Watershed-scale dispersal patterns of juvenile Chinook Salmon (Oncorhynchus tshawytscha) revealed through genetic parentage analysis.

Abstract

Background: For many aquatic taxa, juvenile dispersal from spawning locations to rearing habitats is a critical process influencing individual fitness and population dynamics. However, our understanding of dispersal patterns in naturally spawning fish populations remains largely unknown due to the logistical challenges of tagging and tracking movement at early life stages.

Methods: We quantified dispersal patterns of a spring-run Chinook Salmon (Oncorhynchus tshawytscha) population in NE Oregon, USA using genetic parentage-based tagging to trace juveniles captured from summer rearing habitats back to their maternal parent and associated spawning location (i.e., juvenile origin). We evaluated overall dispersal patterns, longitudinal trends across the watershed, and relationships between dispersal and biophysical factors, including thermal conditions, network-scale abundance estimates, and juvenile size-at-capture.

Results: Overall dispersal of the 1326 juveniles (n sampled = 3388) assigned to a maternal parent (n = 64) was downstream-biased, but we estimated that 32% dispersed upstream and 29% moved into adjacent tributaries after initial mainstem dispersal. Dispersal distances were high relative to those found in other studies, with 25% of parr dispersing more than 0.9 km upstream (max = 10.6 km) and 25% dispersing more than 3.7 km downstream (max = 28.6 km). Analysis of dispersal patterns and potential drivers indicated that (1) dispersal distances, directional bias, and variability showed clear longitudinal trends from downstream to upstream origin locations, (2) temperature was a dominant driver of dispersal, with individuals originating from warmer sections of the mainstem typically moving to cooler mainstem sections or tributaries, and (3) dispersal distance was associated with larger size-at-capture for individuals that dispersed downstream, but not upstream.

Conclusions: The widespread dispersal patterns exhibited in this population, including moving considerable distances upstream, downstream, and into tributaries, suggests that dispersal in naturally spawning fish populations may be more extensive and variable than currently recognized. We found that heterogeneity in biophysical conditions shaped within-population variability and riverscape dispersal patterns with important implications for subsequent fish habitat use, distribution, and size. This study provides an approach to evaluate patterns and drivers of dispersal in naturally spawning populations and inform conservation and restoration planning through better alignment with juvenile fish ecology.