Weir notching is an essential river restoration technique used to improve fish migration across North Wales. Weirs, built long ago for industry or water management, block fish like salmon, sea trout, and eels from reaching important upstream habitats needed for spawning and feeding. By cutting a notch in these weirs, we restore natural river flow, allowing fish to pass through and access critical areas for their life cycle (Larinier & Marmulla, 2004).

Weir notching refers to cutting a "notch" or gap into a weir, to allow a flow of water through which fish can pass. This notching helps create a low flow channel that mimics natural river conditions, allowing fish to navigate upstream to spawn. For species like salmon, which need to move upstream during specific times of the year, such notches provide a critical lifeline (Harris, 2001)​.

Salmon and sea trout migrate upstream to spawn, and without a clear passage, their populations can decline. Notching reconnects these fish to their historic habitats, helping to sustain and boost their numbers (Harris, 2001). Additionally, weir notching is accompanied by the installation of eel passes. European eels, which travel thousands of miles from the Sargasso Sea to reach European rivers, struggle to ascend weirs. Eel passes provide a specially designed route that allows eels to bypass the obstruction and continue their migration, supporting their survival (Environment Agency, 2016).

By allowing fish to move freely, weir notching and eel passes also help restore the river’s overall health. Migratory fish distribute nutrients throughout the river, benefiting other species like birds and mammals. Over time, the river’s ecosystem becomes more resilient and biodiverse. The work being done on the Afon Erch is part of a broader effort to reverse the damage caused by historical human interventions and ensure that these rivers remain vibrant habitats for fish and other wildlife (Freeman et al, 2003).

References

Environment Agency. (2016). Manual for Eel Pass Design. Retrieved from: Eel and elver passes: design and build - GOV.UK (www.gov.uk)

Freeman, M. C., Pringle, C. M., Greathouse, E. A., & Freeman, B. J. (2003). Ecosystem-level consequences of migratory faunal depletion caused by dams. In American Fisheries Society Symposium (Vol. 35, No. January, pp. 255-266).

Harris, J. H. (2001). Altering weir structures and weir operations to be more environmentally friendly. The Proceedings of the Way Forward on Weirs, 66-76.

Larinier, M., & Marmulla, G. (2004). Fish passes: types, principles and geographical distribution-an overview. In Proceedings of the second international symposium on the management of large rivers for fisheries (Vol. 2, pp. 183-206). Bangkok, Thailand: RAP publication.