Session B4: Fine-Scale 2D Acoustic Tracking of the Behaviour of Salmonids to Investigate Delays and Failures in Fish Passage; Implications for Assessing the Efficiency of Fish Passes

Abstract: Many studies of fish passage are limited to consideration of coarse scale efficiency metrics; however the level of resolution they have for explaining changes in these metrics is often limited. In many cases assessments of fish pass performance are instigated due to (and complicated by) factors such as changes in fish pass design, construction of hydropower schemes and/or associated changes in hydro-morphology. In such cases finer scale descriptions of passage behaviour and fish pass proximity behaviour may be required to identify changes in passage efficiency and elucidate the factors influencing them. This presentation describes a five year acoustic tracking study (3 years pre and 2 years post installation of a co-located hydropower scheme with Larinier fish pass) of the 2D behaviour of 138 sea trout and 18 salmon as they approached the fish pass entrance and hydropower outfall on Ruswarp Weir (River Esk, England). The study determined the fine scale (sub-metre resolution) spatial and temporal utilisation of the fish pass pool and approach behaviours; analysing these in relation to river flows, tidal conditions, operation of the hydropower scheme, a change in design of the fish pass and the associated changes in pool morphology. In particular the analysis considered the delays in fish passage and behaviours of fish that were successful or unsuccessful in using the fish pass, and the different conditions they encountered whilst approaching the fish pass. The results showed that alongside a significant increase in the proportion of failed passages (reduced fish pass efficiency from 100% to 69% for sea trout) there was a significant increase in delay of passages and changes in the behaviour of fish and spatial utilisation of the pool. Some of these changes potentially relate to operation of the hydropower whilst others may relate to changes in the pool morphology and fish pass