In Flanders (Belgium) most lowland rivers are straightened and fragmented (locks, weirs, watermills) almost exclusively for economic reasons (shipping traffic, agriculture, hydropower 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. This study is part of the European Community Initiative INTERREG III project IASM. In this cross-border project (Belgium – The Netherlands) mitigating actions are taken to restore free fish migration in the lowland River Mark. As it is important that these mitigating actions, i.e. the building of fish (by)passes, are evaluated on their proper ecological functioning, this case study presents the results of the evaluation of one of four newly built fish passes in the River Mark in Belgium (i.e. fish pass Markweg at Meersel-Dreef). The pool and weir shaped fish pass was monitored between March 13th and July 10th 2006 using a permanent trap at the upstream end (inlet) of the fish pass. The outlet of the fish pass is ideally situated just next to the migration barrier (weir) creating a distinct attraction flow. The Vshaped weirs of the fish pass were made out of big rocks manually embedded in concrete, hereby creating a diverse stream velocity pattern over the weirs. A total of 3768 fish migrated upstream through the fish pass during the study period and were trapped in the fyke net. Fifteen fish species were caught. The dominant fish species was roach (Rutilus rutilus, 52%), followed by gudgeon (Gobio gobio, 41%), perch (Perca fluviatilis, 2%) and rudd (Scardinius erythrophtalmus, 2%). Less common species caught were white bream (Blicca bjoerkna), gibel carp (Carassius auratus gibelio), eel (Anguilla anguilla), pike (Esox lucius), carp (Cyprinus carpio), tench (Tinca tinca), stone loach (Barbatula barbatula), bream (Abramis brama), threespined stickleback (Gasterosteus aculeatus), pumpkinseed (Lepomis gibbosus) and ide (Leuciscus idus). Through the capture of different life stages (juveniles and adults) of different fish species we can assume a good overall ecological functioning of the pool and weir fish pass with good attraction and passage efficiency. |
