Pacific salmon and other migratory Oncorhynchus spp. Furthermore, body size is often associated with the phenology of key life-history events, such as migratory timing, that are predicted to occur when conditions are most favorable (Cushing, 1990). Phenotypic traits that prove beneficial in one environment may incur a fitness disadvantage in the next or vice versa (Gillis, Green, Middleton, & Morrissey, 2008 Schluter et al., 1991 Waples, Teel, Myers, & Marshall, 2004). Life-history trade-offs are predicted to be particularly strong in migratory or metamorphosing species, because individuals are exposed to selection across ecosystems within and among generations. Selection can also be opposing at different stages of the life history, resulting in trade-offs between fitness components, which in turn can underpin life-history divergence and adaptive radiations (Schluter, Price, & Rowe, 1991). These few examples, of many, demonstrate that the expression of body size reflects the balance between fitness costs and gains.Įcological agents of selection that represent the causes of evolution (e.g., predation, parasitism) vary across the landscape and years (Bell, 2010 Calsbeek, Gosden, Kuchta, & Svensson, 2012 Carlson & Quinn, 2007 MacColl, 2011). Furthermore, divergent selection can simultaneously favor both large and small body size in a population depending on the season and alternative strategies (Gross, 1985 Siepielski, Dibattista, & Carlson, 2009). Similarly, obtaining a large size before overwintering can be costly under certain conditions for juvenile Oncorhynchus mykiss (steelhead trout), where relatively warm winter temperatures and low food availability appeared to be more physiologically demanding for large fish (Connolly & Petersen, 2003). For example, hypoxia limits body size in Drosophila melanogaster that otherwise are strongly selected for larger mass (Klok & Harrison, 2009). Yet, despite the fitness benefits of increased body size, countervailing selection can occur as a result of the costs of increased detectability, higher energy requirements, and reduced agility for individuals of a larger size (Blanckenhorn, 2000). Body size is often positively correlated with female fecundity (Coates, 1988 Honek, 1993), can mediate the outcome of territorial aggression (Hastings, 1989 Johnsson, Nobbelin, & Bohlin, 1999 Tokarz, 1985), and affect the probability of starvation and rate of predation (Gliwicz, 1990 Scharf, Juanes, & Rountree, 2000 Sogard, 1997). In many species, traits associated with reproduction and survival are strongly influenced, both directly and indirectly, by body size.

Collectively, these results indicate that “bigger is not always better” in terms of survival and emphasize trade-offs that may exist between fitness components for organisms with similarly diverse migratory life histories.įew phenotypic traits are as important as body size given its influence on fitness across taxa (Brown, Marquet, & Taper, 1993 Choudhury, Black, & Owen, 1996 Sokolovska, Rowe, & Johansson, 2000 Wikelski & Romero, 2003). The absolute magnitude of selection was negatively correlated to mean ocean-entry timing, which may underpin negative selection favoring small age-1 fish, given associations between size and timing of seaward migration. Selection was positive for fish migrating to sea after two years in freshwater (age 2) and in their first year of life (age 0), but negative for fish migrating after 1 year in freshwater (age 1). However, the direction of selection on size consistently varied among age-classes. Results reveal the magnitude of selection on size was very strong (>90th percentile compared to a database of 3,759 linear selection differentials) and consistent among years. Across four cohorts of seaward migrants, we calculated standardized selection differentials by comparing observed size distributions of out-migrating juvenile salmon to back-calculated smolt length from the scales of surviving, returning adults. Here, we quantify the direction and magnitude of natural selection on body size among age-classes of multiple marine cohorts of O. nerka (sockeye salmon). For Pacific salmon (genus Oncorhynchus), most individual studies quantify selection across a narrow range of sizes and ages therefore, uncertainties remain concerning how selection on size may differ among diverse life histories. Due to the mediating role of body size in determining fitness, the “bigger-is-better” hypothesis still pervades evolutionary ecology despite evidence that natural selection on phenotypic traits varies in time and space.

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