Weir removal in salmonid streams: implications, challenges and practicalities

Low-head dams and weirs can greatly limit the distribution and abundance of Atlantic salmon and other migratory salmonids in streams. Weirs can significantly increase the vulnerability of migratory fish to anglers, alter natural migration patterns, and exacerbate the effects of opportunistic predators. Overcrowding of fish at downstream pools can also facilitate the spread of parasites and infectious diseases, magnify the impact of pollution incidents, and increase the risk of mass mortalities, particularly at low flows. Not surprisingly, augmenting the accessible stream area constitutes one of the best ways to restore depleted salmonid populations. In this context, the removal of unused or illegal weirs can be an efficient, cheap solution to increase stream accessibility. Here, I examine some impacts of weirs on Atlantic salmon populations, and document with case studies the removal and breaching of weirs in several Iberian streams.