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In large, interconnected systems such as the Laurentian Great Lakes, little is known about how smallmouth bass (Micropterus dolomieu) activity varies across contrasting habitat types. As an important and widespread predator in southern Boreal waterbodies, understanding how smallmouth bass allocate energy toward movement is critical for linking behaviour to ecosystem processes, particularly when habitat types differ in the energetic costs they impose. Eastern Lake Ontario and the St. Lawrence River form a natural flow gradient across a large geographic region and are supported by an extensive acoustic telemetry network, providing a unique opportunity to quantify how contrasting habitats influence the behaviour of free-swimming fish across diel, seasonal, and spatial scales. To address these knowledge gaps, we used acoustic telemetry and accelerometer data to investigate how smallmouth bass activity patterns and metabolic rate vary across this flow-driven habitat gradient within the Laurentian Great Lakes-St. Lawrence Seaway system. We modelled seasonal and diel activity patterns, estimated space use during high- and low-activity states, and applied established metabolic calibration curves to predict field-based metabolic rate at three distinct sites. Smallmouth bass exhibited strong, location-specific activity patterns across seasonal and diel cycles that were broadly consistent with predicted seasonal changes in metabolic rate. Diel models showed elevated daytime activity, with evidence of crepuscular behaviour, particularly during summer months. Spatial analyses revealed extensive overlap between high- and low-activity states. Smallmouth bass occupying the non-flowing habitats of Lake Ontario typically incurred higher metabolic rates than those residing in the St. Lawrence River for much of the year. This study quantified how the activity and metabolic rate of smallmouth bass can vary between distinct habitats within large, heterogeneous ecosystems. By integrating multi-year behavioural and physiological data, these findings provide new insights into how environmental variables can influence the behaviour of native smallmouth bass.