Interpreting vertical movement behavior with holistic examination of depth distribution: a novel method reveals cryptic diel activity patterns of Chinook salmon in the Salish Sea

Abstract Aquatic organisms exhibit a variety of diel changes in vertical movement that are investigable through the use of biotelemetry. While certain species do not change their movements between day and night, others exhibit diel vertical migration (DVM) or a diverse range of diel activity patterns (DAPs). Consequently, day–night differences in depth distribution may be stark and easily detectable, or more subtle and difficult to identify. To augment the discovery and classification of cryptic diel vertical movement behavior, we developed and utilized a novel method that analyzed entire depth distributions while comparing diel period, season, and season/diel period models. This analysis revealed a seasonally variable DAP previously unreported in Chinook salmon (Oncorhynchus tshawytscha). In summer and fall, Chinook salmon in the Salish Sea juxtaposed shallow and confined daytime movements with nighttime bounce diving. This DAP was reversed in winter, when they occupied deeper sections of the water column during the day, resulting in a substantially overlapping, but more even depth distribution than at night. These results demonstrate the analytical utility of accompanying other metrics with holistic examination and visualization of the entire distribution of depth data. Additionally, we highlight the need to use a framework that answers all of the following questions: (1) does the target species exhibit seasonal variation in depth, (2) does it undergo DVM, (3) if it does not undergo DVM, does it follow a DAP, and (4) if it follows a DAP, what is the difference in vertical movement behavior between diel periods