Heart rate and activity patterns of chinook salmon ( <i>Oncorhynchus tshawytscha</i> ) under steady and unsteady flow conditions

This study investigated how unsteady flow conditions influence the swimming physiology and energetic performance of Chinook salmon using co-implanted heart rate (HR) and acceleration sensors. Fish were monitored for HR, acceleration and overall dynamic body acceleration (ODBA) in two experimental settings: (1) controlled swimming at increasing speeds (0.15-0.90 m s-1) in a swim tunnel under steady and unsteady flow, and (2) free-swimming sentinel fish in tanks under steady and subsequent unsteady flow for 2 weeks each. In experiment 1, HR remained consistently high (81-84 beats min-1) across all speeds under both flow conditions, suggesting limited capacity to further elevate cardiac output. ṀO2 increased from 213±10 to 307±16 and from 225±12 to 330±17 mg kg-1 h-1 under steady and unsteady flow, respectively. Acceleration and ODBA increased linearly with speed and were positively correlated under both flow conditions. In experiment 2, circadian patterns were evident in HR, acceleration and ODBA of the free-swimming fish. Fish exhibited higher daytime and night-time HR and acceleration under unsteady flow compared with steady flow conditions, whereas ODBA remained similar. Regression models based on swim tunnel data accurately predicted acceleration and ODBA in free-swimming fish, indicating consistent relationships between swimming speed and acceleration dynamics. The higher HR and acceleration of free-swimming fish under unsteady conditions indicated a 3-5% increased energetic investment. Overall, this study provides insight into how dynamic flow environments shape the physiological responses of Chinook salmon, informing predictions of fish performance in offshore aquaculture systems.