The Efficiency of a Fish Ladder for Salmonid Upstream Migration in a Swedish Stream Potential Impact of a Hydropower Station on Connectivity and Recruitment
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The Efficiency of a Fish Ladder for Salmonid Upstream Migration in a Swedish Stream Potential Impact of a Hydropower Station on Connectivity and Recruitment Anton Larsson Degree project for Master of Science in Biology Animal Ecology, 30 hec, AT 2016 Department of Biological and Environmental Sciences University of Gothenburg Supervisors: Johan Höjesjö, Lars-Olof Ramnelid, Daniel Johansson Examiner: Charlotta Kvarnemo Abstract Assessments of the function of fish passages are typically rare, although the approach is frequently implemented to mitigate adverse effects of hydropower plants. In this study 249 electro fishing samples from 1979-2015, were used to assess the efficiency of a fish ladder to allow upstream migration of salmonids past a hydropower station in Örekilsälven, Sweden. Densities of brown trout (both young of the year, 0+, and older juveniles, >0+) did not increase in the area upstream the hydropower station after construction of the fish passage; neither did the densities of salmon 0+. >0+ salmon had a higher density upstream the hydropower station after completion of the fish ladder, however this is most likely explained by extensive fish translocations. 0+ salmon were only found in 5 % of the sampling occasions upstream the power station when translocations were removed, whereas 0+ brown trout were found in 44.3 %. No effect of discharge for ascension was found in the study. The efficiency of the passage was determined low and non-satisfactory for Atlantic salmon and brown trout, although the evaluation is more difficult for brown trout as a consequence of resident forms. Smolt models indicate that contemporary smolt escapement of both salmon and brown trout almost exclusively originate from the downstream areas. Improving the hydrological connectivity will probably increase the smolt escapement from the area upstream the power station, but the magnitude will depend on recolonization extent and mortality rates for smolts migrating seawards. Although vast suitable spawning areas exist upstream the hydropower station, natural features, including extensive migration distances and the presence of a lake compose natural constraints in smolt escapement from the upstream area. Future studies should include the aspect of downstream migration as a part of a holistic approach to improve hydrological connectivity. Keywords: Salmonid migration, fish passage, fish ladder, hydrological connectivity, hydropower, electro fishing, smolt production Cover photo: The fish ladder at the hydropower station in Torp, May 2016. Photo by the author. 1 1. Introduction Freshwater ecosystems sustain high biodiversity compared to their limited area, but are currently experiencing rapid declines in biodiversity, the rate exceeding those in the most affected terrestrial ecosystems (Dudgeon et al. 2006). Dams and hydropower stations have been extensively constructed in the last century (Graf 1999, WCD 2000, Nilsson et al. 2005) and created impoundments by blocking transportation of water and altering the natural flow regime of rivers (Nilsson and Berggren 2000, Nilsson et al. 2005, Moore et al. 2012). To date, existing dams retain a volume of more than 10 000 km3 of water, equivalent of five times the volume of all the world's rivers combined (Nilsson and Berggren 2000). Although producing human services such as hydropower and water reservoirs, the foundation of dams modifies streams and freshwater systems resulting in ecological impacts that affect a wide array of taxa e.g. fish (e.g. Franchi et al. 2014, Poulos et al. 2014), amphibians (e.g. Naniwadekar and Vasudevan 2014), macroinvertebrates (e.g. Benitez-Mora and Camargo 2014, Holt et al. 2015) and influencing stream nutrient levels (Zhou et al. 2015). Perhaps the most imminent effect upon building a dam is the construction of a barrier to migration, potentially impeding all movement across the dam and thus reducing hydrological connectivity (Pringle 2001, Pringle 2003). The present study investigates the barrier effect of a hydropower station on salmonid migration and smolt production in the river Örekilsälven on the Swedish west coast. The Atlantic salmon (Salmo salar) and the brown trout (Salmo trutta) are two anadromous fish species migrating from marine environments to freshwater systems to breed and spawn before returning to the ocean (Gibson 1993, Klemetsen et al. 2003). Strictly freshwater forms of brown trout are common (Klemetsen et al. 2003) but typically rarer in Atlantic salmon (Power 1958, Berg 1985, Klemetsen et al. 2003). Anadromous Atlantic salmon have a strong natal homing behaviour, returning to their natal stream for spawning (Stabell 1984, Hansen