Improving yellow eel upstream movements with fish lifts
Authors: Santos JA, Rivaes R, Oliveira J, Ferreira T
JOURNAL OF ECOHYDRAULICS, 2016
Abstract: Fragmentation of the river continuum by high dams has been reported as one of the most important factors believed to contribute to declines of upstream-migrating European yellow-phase eel (Anguilla anguilla). Fish lifts seem to represent the most cost-effective type of fishway for improving longitudinal connectivity for many fish species across these obstacles. However, their performance for eels is unknown, or at least limited, because the gap width in retention screens is generally too large, allowing individuals to escape. To assess the effect of a reduced gap width on eel escape, yellow eel passage was continuously monitored at the Touvedo fish lift (River Lima, Portugal) during two annual periods: (1) August 2011 – July 2012, the pre-treatment period, gap width = 23 mm; and (2) March 2013 – February 2014, the post-treatment period, gap width = 5 mm. After the gap width was reduced, the number of lifted individuals increased more than threefold, while movement seasonality was similar in both periods. Accumulated rainfall and moon illumination were found to be significant environmental triggers, though mean daily flow and the frequency and magnitude of spillway discharges could have accounted for the variation on lifted eel numbers between both periods. Differences in population size structure were also noted between the two periods, principally for the fish-lift samples, with a significantly higher proportion of smaller individuals found during the post-treatment period. Reducing fish-lift gap width may decrease yellow eel escape, thereby expanding the usability of such facilities for this and other small-sized species.