It’s a bear market: evolutionary and ecological effects of predation on two wild sockeye salmon populations

Abstract

Predation can affect both phenotypic variation and population productivity in the wild, but quantifying evolutionary and demographic effects of predation in natural environments is challenging. The aim of this study was to estimate selection differentials and coefficients associated with brown bear (Ursus arctos) predation in wild sockeye salmon (Oncorhynchus nerka) populations spawning in pristine habitat that is often subject to intense predation pressure. Using reconstructed genetic pedigrees, individual reproductive success (RS) was estimated in two sockeye salmon populations for two consecutive brood years with very different predation intensities across brood years. Phenotypic data on individual adult body length, body depth, stream entry timing and reproductive lifespan were used to calculate selection coefficients based on RS, and genetic variance components were estimated using animal models. Bears consistently killed larger and more recently arrived adults, although selection differentials were small. In both populations, mean RS was higher in the brood year experiencing lower predation intensity. Selection coefficients were similar across brood years with different levels of predation, often indicating stabilizing selection on reproductive lifespan as well as directional selection for longer reproductive lifespan. Despite these selection pressures, genetic covariation of morphology, phenology and lifespan appears to have maintained variation in spawner body size and stream entry timing in both populations. Our results therefore suggest considerable demographic but limited evolutionary effects of bear predation in the two study populations.

Genomewide association analyses of fitness traits in captive‐reared Chinook salmon: Applications in evaluating conservation strategies

“…Lastly, two thermotolerance QTL (quantitative trait loci; Everett & Seeb, ; McKinney et al., ) mapped to the same positions as loci associated with age at maturity and return timing on Ots11 and Ots27, respectively. Migration and spawn timings in Chinook salmon are known to exhibit strong phenotypic and genetic correlations in some populations (e.g., Quinn, Unwin, & Kinnison, ), and body size may also be correlated with these traits as it can influence access to spawning grounds and breeding success (Lin et al., ; Schroder et al., ). The colocation of thermotolerance QTL with loci linked to age and return timing is also supportive, as the migration of spring Chinook salmon is correlated with water temperature (Keefer, Peery, & Caudill, ).…”

Modeling local adaptation and gene flow in sockeye salmon

“…The genetic covariance estimated directly from A Creek pedigree data (0.79), and the covariances in the estimated G matrices all indicated a positive genetic covariance between body length and body depth (Lin et al. ).…”

“…Bears are important agents of natural selection on sockeye salmon in streams (Quinn and Kinnison , Lin et al. ), removing approximately 12–96% of adults within each spawning population each year (Quinn et al. ).…”

“…, Lin et al. ). To study how connectivity and selection might affect the potential for phenotypic divergence between ecotypes, we parameterized the model using empirical data collected from sockeye salmon beach‐ and stream‐spawning populations in Little Togiak Lake (Wood River Lakes system, southwestern Alaska).…”

Sexual dimorphism modifies habitat‐associated divergence: Evidence from beach and creek breeding sockeye salmon

“…As a consequence of this reproductive isolation, we might expect strong parallel evolution of the sexes occupying similar environments. In shallow creek habitats, natural selection from bear predation leads to smaller body sizes in both sexes (Cunningham, Courage, & Quinn, ; Cunningham, Ruggerone, & Quinn, ; Lin et al., ; Quinn & Kinnison, ). In addition, in very shallow creeks, larger fish are more likely to “strand” in shallow water and hence die (Carlson & Quinn, ; Cunningham, Courage, et al., ; Quinn, Hendry, & Buck, ).…”