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CROSTON WEIR FISH PASS COMPLETION


When Friends of the River Yarrow (FrY) was conceived back in 2000, a fish pass for Croston weir was one of the first projects FrY wanted to tackle and formed an integral part of Action Plan 1. Croston weir, along with Pincock, Birkacre and Duxbury weirs (listed from lower to upper River Yarrow) were installed for industrial purposes to provide power and water for mills. However, these weirs span the whole width of the river and prevent fish from migrating upstream. This is less of a problem for some fish species that don't need to migrate in order to reproduce, but Salmon and Sea Trout with their more complex life cycle, it poses quite a problem. Adult fish migrate to the upper reaches of rivers in order to deposit eggs in the fast flowing, well oxygenated waters on gravel riffles. These are only found in the upper reaches of rivers. When these weirs were installed, little thought was given to migrating fish and effectively made Salmon and Sea Trout extinct from the River Yarrow where they haven't reproduced since the 1800's.


One of FrY's primary aims is encouraging the return of these migratory fish back to the Yarrow. In order to do this, four fish passes were required; one for each weir to provide a migration route from the tidal reaches at the bottom to the spawning grounds at the top. A design for Croston Weir fish pass was completed in 2003 but several technical difficulties lead to delays. As Croston weir is the lowermost structure for fish to tackle, it's paramount the fish pass is as efficient as possible in order to maximise the number of fish able to use it. The more fish that are able to scale the weir, the more chance they have of reaching the spawning grounds. This required a more technical fish pass design than originally envisaged. While these technical difficulties were overcome, less technical pool and traverse fish passes were installed at Birkacre weir in 2003, and Pincock and Duxbury weirs in 2005.

It may seem strange weirs upstream of Croston were finished first. Croston weir, although it posed a major obstacle for migrating fish, did allow some fish to ascend the weir given certain river conditions. It was a partial barrier rather than total. The weirs upstream posed total barriers. It was therefore decided to concentrate efforts on those weirs upstream while the technical problems with Croston were overcome. The upstream weirs also prevented none migrating fish species, particularly Brown Trout from increasing in population so the projects would benefit those immediately, and migrating species when Croston weir was completed.

In 2006, a final technical design was developed for Croston weir, a Denil type fish pass.

FrY were all set to install the fish pass when a structural problem was identified with the wing wall of the weir where the fish pass would be situated. Water had been seeping through the wall and eroding the earth behind it. 

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