Atlantic Salmon Fishways: the Norwegian Experiences
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INNSENDTE ARTIKLER Atlantic salmon fishways: The Norwegian experiences Av Hans-Petter Fjeldstad, Knut Alfredsen og Torbjørn Forseth Hans-Petter Fjeldstad (Corresponding author: [email protected]) Department of Hydraulic and Environmental Engineering. Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. Knut Alfredsen Department of Hydraulic and Environmental Engineering. Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. Torbjørn Forseth Norwegian Institute for Nature Research. NO-7047 Trondheim, Norway. Sammendrag Tilstanden til alle de 344 norske fisketrappene for bedre funksjon enn trapper ved naturlige hindre. atlantisk laks ble undersøkt, som en kombinert Dette indikerer at trapper med tilstrekkelig tilsyn, studie basert på den norske Fisketrappdatabasen og trapper som administreres av vassdragsregu- og tilleggsopplysninger fra fylkesmennene, samt lanter, gjennomgående vedlikeholdes bedre enn inspeksjon av 89 utvalgte trappeanlegg for ytter- andre trapper. Den viktigste årsaken til at lakse- ligere informasjon om utforming og funksjon. trapper ikke fungerte var fysiske skader på trap- Denne inspeksjonen indikerte at nærmere 70 % pen og manglende vedlikehold. Studien viser at av trappene fungerte godt og at laksetrapper har den norske tradisjonen med kulptrapper antas å åpnet ca. 2000 km nye anadrome elvestrekninger. være godt egnet for voksen laks, og at godt vedlike- Norske laksetrapper består i hovedsak av støpte hold og tilsyn av laksetrapper er et viktig forvalt- kulptrapper i betong, samt en del kulptrapper ningstiltak for framtidig bevaring av mange sprengt i fjell, begge typer med overløpsåpning. norske laksestammer. Studien viste at typisk vannføring i laksetrappene var mindre enn 0,5 m3s-1, men samtidig var også Summary vannføringen i elva liten ved trappelokalitetene. The status of all 344 Norwegian Atlantic salmon I 70 % av tilfellene var årlig middelvannføring i fishways were explored, combining an existing elva ved laksetrappen mindre enn 20 m3s-1. Det national inventory with collection of additional ble ikke funnet noen sammenheng mellom lakse- information from local river authorities and on- trappenes funksjon og trappens lengde eller site inspections of 89 selected sites to provide høyde. Tvert imot viste studien at noen av Euro- more detailed information on design and func- pas største fisketrapper finnes blant godt funge- tionality. Inspections of 89 fishways indicated that rende norske laksetrapper, og at laksen vandrer nearly 70 % were fully functional and that the gjennom mørke tunneler over flere hundre fishways potentially have opened approximately meters strekning. Middelvannføring i elva var 2000 km of anadromous river reaches. With few større ved godt fungerende trapper enn ved lakse- exceptions, Norwegian fishways are pool and weir trapper med dårlig effektivitet. I tillegg hadde type ladders with surface notches, mostly in con- laksetrapper ved menneskeskapte hindringer crete but also a number blasted in rock. Fishway VANN I 02 2013 191 INNSENDTE ARTIKLER discharge was generally smaller than 0.5 m3s-1, recent inventory of the originally 450 self-repro- but at the same time mean annual river flow was ducing Atlantic salmon populations in Norway, small, and smaller than 20 m3s-1 at 70 % of the 344 45 have gone extinct, 32 are threatened with the fishway sites. No relation was found between extinction, 49 are classified as vulnerable and 65 fishway height or length and fishway function. as reduced (Hansen et al. 2007). Contrarily, some of the highest and most sophis- The anadromous life cycle of the Atlantic ticated constructions in Europe are among the salmon implies a particular vulnerability to functional Norwegian fishways, and the salmon anthropogenic threats as it involves migration ascend through long tunnel reaches in complete across several habitats and large areas. The darkness. River flow was larger at sites with func- upstream migration in rivers is the foundation t ional fishways than sites with dysfunctional fish- for the abundance of salmon, both ecologically ways, and fishways at man-made obstacles had and for human recreation and economic output. better function than fishways passing natural During the upstream migration, the salmon may waterfalls. These findings indicate that fishways encounter obstacles and barriers that delay or with sufficient supervision and fishways maintai- block the migration routes (Mills 1989). Con- ned by hydropower companies receive more structions of impassable dams are among the attention than those at other sites. The most most important reasons for the loss of popula- important reasons for fishway dysfunction were tions across the entire species distribution (John- physical damage and lack of maintenance. The sen et al. 2011). Based on the recognition of the study shows that the tradition and development critical need for river connectivity (Knaepkens of pool and weir ladders is considered to be well et al. 2007), fishways for Atlantic salmon is topic suited for adult salmon under Norwegian condi- of increased scientific interest, and occupies tions, and that maintenance of the fishways is an seve ral professional societies, including engi- important management assignment in future neers, ecologists and biologists (Katopodis 1992, conservation of many salmon populations. 