et al. 1993, Keefer et al. 2014). This trait restricts gene flow between different streams and facilitates genetic divergence in rivers (Saunder 1981, Taylor 1991) not necessarily separated by vast geographic distance (Verspoor 1997, Primmer et al. 2006, Vähä et al. 2007). Brown trout has also been considered to have strong natal homing behaviour (Stuart 1957, Ferguson 1989) but recent studies suggest that straying can be considerable (Frank et al. 2012, Degerman et al. 2012a, Östergren et al. 2012). During the last centuries wild stocks of Atlantic salmon have decreased substantially or been extirpated throughout their native range and are at present at the lowest levels in known history (WWF 2001). The sources of decline are several and interconnected, including pollution, overexploitation, acidification, aquaculture and dam constructions (Parrish et al. 1998, WWF 2001); the latter known to have caused multiple extirpations and severe reductions of salmon populations (Limburg and Waldman 2009, Hall et al. 2012, Brown et al. 2013). Trends in population status of brown trout are limited and typically more complicated to assess, but the species seem to experience declines in some areas whereas in other regions it performs better (Pedersen et al. 2012, ICES 2013, Höjesjö et al. in press.A). Due to the anadromous life cycle, dwindling stocks and commercial interest in salmonid species, barrier effects of dams have often been addressed with the construction of different types of fish passages as an attempt to allow upstream and downstream migration and ensure population viability (Clay 1995, Schilt 2007, Calles and Greenberg 2009). Fish passages include a variety of designs, i.e. fish ladders and bypasses, applied depending on location and intention. However, despite extensive construction, studies assessing the function of these 2 passages are scarce (Schmutz et al. 1998, Roscoe and Hinch 2010, Bunt et al. 2012, Hatry et al. 2013) and often indicate low efficiency (Noonan et al. 2012). Studies in this field use confusingly resembling terminology but the difference is crucial to apprehend. The term efficiency will in this report be applied as a quantitative concept defining the proportion of a fish stock successfully migrating upstream a hydropower station, whereas the term effectiveness will be used in a qualitative context, simply stating if target species are able to pass the fish passage at some point (Larinier 2001). Fish passage efficiency is generally determined by two aspects; (1) attraction efficiency meaning the proportion of individuals present downstream the passage able to find the entrance of the fish passage and (2) passage efficiency, the proportion of individuals locating the entrance that successfully ascend the fish passage (Aarestrup et al. 2003, Bunt et al. 2012). In one of the few reviews assessing fish passage efficiency, Noonan et al. (2012), reviewed 65 papers between 1960 to 2011 and concluded that fish passage efficiency was about 50 % on average for both upstream and downstream migration (although salmonids had slightly higher efficiency). Other studies have also shown limited upstream migration success for salmonids (Linløkken 1993, Rivinoja et al. 2001, Croze et al. 2008, Lundqvist et al. 2008) although some fish passages seem to perform better (Bryant et al. 1999, Gowans et al. 1999) indicating potential for improvement if dams are constructed properly. In Sweden, having approximately 2100 dams (Havs- och vattenmyndigheten 2014), documentation and assessment of present fishways is also limited (Rivinoja 2015). One of few reports assessing the efficiency of fishways was conducted in the county of Västra Götaland (Andersson and Bäckstrand 2005). Investigating 62 fish passages (mostly pool and weir passages), the authors concluded that only 53 % of these passages worked satisfactorily (Andersson and Bäckstrand 2005). In other words, there is an increasing need to evaluate and potentially modify fish passages in order to allow efficient migration for fish and other fauna in lotic ecosystems with existing dams. 1.2 Study area Örekilsälven is located on the Western coast of Sweden running 90 kilometres from the county of Dalsland before entering the sea in the Gullmar fjord, close to Munkedal (Fig. 1). It is one of the few rivers in the Western parts of Sweden still supporting a genetically distinct salmon stock of high conservation value (Degerman et al. 1999). Brown trout is also present in the area and is considered of high conservation value (Thorsson 2009). The catchment area covers 1340 km2 and several larger tributaries add to the main stream channel. Forests cover the majority of the catchment area (76 %) whereas the proportion of lakes is limited (3.7 %), the largest lake being Kärnsjön (Fig. 1). Historically, Örekilsälven, has been subjected to human alterations through different dams