Mating patterns and determinants of individual reproductive success in brown trout (Salmo trutta) revealed by parentage analysis of an entire stream living population DIMITAR SERBEZOV,* LOUIS BERNATCHEZ,† ESBEN M. OLSEN*‡ and LEIF A. VØLLESTAD* *Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, P.O. Box 1066, Blindern, 0316 Oslo, Norway, †Institut de Biologie Inte ´grative et des Syste `mes (IBIS), Pavillon Charles-Euge `ne Marchand, Universite ´ Laval, Que ´bec QC G1V 0A6, Canada, ‡Institute of Marine Research, Flødevigen, 4817 His, Norway Abstract Reproductive success and its determinants are difficult to infer for wild populations of species with no parental care where behavioural observations are difficult or impossible. In this study, we characterized the breeding system and provide estimates of individual reproductive success under natural conditions for an exhaustively sampled stream- resident brown trout (Salmo trutta) population. We inferred parentage using a full probability Bayesian model that combines genetic (microsatellite) with phenotypic data. By augmenting the potential parents file with inferred parental genotypes from sib-ship analysis in cases where large families had unsampled parents, we could make more precise inference on variance of family size. We observed both polygamous and monogamous matings and large reproductive skew for both sexes, particularly in males. Correspondingly, we found evidence for sexual selection on body size for both sexes. We show that the mating system of brown trout has the potential to be very flexible and we conjecture that environmental uncertainty could be driving the evolution and perhaps select for the maintenance of plasticity of the mating system in this species. Keywords: assortative mating, Bateman’s gradient, fitness, microsatellite, parentage assignment, sexual selection Received 26 October 2009; revision received 26 May 2010; accepted 27 May 2010 Introduction Studies of mating systems are important for under- standing the processes involved in sexual selection. Lately, genetic parentage analyses have revealed details about the reproductive biology of animals that are elu- sive in observational studies and have improved our understanding of mating systems and sexual selection (Avise et al. 2002; Hughes 1998). In particular, accurate assessment of individual reproductive success, and the factors affecting the variation in reproductive success among reproducing individuals, is crucial for under- standing selective pressures driving adaptation (e.g. Clutton-Brock 2007). Differences in the potential rates of reproduction between the two sexes and variation in quality as part- ners shape the two forms of sexual selection: mate choice and within-sex competition (Andersson 1994). The resultant bias in the operational sex ratio (OSR) is expected to increase the intensity of intrasexual compe- tition, and consequently the variance in reproductive success (Emlen & Oring 1977). Typically, female varia- tion in reproductive success is linked to offspring pro- duction and survival, whereas male reproductive success is constrained by access to mates (Bateman 1948; Andersson 1994). Salmonid fishes exhibit a wide diversity of breeding systems (Gross 1991; Fleming 1998). Within a popula- tion, both female and male reproductive success may vary considerably (Fleming 1998; Garant et al. 2001). As salmonids have aggregate mating systems, competition Correspondence: Dimitar Serbezov, Fax: +47 228 54001; E-mail: d.r.serbezov@bio.uio.no Ó 2010 Blackwell Publishing Ltd Molecular Ecology (2010) 19, 3193–3205 doi: 10.1111/j.1365-294X.2010.04744.x