Reduced flow impacts salmonid smolt emigration in a river with low-head weirs

The impacts of large dams on the hydrology and ecology of river systems are well understood, yet the impacts of low-head structures are poorly known. While impacts of small weirs on upstream-migrating fish have long been mitigated by fish ladders, it is assumed that downstream migration of surface-oriented fishes is unaffected under natural flow regimes. To test this, the effects of low-head weirs and the influence of river flow on the migration of brown trout (Salmo trutta) smolts in the River Tweed, UK, were examined. Movements of acoustic tagged smolts were quantified in 2010 and 2011 using automatic listening stations and manual tracking throughout the migration route. In both years, smolts exhibited major losses, most likely due to predators, with escapement rates of 19% in 2010 and 45% in 2011. Loss rates were greater in 2010 when flows were frequently below Q95 (20% of study period) compared to 2011 when more typical flows predominated (0% of study period below Q95). Smolts experienced significantly longer delays at weirs during 2010 than 2011, associated with the different hydrographs during emigration as well as weir design. Flow comparisons within the study periods and historical records show that low flows experienced in 2010 were not unusual. The swimming behaviour of smolts in relation to flow conditions differed between years, with smolts in 2010 increasing their rate of movement in relation to increasing flow at a faster rate than smolts in 2011. This is the first study to demonstrate river flow impacts on the migration success of wild salmonid smolts at small weirs. Because small weirs are common in rivers and because spring-summer low-flow periods may become more frequent with climate change (based on UKCIP09 models) and altered river hydrology, further research and improved management is needed to reduce the impacts of low river flows in combination with low-head weirs on salmonid smolt migration.