In Flanders (Belgium) most lowland rivers are straightened and fragmented (watermills, locks, weirs) almost exclusively for economic reasons (hydropower, agriculture, shipping traffic and flood protection). The disruption of the longitudinal river continuum has led to ecological catastrophes such as the extinction of several diadromous fish species and isolation/extinction of vulnerable potamodromous species. Mitigating actions are therefore needed to restore fish migration, e.g. the building of fish passage facilities.

In this study the results of the evaluation of a nature-like bypass around a watermill in the river Zwalm are presented. The bypass was monitored from April to June 2007 using a permanent trap located in the fish pass. A total of 1104 fish migrated through the bypass. Twenty-two fish species were caught. The dominant fish species were roach (Rutilus rutilus, 48%) and gudgeon (Gobio gobio, 38%). Other species passing through in considerable numbers were bleak (Alburnus alburnus), eel (Anguilla Anguilla), three-spined stickleback (Gasterosteus aculeatus), perch (Perca fluviatilis) and gibel carp (Carassius auratus gibelio). Diadromous species caught were river lamprey (Lampetra fluviatilis) and eel.

The overall efficiency of a fish pass is ruled by its attraction and passage efficiency. The outlet of the fish pass, approximately 60m downstream of the migration barrier (weir), is not ideally situated. The bypass can only be made attractive if all or at least the majority of the water is flowing through the bypass channel. Under these circumstances upstream migrating fish are attracted to the fish pass and not to the weir. Due to abnormal dry weather conditions during the first half of the study period (0mm precipitation in April 2007) the majority of the discharge water was flowing through the bypass channel. During this period attraction towards the weir was probably negligible. The outlet construction of the fish pass consists out of a concrete dam with a vertical slot. Due its superficial position the attraction flow penetrates only superficially into the headstream of the river. High stream velocities (> 2ms^-1) were measured in the vertical slot. Stream velocities that are probably critical for certain species and/or life stages.

Better management off the weir is needed to assure permanent fish migration. Re-dimensioning of the fish pass, e.g. a bypass with a winter- and summer bed, is needed to allow higher water discharges through the fish pass during periods with increased river discharge. Technical adaptations on the outlet construction are necessary to improve the attraction and passage efficiency of the fish pass.