Effect of trash diverters and overhead cover on downstream migrating brown trout smolts
Introduction
Hydropower stations, with their dams and turbines, constitute barriers and sources of mortality for downstream migrating smolts. The actual route selected, which is associated with a certain risk of mortality, is thought to be strongly influenced by flow conditions, where smolts are believed to select the path with the highest flow. Typically, the fish pass the dams by either swimming through turbines, spillways or some type of bypass system (Clay, 1995). There are essentially two types of turbines, the Francis and the Kaplan turbine, which are used in low-head power plants. Fish passing through Francis turbines, which possess many runner blades with little space between them, typically experience a higher mortality than fish passing through Kaplan turbines, with its rather few runner blades and more space between the blades (Montén, 1985). In addition, the risk of being struck by a runner blade increases with increasing fish body size (Clay, 1995, Rivinoja, 2005). Fish may also die or incur damage as they are impinged against the bar racks that are often placed just upstream of turbine intakes (Calles et al., 2010). Spillways are also associated with mortality risks, either directly due to fish free-falling against concrete structures before re-entering the water (Calles and Greenberg, 2009) or indirectly due to pressure changes and gas supersaturation (Coutant and Whitney, 2000).
There are various methods for diverting fish from turbine intakes (Clay, 1995). One such way is use of a ‘behavioral barrier’, whereby one elicits a behavioral response by the fish, using for example sound, so that they select a route associated with a low risk of injury or death. Such methods have had variable success as they depend on local conditions that permit active choice as well as on the stimulus having a strong and consistent effect. A second method of diverting fish is to construct mechanical barriers, using for example meshed grating or nets to force the fish to swim along a particular route. Yet another way of preventing fish from entering turbines is to capture them in traps and transport them past the power plant.
When it concerns behavioral barriers, these can be used to either attract or divert fish. A number of studies have shown that downstream migrants react to or are affected by illumination (Larinier and Travade, 1999, Kemp et al., 2008, Kemp and Williams, 2009). For example, Kemp and Williams (2009), who worked with several species of juvenile Pacific salmonids, showed that more individuals approached, and either passed or rejected an artificial channel with a submerged weir when the area was artificially illuminated than when it was not. Based on this result, Kemp and Williams (2009) suggested that the design of fish passages may need to take into account the response of fish to illumination. Even use of overhead cover in the design of fish passage facilities may be warranted (Kemp et al., 2005, Kemp et al., 2006). Kemp et al., 2005, Kemp et al., 2006, for example, showed that approximately 75% of chinook salmon (Oncorhynchus tshawytscha Walbaum) avoided covered channels in controlled laboratory tests.
In many cases, bypass facilities have been implemented without evaluating their effectiveness or if effectiveness has been evaluated, guidance efficiency has been shown to be low (Kemp et al., 2008, Larinier, 1998). Moreover, many of these diversions are expensive. Here we evaluate the use of two relatively inexpensive measures, trash deflectors and overhead cover, to guide brown trout (Salmo trutta L.) smolts away from turbine intakes. We test whether guidance away from turbine intakes can be increased by simple modifications of extant trash deflector systems and by use of overhead cover at the often illuminated canals just upstream of turbine intakes. This study was conducted at two power plants, upper Finsjö with two twin-Francis turbines and lower Finsjö with a Kaplan turbine, in the Emån River, southern Sweden. We also expected mortality for the fish that passed through the Frances turbine to be higher than for those fish that passed through the Kaplan turbine.
Section snippets
Study area
The study was conducted in the Emån River (57°07′59″N; 16°30′00″E), a lowland river dominated by forest and agricultural land, with a catchment area of 4472 km2. The mean annual discharge is 30 m3 s−1 and generally varies between 6 and 107 m3 s−1. The river has been regulated for approximately 100 years and has a long history of supporting a recreational and commercial fishery (Klippinge, 1999). This study was conducted at two hydroelectric facilities in Finsjö, situated approximately 30 km upstream
Results
In total, 46 smolts were marked with radio-transmitters, but two of the fish were excluded as they were observed to have fungal infections when recaptured in traps. The 44 remaining smolts were 153–270 mm long and weighed 31.9–180.4 g. Two of these were never recorded on the loggers.
The fate of the smolts differed at upper and lower Finsjö. A significantly larger number of successful passages occurred at lower Finsjö than at upper Finsjö (Fig. 2). This was in large part due to the substantially
Discussion
There was a large difference in mortality between upper and lower Finsjö. As shown previously for Emån, mortality was higher for upper Finsjö than for lower Finsjö (Calles and Greenberg, 2009, Greenberg and Calles, 2010). A salient difference between the two power plants is the presence of twin-Francis turbines at upper Finsjö and a single Kaplan turbine at lower Finsjö. In general, mortality in Francis turbines is higher than in Kaplan turbines (Montén, 1985, Calles and Greenberg, 2009). At
Acknowledgements
Hanna Karlsson, Anders Robertsson and Carl-Johan Månsson are thanked for help in the field. The research presented in this paper was carried out as a part of the Swedish R&D program ‘Hydropower—Environmental impact, remedial measures and costs in existing regulated waters’, which is financed by Elforsk, the Swedish Energy Agency, the National Board of Fisheries and the Swedish Environmental Protection Agency. The study was performed under license from the Swedish Animal Welfare Agency
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