Variation in spawning phenology within salmon populations influences landscape‐level patterns of brown bear activity

Abstract

AbstractAnimal consumers track spatial variation in resource phenology (i.e., resource waves) to prolong their access to ephemeral foods. While recent work has revealed how animals move across landscapes to exploit phenological variation among discrete foraging patches, much less is known about how variation nested within patches influences the spatiotemporal pattern of foraging opportunities and the behavior of consumers. Local, within‐patch, variation in phenology influences levels of resource ephemerality and could dictate how frequently consumers must move to continuously exploit a pulsed food source. Here, we explore how within‐site (stream) phenological variation relates to the duration of salmon (Oncorhynchus nerka) runs and its consequences for brown bear (Ursus arctos middendorffi) foraging behavior. We accurately quantified salmon run duration across a large number of spawning streams (21 site‐year combinations). We found that salmon run duration varied threefold among spawning sites and that the source of prolonged runs was within‐site variation in spawning phenology (i.e., the timing of arrival at spawning grounds). Although the estimated reproductive lifespan varied among sites, a simulation suggested that reproductive lifespan has little influence on salmon run duration. Salmon run duration strongly predicted the duration of site occupancy by bears, demonstrating that phenological variation within salmon populations compliments among‐population variation to alleviate time constraints on salmon consumption. To explore whether within‐population variation in salmon phenology was related to spatial variation in habitat conditions (as is the case with among‐population phenological variation), we monitored water temperature, salmon availability, and bear activity across a longitudinal gradient in Connecticut Creek, the study stream with the most prolonged salmon run. Spawn timing varied spatially, occurring first in cold headwater reaches and later in warmer downstream reaches. Patterns of bear presence closely tracked this spawning sequence, suggesting they “surf salmon waves” not only across landscapes but also within spawning sites. However, a coarser analysis across multiple sites suggests phenological variation within salmon populations may not always be spatially structured. Our results demonstrate one way in which local variation in phenology can influence consumer foraging behavior, highlighting the need to understand the causes and consequences of phenological variation at multiple scales.