Conserving iteroparous fish stocks in regulated rivers: the need for a broader perspective!

Abstract

Hydroelectric dams and weirs often represent barriers for a wide range of migratory fish species and may cause population fragmentation (Jungwirth 1998; Lucas & Baras 2001; Heggenes & Roed 2006). In general, the migratory system of diadromous fishes requires both upand downstream migrations during the life cycle. Anadromous and potamodromous fish, such as Atlantic salmon, Salmo salar L., brown trout, Salmo trutta L., steelhead, Oncorhynchus mykiss (Walbaum) and grayling, Thymallus thymallus L., exhibit a varying degree of iteroparity and perform repeat migrations between habitats (Fleming 1996; Klemetsen, Amundsen, Dempson, Jonsson, Jonsson, O Connell & Mortensen 2003; Keefer, Wertheimer, Evans, Boggs & Peery 2008). Catadromous eels (Anguilla spp.) and most Pacific salmon (Oncorhynchus spp.) are semelparous, but are still vulnerable to migration problems (Jansen, Winter, Bruijs & Polman 2007; Ferguson 2008). When human encroachment creates obstacles for migratory individuals, the benefits gained from migration may be reduced or even eliminated. Increased mortality may occur from contact with turbine blades, pressure changes in powerhouses and traumas caused by other artificial migration routes (e.g. Cada 2001; Ferguson 2008). As a consequence, the expression of iteroparity among migrating fish species may decrease or even disappear. Thus, residency may replace migratory life history strategies in some species (Bohlin, Pettersson & Degerman 2001). Maintaining or restoring migratory fish populations in regulated rivers depend on implementing measures at the right scale. Establishing fishways are obvious measures to mitigate the negative impact caused by dams and weirs, but numerous studies document migratory problems and selectivity in fishways (Laine, Jokivirta & Katapodis 2002; Carlsson, Aarestrup, Nordwall, Naslund, Eriksson & Carlsson 2004; Antonio, Agostinho, Pelicice, Bailly, Okada & Dias 2007). However, on a broad spatial as well as a temporal scale, fishways and other efforts to aid upstream migration represent only one element in the overall problem complex encountered by migrants after hydropower development. In this paper, temporal and spatial challenges regarding iteroparous fish migrations in regulated rivers are examined. Traditionally, measures to mitigate barriers to fish migration focused on a very narrow time-window: the migration towards spawning grounds. Maintaining the spawning run in regulated rivers by constructing suitable fishways at migration barriers is of major importance for reproduction. However, iteroparous species need to pass the same barriers in a downstream direction after spawning. This may be a problem, particularly for autumn spawners (e.g. the genera