Sibling-size variation in brown trout Salmo trutta in relation to egg size and stream size F. GREGERSEN*†, L. A. VØLLESTAD*‡, E. M. OLSEN*§ AND T. O. HAUGEN* k *Department of Biology, Centre for Ecological and Evolutionary Synthesis (CEES), University of Oslo, P. O. Box 1066 Blindern, N-0316 Oslo, Norway, †Oppland County Governor, Environmental Protection, N-2626 Lillehammer, Norway and §Institute of Marine Research, Flødevigen, 4817 His, Norway (Received 13 May 2008, Accepted 9 January 2009) Sibling-size variation (SSV), estimated as the coefficient of variation of egg size, was investigated for 13 populations of brown trout Salmo trutta. SSV was negatively correlated with mean egg size both at the population and individual levels. After correction for the effect of mean egg size, SSV was also negatively correlated with stream size. These results provide new information about how salmonid SSV can vary at different ecological scales (individual, population and region). The results are discussed in light of competing theories for explaining SSV: (1) the passive effect hypothesis, stating that egg size variation follows passively from selection on egg size and (2) the bet-hedging hypothesis, stating that high SSV is adaptive in unpredictable environments. # 2009 The Authors Journal compilation # 2009 The Fisheries Society of the British Isles Key words: bet hedging; life history; reproduction; selection; trade-off. INTRODUCTION Offspring size is a major component of maternal fitness, where maternal fitness is determined by female lifetime production of viable offspring (Stearns, 1992; Roff, 2002). In many species, juvenile viability is higher for larger individuals (Kamler, 2005; Hurst, 2007), especially in situations with high density-dependent interaction levels (e.g. competition for space or food), leading to selection for competitive ability (Parker & Begon, 1986; Sinervo et al., 2000). A large off- spring size (and thus egg size) should thus be selected for when there is strong intra-cohort competition for resources (food and space). In fishes, juvenile body size, survival and fitness are often positively correlated with egg size (Kamler, 2005). This is especially so for territorial salmonids (Hutchings, 1991; Elliott, 1994; Einum & Fleming, 1999; Haugen & Vøllestad, 2000; Olsen & Vøllestad, ‡Author to whom correspondence should be addressed. Tel.: þ47 22854640; fax: þ47 22854001; email: avollest@bio.uio.no kPresent address: Norwegian Institute for Water Research, Gaustadall  een 21, NO-0349 Oslo, Norway. Journal of Fish Biology (2009) 74, 1259–1268 doi:10.1111/j.1095-8649.2009.02194.x, available online at http://www.blackwell-synergy.com 1259 # 2009 The Authors Journal compilation # 2009 The Fisheries Society of the British Isles