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.
The Green Sturgeon Acipenser medirostris is an anadromous, long-lived species that is distributed along the Pacific coast of North America. Green Sturgeon is vulnerable to global change due to its sensitive life history and few spawning locations. The persistence of Green Sturgeon is threatened by habitat modification, altered flows, and rising river temperatures. The southern Distinct Population Segment was listed as threatened under the U.S. Endangered Species Act in 2001 due to persistent stressors. Despite increased research efforts after the species was listed, substantial gaps in basic population information for Green Sturgeon remain. We present the only known information on age structure and growth of a threatened population of Green Sturgeon. By analyzing archived fin rays that were collected from 1984–2016, we revealed highly variable growth among individuals. We detected several age classes from 0–26 years and found similar growth rates of Sacramento River Green Sturgeon compared to northern populations. Though limited, this analysis is an important first step to understanding Green Sturgeon population dynamics and highlights critical research needs.
Management objectives for long-lived species are difficult to define as many taxa have delayed maturity and variable recruitment. White Sturgeon Acipenser transmontanus is an example of a species with a complex life history that complicates long-term status monitoring and establishment of management objectives. Historically, White Sturgeon in the Sacramento-San Joaquin River basin have been monitored by tracking the abundance of age-15 individuals as outlined by the Central Valley Project Improvement Act. However, infrequent recruitment complicates progress toward Central Valley Project Improvement Act management objectives, as abundance of a single cohort fails to represent overall population trends. By using a Leslie population matrix, we demonstrate that the probability of reaching the Central Valley Project Improvement Act objective of 11,000 age-15 White Sturgeon is highly unlikely. We propose an alternative metric of 155,000 adults, which better represents overall population trends of White Sturgeon in the Sacramento-San Joaquin River basin, can be efficiently monitored, and can support both the goal of the Central Valley Project Improvement Act and management objectives.
1. Predicting effects of climate on fitness-linked phenotypic traits, such as body size, is important for the management and conservation of species in the face of global change. During sensitive life stages, small changes in mean trait values can have large effects on survival and population productivity.2. The transition from freshwater to saltwater by migrating anadromous fishes such as Pacific salmon (genus Oncorhynchus) is a critical life history transition, where survival is mediated by the size of migrating individuals. For salmon that spend extended periods rearing in freshwater, the size at ocean entry (i.e., smolt length) may be sensitive to changes in freshwater conditions shaped by biotic and abiotic factors, yet long-term phenotypic time series for exploring these responses are rare.3. We reconstructed a four-decade time series of smolt length from archives of returning adult scales to quantify population-specific responses to climate and conspecific density in a small watershed. 4. Dynamic linear modeling found that the relationship between a proxy for cohort density and smolt length at ocean entry was consistently negative, suggestive of competition. In contrast, there was a positive, yet dynamic, relationship between a proxy for density of emerging fry during the second summer of growth and smolt length.5. The effect of temperature on smolt length was not consistent between two populations of sockeye salmon O. nerka that likely use distinct habitats within the watershed. A positive relationship between smolt length and temperature was only detected for the less abundant, early returning population.6. Predictions of smolt length showed variable responses under scenarios of increasing temperature and high and low densities of conspecifics. Collectively, these results reveal population-specific responses to temperature and density, suggesting that local habitat conditions may filter larger-scale climate drivers.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.