Abstract
The maintenance of a portfolio of life histories promotes population resilience in heterogenous environments by buffering the risks of a discrete behavior. Populations of Pacific salmon (Oncorhynchus spp.) are regarded for their life history variability, especially in reference to migratory behavior. Long distance ocean migrations are a key component of the Pacific salmon life cycle and consequently this behavior has been well studied. However, species of Pacific salmon additionally display a propensity for shorter distance migrations during juvenile freshwater residency. The exhibition of these juvenile migratory strategies is temporally and spatially variable, but, compared to ocean migration, has received less research focus. With the goal of developing a better understanding of this type of behavior, we studied migratory diversity across a population of Spring Chinook salmon within the Middle Fork Salmon River’s largest tributary, Big Creek. Discrete behavior was associated with the timing of migration, with one group (downstream rearing or DSR) migrating to downstream freshwater habitat during their age-0 Summer-Fall, where they rear overwinter before continuing migration to the ocean during their age-1 Spring. Another group (natal-reach rearing or NRR), remains in their natal habitat for the entirety of their juvenile life stages prior to oceanic migration, which also occurs during their age-1 Spring.
Our objectives were to 1) assess the manner in which abiotic and biotic conditions experienced during early life stage rearing influence dispersal, 2) examine short-term and long-term fitness and survival consequences of exhibiting discrete behavior, and 3) use otolith microchemistry to develop a finer scale understanding of these migratory strategies.
Our results suggest that discrepancies in habitat use that result from dispersal impact juvenile fitness, which in turn influences success through the rest of an individual’s life cycle. Additionally, that life history behavior is more variable than previous analysis revealed. This information can be used to advance understanding of how juvenile salmon utilize habitat during freshwater residency, consequently better informing population management and restoration.