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 (AC) sensors. Fish were monitored for HR, AC, 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 two weeks each. In experiment 1, HR remained consistently high (81-84 bpm) across all speeds under both flow conditions, suggesting limited capacity to further elevate cardiac output. MO2 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. AC and ODBA increased linearly with speed and were positively correlated under both flow conditions. In experiment 2, circadian patterns were evident in HR, AC, and ODBA of the free-swimming fish. Fish exhibited higher daytime and nighttime HR and AC under unsteady flow compared to steady flow conditions, while ODBA remained similar. Regression models based on swim-tunnel data accurately predicted AC and ODBA in free-swimming fish, indicating consistent relationships between swimming speed and acceleration dynamics. The higher HR and AC 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.