Large-scale and long-term decrease in fish growth following the construction of hydroelectric reservoirs Göran Milbrink, Tobias Vrede, Lars J. Tranvik, and Emil Rydin Abstract: Hydroelectric reservoirs retain large volumes of water and have a global impact on sea level, elemental cycles, and biodiversity. Using data from a total of 90 historical and recent surveys in nine regulated and eight unregulated alpine and subalpine lakes, we show an additional large effect of reservoirs, i.e., that impoundment causes drastically decreased fish growth and thereby great negative consequences for inland fisheries in Scandinavia. Following a long period (40– 65 years) after impoundment, the length and mass of Arctic charr (Salvelinus alpinus) of the single age class 4+ years was, on average, 35% and 72% lower, respectively, in impounded versus natural lakes in northern Scandinavia. The effect was stronger at higher altitudes and can be mitigated by addition of inorganic nutrients. We suggest that the decreased fish growth is a consequence of lowered ecosystem productivity, oligotrophication, caused by impoundment, resulting in erosion and loss of the littoral ecosystem as well as delayed flooding and leakage of nutrients from the riparian zone until after the growing season. Résumé : Les réservoirs d’ origine anthropique retiennent d’importants volumes d’eau et ont un impact global sur le niveau de la mer, les cycles des éléments et la biodiversité. À l’aide de données provenant d’un ensemble de 90 inventaires du passé et inventaires actuels dans neuf lacs alpins et subalpins munis de barrage et huit lacs sans barrage, nous démontrons un effet supplémentaire important des réservoirs, à savoir que les transformations en réservoir diminuent considérablement la croissance des poissons et qu’elles ont ainsi de sérieuses conséquences négatives sur les pêches intérieures de la Scandi- navie. Après une longue période (40–65 ans) après la transformation en réservoir, les longueurs et les masses des ombles chevaliers (Salvelinus alpinus) de la seule classe d’âge 4+ sont respectivement 35 % et 72 % inférieures dans les lacs- réservoirs par rapport aux lacs naturels dans le nord de la Scandinavie. L’effet est plus marqué aux altitudes plus élevées et peut être mitigé par l’addition de nutriments inorganiques. Nous croyons que la croissance réduite des poissons est une conséquence de la production plus basse de l’écosystème, donc de l’ oligotrophisation, causée par la transformation en réser- voir qui produit une érosion et une perte de l’écosystème littoral, ainsi qu’une inondation retardée et une perte des nutri- ments dans la zone riveraine jusqu’après la saison de croissance. [Traduit par la Rédaction] Introduction The majority of the large rivers of the world are subject to impoundment (Nilsson et al. 2005). Hydroelectric reservoirs retain large volumes of water and have a global impact on sea level (Chao et al. 2008), elemental cycles (Humborg et al. 1997; St. Louis et al. 2000), and biodiversity (Poff et al. 2007). In Scandinavia, most rivers are regulated for hydro- electric power production. In the catchments of the eight ma- jor regulated rivers in northern Sweden (41% of the total area of Sweden), there are 90 reservoirs larger than 1 km 2 that are classified as being substantially or heavily affected by the change in water amplitude according to current environmen- tal quality criteria. These reservoirs cover 3200 km 2 , corre- sponding to 22% of the regional lake area and 8% of the total Swedish lake area (data from Swedish Meteorological and Hydrological Institute). Likewise, in Norway, 6000 km 2 are affected, representing 40% of the Norwegian freshwater surface area (Aass 1984). Water level fluctuations in the Swedish reservoirs are on average 7 m, but can be as large as 35 m. Drawdown occurs mainly during winter, which re- sults in a heavy erosion of the littoral zone. It has been sug- gested that regulated ecosystems will gradually become oligotrophic (i.e., biological production declines below pre- impoundment levels) owing to habitat destruction and altered hydrology (Stockner et al. 2000; Rydin et al. 2008). Accord- Received 21 September 2010. Accepted 7 September 2011. Published at www.nrcresearchpress.com/cjfas on 6 December 2011. J2011-0105 Paper handled by Associate Editor Bror Jonsson. G. Milbrink.* Department of Ecology and Genetics, Animal Ecology, Uppsala University, Norbyvägen 18D, SE-752 36 Uppsala, Sweden. T. Vrede.* Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Sweden. L.J. Tranvik. Department of Ecology and Genetics, Limnology, Uppsala University, Sweden. E. Rydin. Department of Ecology and Genetics, Erken Laboratory, Limnology, Uppsala University, Sweden. Corresponding author: Göran Milbrink (e-mail: goran.milbrink@ebc.uu.se). *First authorship is equally shared between G. Milbrink and T. Vrede. 2167 Can. J. Fish. Aquat. Sci. 68: 2167–2173 (2011) doi:10.1139/F2011-131 Published by NRC Research Press Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Steve Cramer on 01/20/12 For personal use only.