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Abstract: Avoidance of abrupt accelerations of flow exhibited by downstream migrating fish at screens used to divert them, or at fishway entrances, can cause delay and adversely impact efficiency. The use of alternative stimuli to attract fish and mask the unwanted deterrent effects associated with velocity gradients is of interest to those working in fish passage engineering. The influence of a continuous light source on the downstream movement of brown trout (Salmo trutta) as they encountered accelerating velocities created by a constricted channel in an experimental flume under three discharge regimes was assessed. It was predicted that: (1) in the absence of a light source, behavioural responses typical of downstream moving salmonids would be elicited on encountering velocity gradients, and that these responses would be initiated at some threshold spatial velocity gradient relative to body length and (2) light would act as an attractant and mask the deterrent effects of a velocity gradient and thus reduce delay. Typical avoidance behaviours, e.g. rheotactic switches in orientation or retreating upstream before re-approaching a velocity gradient, were common. The spatial velocity gradient threshold at which a response was initiated when dark was similar (ca. 0.4cms−1 cm−1) independent of discharge. Fish responded farther upstream at a lower spatial velocity gradient threshold (ca. 0.2cms−1 cm−1) in the presence of both mechanosensory and visual cues when light. Contrary to the second prediction, downstream movement was further delayed by the addition of a light stimulus. The findings support an alternate hypothesis, that responsiveness (avoidance) can be enhanced when multimodal stimuli are presented. [Copyright &y& Elsevier]

Artificial light at night (ALAN) is considered one of the most pervasive forms of environmental pollution. It is an emerging threat to freshwater biodiversity and can influence ecologically important behaviours of fish. The European eel (Anguilla anguilla) is a critically endangered catadromous species that migrates downstream to the ocean to spawn in the Sargasso Sea. Given the pervasive nature of ALAN, many eel will navigate through artificially lit routes during their seaward migration, and although considered negatively phototactic, their response has yet to be quantified. We investigated the response of downstream moving European eel to simulated ALAN using a Light Emitting Diode unit in an experimental flume. We presented two routes of passage under: (1) a dark control (both channels unlit), (2) low ALAN (treatment channel lit to ca. 5 lx), or (3) high ALAN (treatment channel lit to ca. 20 lx). Eel were: (i) more likely to reject an illuminated route when exposed to high levels of ALAN; (ii) less likely to select the illuminated channel when given a choice; and (iii) passed downstream more rapidly when the illuminated route was selected. This study quantified the response of the critically endangered European eel to ALAN under an experimental setting, providing the foundations for future field based research to validate these findings, and offering insight on the ecological impacts of this major environmental pollutant and driver of global change. Image 1 • An experiment assessed the impact of ALAN on a critically endangered fish. • Fish behaviour was affected by ALAN. • Individuals were less likely to select to pass an illuminated route when given the choice. • Individuals passed downstream faster when the illuminated route was selected. Eel more frequently rejected a route when exposed to simulated light pollution, were less likely to select the illuminated route given a choice and passed faster when the illuminated route was selected. [ABSTRACT FROM AUTHOR]

Globally, gauging weirs are commonly installed in rivers to monitor flows. They have widespread negative impacts on the upstream movement of fish due to the barriers created by the unfavourable hydraulic conditions on the downstream face. There is a need to develop simple low-cost solutions to enhance multi-species fish passage, while not impacting the ability of these structures to gauge flow or increasing their potential to accumulate debris. This study investigated the use of an array of Cylindrical Bristle Clusters (CBCs), mounted on the downstream face of Crump weirs, to improve upstream passage of multiple species of fish while maintaining gauging accuracy. The simplicity and modularity of the design helps improve cost effectiveness and ease of installation. Laboratory tests with roach (Rutilus rutilus) showed that the passage efficiency of a Crump weir was increased from 0% (control) to ≈30% when retrofitted with CBCs. Swim path analysis indicated fish utilised low velocity zones in the wake of clusters to facilitate passage. Time taken to pass the weir and length of the swim path were greatest under the highest cluster density, where manoeuvrability was most constrained. Following these promising experimental results, the fish pass efficiency of a weir retrofitted with a staggered array of CBCs, was evaluated in the field. Upstream passage of a number of non-salmonid fish, including chub (Squalius cephalus) and roach, was monitored using Passive Integrated Transponder telemetry at a 7 m long, 1.2 m wide, Crump weir with a 1:5 downstream slope. Overall passage efficiency, considering all species, increased from 2% to 14% when the fish pass was installed, and for chub from 0 to 52%. Debris accumulation was minimal during the test period. Assessment of the impact of a variety of CBC array densities on gauging was also undertaken. As predicted using theory, there was no effect on gauging when arrays were placed downstream of the point at which flow regime changes from sub- to supercritical. This study highlights the potential for a staggered array of CBCs to improve the upstream passage of multiple species of fish at gauging weirs, common barriers to fish migration throughout the world, without affecting the accuracy of flow gauging. [ABSTRACT FROM AUTHOR]