropower outfalls with co-located fish-passage entrances are recommended, utilising turbine flows to attract fish towards the fish pass. The present study used acoustic telemetry to understand the performance of a co-located Larinier fish pass at a low-head hydropower scheme at a weir on the tidal Yorkshire Esk, England. The majority of the sea trout (anadromous Salmo trutta L.) individuals that approached the impediment were attracted to the hydropower and the co-located fish pass. Fish ascended through the pass under a wide range of river flows, tide heights, downstream river levels and hydropower flows, and there was no evidence that the hydropower operation affected fish-pass ascent. The information presented is urgently required to inform management decisions on the operation of hydropower schemes during the migratory period of salmonid fish, and help determine best-practice designs and operation at these facilities.