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Session A6: Five Years Monitoring of the Original "Stairs Pipe" Fish Pass Assess the Complete Reconnection and Natural Function of a Nursery Tributary for its Main River

Abstract: The sedentary trout population dynamic of a main river and its tributary has been studied in the Ardenne (Belgium) for more than fifty five years. It shows a population division between the nursery brook dominated by the juveniles and the river by the adults. Every winter, several hundreds of spawners migrate to the brook to reproduce and when the offspring fingerlings are 1 or 2 years old, they swim down to the river for a quicker growth. This ensures the river population sustainability. Close to this well documented system, another one is disturbed by an obstacle preventing any spawning run. The result is an isolated brook population made of all age class with an evident lack of numerous juveniles. The obstacle consists in a fall added to a sloped culvert. Two fish traps downstream to the fall showed very few emigrating fingerlings and much less spawners attempting to jump the obstacle than at the well connected brook. In 2008 an original fish pass was built to bypass the obstacle. It is a “stairs pipe” made of standard particular pipe elements provided with an inclined baffle. Alternating the position of these elements creates a sinuous flow that breakes the water velocity. Secondly it creates every two meters a small jump and an intermediate rest place with a slow backwater current. So this 24m long fish pass can be very easily crossed. The reconnection results have been monitored for five years by migrating fish trapping and population inventories. It showed the tributary natural function rapid recovery. The migrating spawners came rapidly nearly as numerous in the reconnected brook as in the naturally connected one. The brook population structure changed also significantly with a relative increase of the juvenile fraction. Consequently, the tributary contribution to the main river recruitment was also significantly boosted.
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