Journal of Fish Biology (2014) 84, 503–512 doi:10.1111/jfb.12300, available online at wileyonlinelibrary.com Sex identification and PIT-tagging: tools and prospects for studying intersexual differences in freshwater fishes K. Hulth´ en*†, B. B. Chapman*, P. A. Nilsson*, L.-A. Hansson*, C. Skov‡, H. Baktoft‡, J. Brodersen§ and C. Br¨ onmark* *Department of Biology, Lund University, Ecology Building, 223 62 Lund, Sweden, ‡National Institute of Aquatic Resources, Technical University of Denmark (DTU), Vejlsøvej 39, 8600 Silkeborg, Denmark and §Department of Fish Ecology and Evolution, EAWAG Swiss Federal Institute of Aquatic Science and Technology, Center of Ecology, Evolution and Biochemistry, Seestrasse 79, CH-6047 Kastanienbaum, Switzerland (Received 13 August 2013, Accepted 14 November 2013) This study evaluated a technique to allow the long-term monitoring of individual fishes of known sex in the wild using sex confirmation in close proximity to the reproductive period combined with individual tagging. Hundreds of partially migratory roach Rutilus rutilus were tagged with passive integrated transponders (PIT) following sex determination in spring and various performance measures were compared with fish tagged outside the reproductive period in autumn. Short-term survival was >95% for R. rutilus sexed and tagged under natural field conditions. Total length (L T ) did not affect the probability of survival within the size range tagged (119–280 mm), nor were there differences in timing of migration the following season between individuals sexed and tagged in spring and individuals tagged in autumn (i.e . outside the reproductive period). Also, a similar per cent of R. rutilus sexed and tagged in spring and tagged in autumn migrated the following season (34·5 and 34·7%). Moreover, long-term recapture data revealed no significant differences in body condition between R. rutilus individuals sexed and tagged in spring, individuals tagged in autumn and unmanipulated individuals. The observed sex ratio of recaptured fish did not differ from the expected values of equal recapture rates between males and females. Hence, there is no observable evidence for an adverse effect of tagging close to the reproductive period and therefore this method is suitable for studying intersexual differences and other phenotypic traits temporarily expressed during reproduction at the individual level in fishes.  2014 The Fisheries Society of the British Isles Key words: females; males; sex; sex determination; telemetry. INTRODUCTION In many animals, including fishes, there are intersexual differences in many aspects of life history, behaviour and physiology (Hanson et al., 2008). In order to fully understand the biology of fishes, it is therefore of fundamental importance to take sex into account. Most research on intersexual variation in fishes has focused upon species in which sex can be readily determined from external characteristics, such as the dichromism in some poeciliids (Endler, 1983), making the current body of †Author to whom correspondence should be addressed. Tel.: +46 767 864 997; email: kaj.hulthen@biol.lu.se 503  2014 The Fisheries Society of the British Isles