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Utfall av Trollheim kraftverk i juli 2008. Effekter av fiskebestandene i Surna


An accidental stop in the turbine in the Trollheim Hydropower Plant on 27th July 2008 caused a
drop in downstream discharge in the River Surna from 21 to 3 m3/s during 100 min. River discharge
was reestablished after three hours. The drop caused large dewatered areas (26 % of
total wet area before the drop), but HEC-RAS simulations indicated that the ramping rates
were relatively low (less than 20 cm per hour for the whole drop period in the modeled transects).
Thus, a high proportion of the fish may have avoided stranding. Stranding mortality was
estimated at approximately 14.000 0+ salmon and 3.600 older juveniles (70 % 1+), or the
equivalent of approximately 3000 smolts. The loss constitutes less than 3 % of the future smolt
production downstream the power station (estimated by up-scaling from densities at electrofishing
station, via mesohabitats to the whole river stretch) during 2009-2012. About one third
of the smolts in the River Surna has been estimated to be produced in areas below the power
plant. Between 3000 and 15 000 0+, and an unknown number of older brown trout likely also
died from stranding, and the effect was probably larger for the juvenile population of brown
trout than Atlantic salmon. All the above estimates are uncertain. Additional releases of water
from the reservoir during, and one week after the drop, likely had no effects on the fish populations.
The diversity of the zoobenthos communities in the river below the outlet of the power plant is
low, likely due to large and frequent variation in discharge. The accidental stop in 2008 was
probably particularly damaging, due to the large dewatered areas and low minimum discharge
(far below the minimum allowed residual flow at 15 m3/s). This may temporarily reduce biomass
and diversity of zoobenthos and thus prey availability for fish.
Analysis of discharge data during the period from 2000 to 2008, show that despite measures
implemented in the power plant (from 2006), there are several events when ordinary reductions
in power production cause a drops in water level, with similar or higher risks of stranding mortality
than the accidental stop in 2008. Stranding remains a major problem for the fish population
in the River Surna, and there is great potential for improvements by changes in the operation
of the power plant.
While the accidental stop in 2008 likely did not cause very high fish stranding mortality, such
drops are generally harmful for the fish population and the river ecosystem as a whole.
Norge, Møre og Romsdal, Surnadal, ferskvann, elv, laks, Salmo salar, aure, Salmo trutta, bunndyr, effektstudie, stranding, vassdragsregulering, Norway, hydropower, freshwater, regulated river, Atlantic salmon, brown trout, stranding mortality, zoobenthos
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  • Other / Statkraft Energi AS




  • Torbjørn Forseth
  • Morten Stickler
  • Ola Ugedal
  • Håkon Sundt
  • Gunnbjørn Bremset
  • Tommi Linnansaari
  • Nils Arne Hvidsten
  • Atle Harby
  • Terje Bongard
  • Knut Alfredsen


  • Norwegian Institute for Nature Research
  • SINTEF Energy Research
  • Norwegian University of Science and Technology
  • SINTEF Energy Research / Energisystemer




Norsk institutt for naturforskning





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