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

Salmon and sea trout spawning migration in the River Tweed: telemetry-derived insights for management

Management of multiple exploited stocks of anadromous salmonids in large catchments requires understanding of movement and catchment use by the migrating fish and of their harvesting. The spawning migration of sea trout (Salmo trutta) and Atlantic salmon (Salmo salar) was studied in the River Tweed, UK, using acoustic telemetry to complement exploitation rate data and to quantify catchment penetration. Salmon (n = 79) and sea trout (n = 65) were tagged in the tidal-influenced Tweed in summer–autumn. No tagged salmon left the river before spawning, but 3% (2010) and 8% (2011) of pre-spawning sea trout dropped out. Combined tag regurgitation/fish mortality in salmon was 12.5%, while trout mortality was 6% (2010) and 0% (2011). The estimated spawning positions of salmon and sea trout differed; tagged salmon were mostly in the main channel while trout occurred mostly in the upper Tweed and tributaries. Early fish migrated upstream slower than later fish, but sea trout moved through the lower-middle river more quickly than salmon, partly supporting the hypothesis that the lower exploitation rate in autumn of trout (1 vs 3.3% for salmon) there is generated by differences in migration behaviour

The status of Arctic charr Salvelinus alpinus in Britain and Ireland

The Arctic charr occurs in lakes across Britain and Ireland and was previously described here as 15 separate species. Most authorities now agree that all these stocks belong to a single polymorphic species complex Salvelinus alpinus (L.). This fish is given little protection in British and Irish law and there has been a steady loss of natural populations in recent years in all the countries concerned. A few new stocks have been created either intentionally or accidentally. In Scotland, only a small proportion of the 258 recorded natural populations has been studied and at least 12 of these are now extinct. There are at least four introduced populations originating from native Scottish stocks, but the fate of stocks introduced from Canada for aquaculture is uncertain. In England, there are eight extant populations in Cumbria and four others extinct. The status of introduced stocks in England is uncertain but there is probably one population surviving in Yorkshire. In Wales, eight lakes with resident Arctic charr populations have been recorded, three of these populations are natural, one is extinct and four have been introduced. In Ireland, of the 74 known populations, approximately 30% are extinct. There is no evidence to indicate that introduced stocks (some of them from Iceland) in a small number of lakes have survived there. A range of factors is involved in the extinction of populations and these include pollution, eutrophication, acidification, afforestation, engineering, exploitation, aquaculture, introductions and climate change. Much research remains to be done and unique stocks of this valuable species will continue to be lost unless positive action is taken through local conservation management backed by appropriate national legislation