Direction and magnitude of natural selection on body size differ among age‐classes of seaward‐migrating Pacific salmon

Abstract

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. 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. 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). 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. 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. However, the direction of selection on size consistently varied among age‐classes. 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). 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. 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.

Climate and conspecific density inform phenotypic forecasting of juvenile Pacific salmon body size

“…The South Olga lakes system on the southern end of Kodiak Island, Alaska, USA supports one of the largest sockeye salmon Oncorhynchus nerka runs in the Kodiak Archipelago and has a long‐term average sockeye salmon combined return of approximately half a million fish (Finkle & Loewen, 2015; Jackson et al., 2012). South Olga lakes includes Upper Olga Lake, an oligotrophic lake with a maximum depth of 70 m, and Lower Olga Lake, a mesotrophic lake with a maximum depth of 2 m (see Ulaski et al., 2020). Adult sockeye salmon have been enumerated and sampled for age and length at the Upper Station weir since 1928 (Finkle & Loewen, 2015).…”

“…In addition, we measured the total distance from the focus of the scale to the edge of the scale (Ruggerone et al., 2007). Random checks between two trained readers occurred for approximately 5% of adults scales (see Ulaski et al., 2020). We used the Fraser‐Lee equation (Fraser, 1916; Lee, 1920) to back‐calculate smolt length from scales of returning adults: where L i is the length of the fish at ocean entry, L c is the length of the adult fish when the scale was collected, S i is the radius of the scale at ocean entry, and S c is the total radius of the scale.…”

“…Random checks between two trained readers occurred for approximately 5% of adults scales (see Ulaski et al, 2020). We used the Fraser-Lee equation (Fraser, 1916;Lee, 1920) to back-calculate smolt length from scales of returning adults:…”

“…However, back-calculated smolt length of the early and late run were highly correlated, suggesting that it is unlikely variation in mean smolt length is explained by spurious variation in sub-sampling and back-calculation methods. In addition, it appears that selection on size for age-2 smolts is relatively consistent among years (Ulaski et al, 2020); therefore, growth patterns of surviving adults should represent relative trends in smolt length. We have demonstrated that despite the ecological noise of size-selective survival and a relatively small sample size per year, the effects of climate and density on juvenile growth can still be detected and potentially used to inform management of these Year of ocean entry Coefficient Early Late populations.…”

“…Body size at this life‐history stage has been identified as a key trait in determining marine survival (Foerster, 1954; Koenings & Geiger, 1993; Ward et al., 1989). Although the ecological agents of selection are not fully described, higher survivability of larger smolts is presumably linked to increased escape ability, faster growth, and a shorter time to attain a size less susceptible to predation and starvation (Heintz & Vollenweider, 2010; Sogard, 1997; Ulaski et al., 2020). Therefore, the conditions mediating growth that juvenile salmon experience in freshwater may have direct consequences for marine survival.…”