2005; Gowans et al. 2003; Larinier 1998; Key words: Salmo salar, migration barrier, Thorstad et al. 2008). fishways, upstream migration, fishway efficiency. Construction of fish passage facilities is widely used to mitigate migration barriers (Kato- Introduction podis 2005). The first documented fish passes in The Atlantic salmon has fascinated humans for the 18th century (Landmark 1884), were simple thousands of years and is one of the most prized constructions and likely used as a measure to and exploited species worldwide (Aas et al. 2011). increase population size and fishing opportuni- Historically, Atlantic salmon was distributed in ties. Since then, anthropogenic impacts and more than 2600 watersheds on both sides of the industrial development have fragmented river North Atlantic (WWF 2001), but now popula- systems in the majority of large rivers around the tions are declining within the whole distribution world (Nilsson et al. 2005) and parallel to this range and numerous anthropogenic impacts con- development a large number of fish passes of tribute to this trend (ICES 2011). Nearly 90 % of various technical designs have been constructed the known, healthy populations are now found in (Clay 1995). Despite the Atlantic salmon’s four countries (Norway, Iceland, Ireland and impressive capability of leaping waterfalls, the Scotland) while 85 % of the populations in the migration success through man-made fish passes other countries are categorised as critical, vulner- is often small or involves significant delays able or endangered (WWF 2001). Based on the (Rivinoja et al. 2001). The reason for this is number of distinct populations and total occur- obvio usly complex and depends on a set of rence, Norway presently represents the core area parameters, both general and site specific. for the Atlantic salmon but also here, negative Adult Atlantic salmon appear to display a trends are highly noticeable. According to a high degree of local homing within the river 192 VANN I 02 2013 INNSENDTE ARTIKLER (Harden Jones 1968; Stasko et al. 1973). Conse- different physical aspects must be considered in quently, full recruitment of new production each different case, such as entrance design, areas only by construction of fish passes may water discharges in the fish pass and in the river, take decades (Berg 1964, 1966), explaining why and adaption to local dynamics including floods, initial fish pass migration can be deceptively ice jamming and sediment exposure. limited. The conditions in the opened reaches, Construction of fishways for Atlantic salmon such as availability of spawning and rearing has long traditions in Norway. The first one was habitats and presence of competitors or preda- opened in 1872 and is still in use. Since then, tors, will confine the expansion progress of the approximately 500 fishways have been built, upstream salmon population and subsequently mostly for Atlantic salmon migration (Grande the migration motivation to these new areas. The 2010) including a variety of designs, from minor ability of swimming and leaping is strongly riprap and blasting works to comprehensive dependent on water temperature and cold water combinations of concrete towers and rock tun- can delay migration past waterfalls or fish passes nels, the tallest nearly 50 meters high (Mathis- significantly (Bell 1973). Local hydraulic condi- fossen waterfall, 46.5 m). However, considering tions around a migration obstacle depend on the large number of technical solutions world- discharge and fish passing often takes place in wide (e.g. Denil, lifts, Borland and vertical slots), certain discharge windows (Jensen et al. 1986; Norwegian fishways are almost exclusively pool Rivinoja et al. 2001). Water quality, turbidity, and weir type ladders with surface notches. Also, cloud cover, atmospheric pressure and air tem- they are designed for relatively small discharges perature are additional factors that can influence (0.2-0.5 m3s-1) and with a 0.5 meters drop bet- migration, and these variables are often related ween each pool. Partly, this is a result of a tradi- to river discharge (Banks 1969). Intrinsic factors, tion and not scientific