As we come to the end of Developing the Natural Aire, SImon Watts explains why building fish passes matters.
All fish need to move to feed, breed and shelter. Even fish who spend their whole lives in the river can travel considerable distances looking for the ideal spot. Barbel have home ranges up to fifteen miles. Many freshwater fish migrate much, much further. For migratory species, like Atlantic salmon and the European eel, access to the sea is essential. The Industrial Revolution has left us with a fragmented river. Weirs built to harness the power of water for mills, factories and for navigation, block the way. Fish trapped in their sections are vulnerable to pollution or predation. And species like Atlantic salmon trying to return to our cleaner rivers cannot reach spawning ground in the shallow headwaters.
Whilst trout and Atlantic salmon can sometimes be seen leaping up waterfalls, weirs present a unique challenge. They are often long and steep. Their surface is smooth with little variation in the flow of water. The deep pools found at the base of waterfalls which fish can leap from are replaced by shallow aprons of concrete and stone. Many coarse fish species cannot jump obstacles.
The Aire Rivers Trust works with partners such as the Environment Agency and landowners to overcome these obstacles. The ideal solution is weir removal which restores natural river habitat (returning the artificial ponded section upstream back to a more natural system). But in some locations this proves impossible. Weirs form part of the urban fabric of our landscape. Some are valued for their heritage or for their amenity value. Buildings are built close to them and complicate their removal.
Yorkshire Water explains why we need them and how fish passes work
Mid construction footage of the new fish pass at Kirkstall Abbey Weir as part of our Developing the Natural Aire project
An explanation of how an eel pass works from a Canal and Rivers Trust project similar to our Developing the Natural Aire
Where weirs cannot be removed, the solution is engineering. Records show rough fishways built in France during the 1600s. Here bundles of branches were used to create steps in steep channels to bypass obstructions. Engineered fish pass design has come a long way since the first one was patented in 1837 in New Brunswick, Canada. All designs seek to make weirs passable to fish. The majority of modern fish passes on the River Aire are Larinier Passes. Metal baffles in the base of them slow the flow over the weir that fish can swim or leap through.
The reconnection of 60km of habitat in the River Aire by the Developing the Natural Aire project will allow the return of Atlantic salmon, sea trout and other migratory fish. Monitoring conducted alongside the project has recorded a brown trout travelling from Armley Mills to Cross Flatts in Bingley (and most likely beyond) in a huge 80-kilometre journey to search for new areas to feed, breed and shelter in. This journey would have been impossible before Developing the Natural Aire as weirs like Kirkstall Abbey were completely impassible to fish.
