Guiding downstream migrating Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) of different life stages in a large river using bubbles

Salmonid repeat spawners are precious individuals for wild populations due to their high fecundity and previous spawning experience, making them important in environmental policy. However, repeat spawners rarely exist above hydropower dams in regulated rivers as the mortality of post-spawners (kelts) when passing through turbines during downstream migration is very high. To mitigate this problem, there are different technical solutions that potentially guide fish toward available fishways. Bubble barriers represent one alternative to costly physical guiding structures, but the efficiency of bubbles for guiding downstream migrating kelts has not been tested. In this study, we evaluate a 100 m long bubble barrier in guiding salmonids—both smolts and kelts—away from the main current and toward an alternative fishway in Ume River, a large regulated river in northern Sweden. We used both acoustic telemetry and sonar to measure the guiding effect of the bubble barrier for downstream migrating fish. We found that more than twice as many salmonids chose the alternative fishway when the bubble barrier was turned on. This was true both for smolts and kelts, suggesting that bubble barriers can be used to guide salmonids of different life stages in rivers with flow rates over 500 m³ s⁻¹. Indeed, our study indicates that bubble barriers are low-cost structures that could be rapidly applied in many regulated rivers to support salmonid migration.     

The salmon-peloton: Hydraulic habitat shifts of adult Atlantic salmon (Salmo salar) due to behavioural thermoregulation

In recent decades, there has been an increase in conservation and restoration projects targeting Atlantic salmon (Salmo salar – AS), as populations in eastern Canada decline. Missing however, is an understanding of thermo-hydraulic habitat use by adult AS during summer, and thus the actual benefits of altering in-river physical structures. Here, we illustrated how optical and thermal infrared (TIR) imagery acquired from a UAV can be used in concert with in-situ depth and velocity data to map adult AS and develop models of thermo-hydraulic habitats in the Miramichi River, New Brunswick. We found during normal temperature conditions (<19°C) boulder proximity, depth, velocity, and Froude number, a non-dimensional hydraulic metric, were key parameters that characterized adult AS habitat. However, during behavioural thermoregulation events (>19°C), proximity to the cool thermal plume and Froude number were critical controls on habitat use. We also observed AS forming a distinct geometric formation during behavioural thermoregulation events, and term this formation a “thermal-peloton”; in reference to competitive cycling in which groups of cyclists pack together. The primary function of the peloton is undoubtedly to reduce thermally induced stressed; however, we conceptualize that the geometry of the peloton attenuates hydraulic-drag, and reduces the energetic expenditure of individuals practicing behavioural thermoregulation. These data provide a blue print for Atlantic salmon restoration work. The use of UAV-based sensors has the potential to initiate a paradigm shift for river sciences.     

Causes of mortality of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts in a restored river and its estuary

During October 2000 to August 2002, the River Skjern Nature Project was implemented by removing dykes and re‐meandering 20.5 km of the lower canalized river. As a consequence the length of the river stretch increased to 23 km. A lake of 250 ha developed in the river valley 5 km upstream from the river mouth because of subsiding soils caused by reclamation and drainage since the 1960s. Using radiotelemetry, the mortality of wild Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts in the River Skjern and its estuary was investigated prior to and after the implementation of the project. Altogether, 77 Atlantic salmon and 66 brown trout smolts were caught, tagged and released in the river upstream of the restoration project during the spring of 2000 and 2002. The in‐river smolt mortality was more than double in 2002 for both Atlantic salmon and brown trout compared with 2000. This was primarily due to bird predation in 2002 which was not observed in 2000. The in‐river bird predation in 2002 was mediated by the new lake, which quickly became an important bird rest area. Estuarine mortality mainly caused by cormorants (Phalacrocorax carbo sinensis) differed significantly between species, but was high for both Atlantic salmon (39%) and brown trout (12%) in both years of investigation. The aggregated smolt mortality in the river and in the estuary (48%) may threaten an indigenous self‐sustaining Atlantic salmon population in the River Skjern. When planning river restoration projects, caution should be used, especially where permanently flooded floodplains (lakes) develop due to subsiding soil. In situations where rivers pass directly through newly developed lakes, migratory species such as Atlantic salmon and brown trout may be severely affected due to increased exposure to predation from predatory fish and birds. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of decreased temperature on growth and smoltification of juvenile atlantic salmon (salmo salar) and brown trout (salmo trutta) in a norwegian regulated river

The lowermost 20 km of the River Surna, northwestern Norway receives cool water during summer from a hypolimnetic release mountain reservoir. In this part of the river, yearlings of both Atlantic salmon and brown trout are significantly smaller compared with those in the upper section of the river, which is unaffected by the cold water release. The slower growth below the power station causes both Atlantic salmon and anadromous brown trout to smoltify one year later compared with fish in the upper section of the river. This leads to higher mortality and therefore lower production of both adult salmon and trout.