Effects of population density and environmental conditions on life‐history prevalence in a migratory fish

Abstract

Individual variation in life‐history traits can have important implications for the ability of populations to respond to environmental variability and change. In migratory animals, flexibility in the timing of life‐history events, such as juvenile emigration from natal areas, can influence the effects of population density and environmental conditions on habitat use and population dynamics. We evaluated the functional relationships between population density and environmental covariates and the abundance of juveniles expressing different life‐history pathways in a migratory fish, Chinook salmon (Oncorhynchus tshawytscha), in the Wenatchee River basin in Washington State, USA. We found that the abundance of younger emigrants from natal streams was best described by an accelerating or near‐linear function of spawners, whereas the abundance of older emigrants was best described by a decelerating function of spawners. This supports the hypothesis that emigration timing varies in response to density in natal areas, with younger‐emigrating life‐history pathways comprising a larger proportion of emigrants when densities of conspecifics are high. We also observed positive relationships between winter stream discharge and abundance of younger emigrants, supporting the hypothesis that habitat conditions can also influence the prevalence of different life‐history pathways. Our results suggest that early emigration, and a resultant increase in the use of downstream rearing habitats, may increase at higher population densities and with greater winter precipitation. Winter precipitation is projected to increase in this system due to climate warming. Characterizing relationships between life‐history prevalence and environmental conditions may improve our understanding of species habitat requirements and is a first step in understanding the dynamics of species with diverse life‐history strategies. As environmental conditions change—due to climate change, management, or other factors—resultant life‐history changes are likely to have important demographic implications that will be challenging to predict when life‐history diversity is not accounted for in population models.

Modular switches shift monarch butterfly migratory flight behavior at their Mexican overwintering sites

“… 62 Across these migrants, while seasonal cues (e.g., photoperiod) mediated through neurohormones often control transitions between migratory and non-migratory states, particularly migration initiation, other factors control more dynamic transitions. Density dependence (e.g., in elk, 63 deer, 64 and salmon 65 ), resource limitation (reviewed in a study by Chapman et al. 66 for vertebrates; and Menz et al.…”

Incorporating life history diversity in an integrated population model to inform viability analysis

“…However, juveniles exhibit life history diversity: a portion rear within their natal tributary for the entirety of their freshwater juvenile rearing period, while others emigrate to downstream rearing areas at various times during their first year of life (Buchanan et al 2015). Specifically, we consider the four juvenile LHPs identified in Sorel et al (2023b): three LHPs that emigrate during their first year of life (downstream-rearing LHPs) in spring (spr.0), summer (sum.0), and fall (fal.0) and rear in mainstem habitats within the Wenatchee River system, and one LHP that emigrates at age 1 (natal-reach-rearing LHP) in spring (spr.1). All four LHPs migrate downstream through the Columbia River as 2-year-olds during the spring following hatching.…”

“…Screw traps--we used estimates of the abundance of emigrants expressing different juvenile life history strategies generated from data collected at screw traps (Sorel et al 2023b). A sample of juvenile emigrants from natal tributaries was captured in rotary screw traps operated downstream of spawning habitat.…”

“…To estimate capture probability in the traps, mark-recapture trials were conducted wherein tagged fish were released upstream of traps and recaptures were recorded. The catch and mark-recapture data were used to estimate the abundance of daily emigrants (Sorel et al 2023b).…”

“…Daily emigrant abundance estimates of each age were summed within discrete time periods each year that represented the four distinct juvenile LHPs--the three downstream-rearing LHPs (spr.0, sum.0, and fal.0) and the natal-reach-rearing LHP (spr.1; Sorel et al 2023b). To account for uncertainty in estimates of the abundance of emigrants from each natal stream expressing each LHP each year, lognormal distributions were developed using a parametric bootstrap of daily abundance by LHP (Sorel et al 2023b).…”