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During the last century, the majority of Swedish rivers were heavily channelized to facilitate timber floating. This dramatically altered in-stream habitat by reducing the structural heterogeneity and increasing water velocity, which in turn had effects on the aquatic fauna. Today, extensive efforts to restore channelized rivers are being made by adding boulders to the streambed. In northern boreal rivers, evaluations of such restorations are often based on the response of trout and salmon populations. Few studies have investigated the effect on European grayling (Thymallus thymallus L), despite it being an important and vulnerable gamefish in both North America and Europe. This 6-year study investigated the effect of streambed restoration on grayling in a channelized, regulated (hydro-peaking) river in northern Sweden open to recreational fisheries. No effects on habitat preference, size, body condition, abundance or survival of grayling could be detected. The results are discussed based on grayling habitat preferences and behaviour, and give perspectives on streambed restoration in river conditions common to many Scandinavian grayling rivers today. The study is one of very few that presents multiyear abundance and survival estimates of grayling in a large boreal river.

Using radio telemetry, the present study simulated the escape of 48 adult rainbow trout Oncorhynchus mykiss from a net-cage fish farm in the Lake Ovre Fryken, Sweden. The post-release dispersal of O. mykiss was fast, showed long-range dispersal behaviour, low winter survival and lacked the ability to find suitable spawning habitats. Thus, the present study suggested that reproducing for the first time may be an obstacle to the establishment of escaped farmed O. mykiss.

Post-spawning migrations of sea trout (Salmo trutta L.) in two northern Swedish river systems, Vindelalven and Pitealven, were studied. Telemetry was used to evaluate spawning locations, post-spawning mortality, overwintering survival/habitat selection/location, partitioned natural mortality from dam passage mortality and overall survival from post-spawning to the sea entry. Fifty-eight sea trout (LT = 50–86 cm) were radio-tagged at the beginning of their spawning migration from June to September in 2003 and 2004, and their spawning in September and October each year was monitored. In total, 91% (n = 53) survived spawning and were thus defined as kelts. Of these, 92% (n = 49) overwintered under ice-cover in deep, slow flowing sections of the rivers. Overwintering mortality was low, on average 8%. The main seaward migration occurred during May and June and was initiated when ambient water temperatures exceeded 4–6°C, independent of the start of the spring flood when flows rose to 1000 m3 s−1. The highest downstream migration speed was approximately 25 km 24 h−1 and migrations mainly took place at daytime. During seaward migration, the kelts encountered hydropower stations that lacked bypass systems for fish passage, which delayed their downstream migrations. Passage mortality at the power stations was 69 and 25% in the Vindelalven and Pitealven, respectively. These losses, combined with the naturally high winter survivals of kelts at the unregulated river parts, underline the importance of developing safe passage routes for kelts at hydropower facilities for conserving sea trout populations in regulated rivers. Copyright © 2008 John Wiley & Sons, Ltd.

Adult Atlantic salmon (55–116 cm total length) with PIT-tags (n= 318) and both PIT-tags and gastric radio-tags (n= 127) in a regulated river showed similar upstream migration success, yet radio-tag ...

Many Baltic salmon rivers have lost their natural juvenile production due to human activities blocking or reducing access to their spawning grounds, e.g. damming, power generation, partial hinders, etc. One such hindrance is a hydroelectric complex located in the lower reaches of River Umealven in northern Sweden. Water from the forbay created by the dam Norrfors is directed to the Stornorrfors power-station. At times, 100% of the river water is directed to the power-station. Water from the power-station then flows via a tunnel and outlet channel to the river. From the point of the tunnel's discharge into the river, the old river bed acts as a bypass channel directing migrating adult fish to a fish ladder located at the base of the dam. In this study, the effect that an additional turbine, that was installed at the power-station in 1986, had on fish passage run-time was examined. Changes in run-time were compared for two periods 1974–1985 and 1986–1995. In 1997, 55 wild and 25 hatchery salmon were captured in the Umealven estuary, radio tagged with uniquely coded tags, and tracked upstream. Both manual and automatic loggers were used to locate fish daily. The main findings show that only 26% of the wild salmon and none of the hatchery salmon found the fish ladder. It is suggested that the salmon followed the main water discharge from the power-station outlet and are thus directed away from the entrance to the bypass channel leading to the fish ladder. Salmon respond by moving upstream or downstream depending on the current flow regimes. The bypass channel consists of partial hinders that may explain why it takes on average 52 days for the salmon to migrate 32 km from the estuary to the fish ladder. Adding a fourth turbine at the power-station did not appear to have changed the timing of the migration or the seasonal distribution of the migrating wild salmon through the fish ladder. There was no significant effect of the fourth turbine on the duration of the middle 50% of the run in any weight-class for wild salmon. Daily turbine discharges or ambient water temperature and daily counts of wild salmon at the fish ladder were not related. Copyright © 2001 John Wiley & Sons, Ltd.

Downstream migrations of Atlantic salmon (Salmo salar) and sea trout (S. trutta) smolts were monitored in the in northern Sweden to evaluate the river’s potential as a national index river for salmonid populations in the Baltic region, and to acquire information for stock conservation plans. A rotary screw trap was installed and operated from 2005 to 2008. Recapture rates of tagged smolts were high (from 8% to 31%), which allowed smolt abundances to be estimated. Between 2,600 and 3,900 salmon and 500–1,500 trout emigrated from the river each year. Genetic analyses made it possible to identify the origin of salmon and trout smolts and to determine whether they originated from the Savaran stock or from stocks in neighbouring rivers. In 2005 and 2006, most of the salmon were from the Savaran stock, which previously had been thought to be extinct. By contrast, 58% and 52% of the salmon analysed in 2007 and 2008, respectively, originated from stocks other than Savaran. The density of salmon and trout 0+ parr (i.e. recruitment success) based on electro-fishing was a poor predictor of smolt abundance 2 to 3 years later. The smolt trap data combined with genetic analyses provided valuable information to develop stock conservation plans.

Due to hydropower development, the upstream migration of wild anadromous salmon and brown trout is impaired in many European rivers, causing negative effects on the long-term survival of natural salmonid populations. This study identified problems for Atlantic salmon during upstream migration in a regulated river in northern Sweden, Umealven (mean flow: 430 m3 s−1). Tagging from 1995 to 2005 involved radio tags (n = 503), PIT tags (n = 1574) and Carlin tags (n = 573) to study the spawning migration of salmon from the coast past the regulated section of the river to a fish ladder at the dam/spillway 32 km upriver. The results demonstrate that migration success from the coast to the fish ladder varied between 0% and 47% among years, indicating an average loss of 70% of potential spawners. Discharge from the turbines attracted the salmon away from the bypass route. Echo-sounding in the turbine outlet showed that salmon were normally found at 1–4 m depths. They responded with upstream and/or downstream movements depending on flow changes; increased spill in the bypass channel attracted salmon to the bypass. Once in the bypass channel, salmon could be delayed and had difficulties passing the first rapid at high spills. Additional hindrances to upstream migration were found at rapids and the area of the fish ladder, located further upstream in the regulated river section. The average migration duration was 44 days from the estuary to the top of the fish ladder, with large variation among individuals within years. Modelling the salmon population dynamics showed a potential population increase of 500% in 10 years if the overall migration success could be improved from the current 30% to levels near 75%. Consequently improved migration facilities at the regulated river section should be implemented to achieve a long-term sustainability of these threatened anadromous salmonids.