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 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.
The solution in these cases is engineering. Records show of rough fishways built in France during the 1600's. 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 will allow the return of Atlantic salmon, sea trout and other migratory fish. In 2010, Environment Agency staff caught an Atlantic salmon attempting to return to the Aire. Now they have a way home. Our actions here in Yorkshire are part of a great migration that takes them from shallow waters like Gargrave to the Atlantic Ocean, Greenland and back.