Fragmentation of the river continuum by high dams and subsequent reduction in habitat availability has been cited as an important factor contributing to population decline of inland stocks of European eel. Eels have typically difficulty using traditional fishways because water velocities and turbulence limit their upstream movements. However, when it comes to use fish lifts, performance is often unknown. 

Upstream yellow eel passage was continuously monitored at the Touvedo fish lift (Lima River, northwest Portugal) by a combination of video-recording and electrofishing sampling during two annual sampling periods to analyze the effect of reduction of the free gap between the bars of the trapping cage on eel escapement: i) August 11 to September 12, the pre-treatment phase when the free gap was 23 mm and ii) March 13 to February 14, the post-treatment phase when the free gap was reduced to 5 mm.

Upon reduction of the free gap of the trapping cage, the number of lifted individuals increased more than threefold (1207 to 3852 individuals, Wilcoxon match-paired test, Z=2.31, P=0.021), though the seasonality of movements and the environmental triggers of migration remained similar during both periods. Differences in population size–structure were also noted between the pre- and post-treatment phases (Kolmogorov-Smirnov test, Z=5.357, P< 0.001), with a significant higher proportion of smaller individuals found during the post-treatment.

Reducing the free gap between retention bars of fish lifts may decrease escapement of small yellow eels, therefore expanding the use of such facilities for this and other small-sized species individuals.