Prespawn Migration Patterns of Adult Spring Chinook Salmon in the Terminal Reaches of a Highly Altered Interior Stream

Spring-run Chinook salmon in the Pacific Northwest can be vulnerable to prespawn mortality because they exhibit a premature migration life history, often holding in warm, degraded natal tributaries for months prior to spawning. Premature migration may result in high prespawn mortality if adequate holding conditions are not present in the terminal reaches of natal streams. This study investigated the fine-scale movement patterns and survival of prespawn, hatchery-origin spring Chinook salmon in the terminal reaches of the Upper Grande Ronde River, Oregon. We tracked fish implanted with radio transmitters and constructed a one-dimensional Hidden Markov Model using the river kilometer of fish detections and two behavioral states, move and hold, to quantify the influence of environmental factors on prespawn movement patterns. Higher temperatures were generally associated with increased movement behavior, though some fish continued holding or switched from moving to holding at peak water temperatures. Larger fish were associated with an increased frequency of transitioning between behavioral states. Our results suggest three tactics for behavioral thermoregulation: (1) rapid movement to cold terminal reaches prior to thermal stress exposure in lower reaches; (2) sprinting, whereby fish initiate movement during thermal stress; and (3) stalling, whereby fish exhibit prolonged holding below spawning grounds during thermal stress. Stalling behavior in the warmest, most downstream reaches was associated with the highest prespawn mortality. Our findings suggest that some migration strategies within spring Chinook salmon populations may be more susceptible to prespawn mortality as climatic conditions continue to warm. Key Points In response to high temperatures, some prespawn spring Chinook increased movement while others continued to hold. Three behavioral tactics were observed: rapid migration at low temperature and sprinting and stalling at high temperature. Prespawn mortalities were associated with increased holding time in locations with temperatures above thermal tolerances.