| Summary: | Abstract: Different types of fishways have been implemented worldwide to mitigate the impact of dams on diadromous and potamodromous fish species. Among these, fish lifts stand as the most cost-effective mitigation measures for high dams. The present study examines fish passage through the Touvedo fish lift (River Lima, northwest Portugal) along a 2.5 year period with the goal of: i) evaluating the conditions in terms of seasonal/daily fish movements patterns and to preliminary assess the overall efficacy of the facility (pre-treatment period, 1 year); ii) implementing specific measures to improve the efficacy previously recorded (6 months) and iii) evaluating the present conditions of the facility after measure implementation (post-treatment period, 1 year). Monitoring was continuously performed by using a combination of monthly video-recording and downstream electrofishing. A total of 3389 fish were transferred by the lift during the pre-treatment period, of which 89.7% corresponded to Iberian nase (54.1%) and European eel (35.6%), which migrated mainly during summer and beginning of autumn, independently from time of day. Mean overall efficacy was higher for potamodromous cyprinid species (0.42-0.44) and lower for the eel (0.10). Differences between fish size, downstream and in the fish lift, were also noted. After implementing a set of specific measures, the post-monitoring annual period revealed a significant increase of eel abundance, which increased more than threefold (1207 to 3852 individuals), though the timing of movements remained similar as of the pre-treatment period. An overall increase in fish lift efficacy was also noted for eel (0.21) and potamodromous cyprinids (0.44-1.35). Differences in fish size also decreased. The current work revealed that the use of the Touvedo fish lift by eels and medium-sized cyprinid species can be improved by reducing the gap between retention bars in the fish lift and by improving the attraction flow at the fish entrances.
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