Journal of Fish Biology (2012) 81, 642–663 doi:10.1111/j.1095-8649.2012.03363.x, available online at wileyonlinelibrary.com A concept for improving Atlantic salmon Salmo salar smolt migration past hydro power intakes H. P. Fjeldstad*†, I. Uglem‡, O. H. Diserud‡, P. Fiske‡, T. Forseth‡, E. Kvingedal‡, N. A. Hvidsten‡, F. Økland‡ and J. J¨ arnegren‡ *Department of Hydraulic and Environmental Engineering. Norwegian University of Science and Technology, NO-7491 Trondheim, Norway and ‡Norwegian Institute for Nature Research, NO-7485 Trondheim, Norway In this study, cost effective (in terms of reducing loss of power production) measures for increasing bypass migration of Atlantic salmon Salmo salar were developed and tested by establishing statis- tical models for timing of smolt migration and favourable diversion of water to the bypass. Initial tracking of radio-tagged smolts showed very low bypass migration under normal hydropower opera- tions. Bypass migration increased when bypass discharge was experimentally increased and a model was developed that described relationships between total river discharge, bypass diversion and smolt migration route. Further improvements were obtained by installing two strobe lights at the power- production tunnel entrance that increased bypass migration during the night, but not during daytime. According to the behaviour of radio-tagged fish, the implemented measures contributed to increas- ing the annual percentage of bypass migration from 11 to 64%, and according to model predictions to 60–74% when the hydropower facilities were operated according to the developed models. To ensure correct timing of discharge diversion a smolt migration model was developed based on environmental variables that could successfully predict the general pattern of migration timing. The concept presented for improving smolt migration past hydropower intakes should be applicable in many systems where migration past hydropower installations cannot easily be solved by screening systems.  2012 The Authors Journal of Fish Biology  2012 The Fisheries Society of the British Isles Key words: bypass migration; diverted flow; downstream migration; migration timing; modelling; telemetry. INTRODUCTION Loss of habitat connectivity due to construction of dams is a major threat to riverine fish populations (Ward, 1989; Larinier, 2001). On a global scale >40 000 dams higher than 15 m have been built (World Commission on Dams, 2000). These dams restrict fish migrations and alter the natural habitat for many fish species. This problem is particularly evident for anadromous fishes for which viable populations depend on successful migration from breeding grounds to the ocean and return migrations to spawning grounds. Upstream migrations across dams may be ensured by construction of fish ways for which there are many design solutions (EPRI, 2002). These may also †Author to whom correspondence should be addressed. Tel.: +47 73597166; email: hans-petter. fjeldstad@sintef.no 642  2012 The Authors Journal of Fish Biology  2012 The Fisheries Society of the British Isles