Marine heatwaves shape size-dependent thermal exposure, habitat use and marine residency in Arctic char (Salvelinus alpinus).

Marine heatwaves (MHWs) are increasing in high-latitude oceans, yet behavioural responses of anadromous fishes to these potential stressors during short marine feeding seasons remain poorly understood. We combined acoustic telemetry (internal temperature and depth) with satellite-derived sea surface temperature data to quantify Arctic char (Salvelinus alpinus) responses to MHWs in the coastal Labrador Sea across two contrasting summers: 2023, with frequent early-season MHWs, and 2024, dominated by a late-season event. Using non-MHW detections, we fit seasonal baselines for each fish and calculated temperature and depth anomalies. Char tended to occur in inner habitats (estuaries/fjords), though 2024 showed greater among-individual variability in outer (coastal/headland) habitat use. Internal temperatures increased relative to baseline during MHW onset (median +0.64°C) with modest persistence post-onset (+0.23°C), whereas depth anomalies showed no consistent departures. Temperature anomalies increased from pre to during MHWs across body sizes and were strongest for larger fish during onset. In contrast, MHW conditions increased the probability of inner-habitat detection primarily for smaller fish after controlling for seasonality. Migration timing differed between years in a size-dependent way, with larger fish leaving the marine environment and entering rivers earlier in 2023. Overall, Arctic char responses did not provide evidence that they sought out thermal refuges during MHWs. However, size-dependent shifts in habitat use and migration timing indicate that MHWs can still alter marine habitat trade-offs, likely through indirect ecological factors (e.g., prey availability). Arctic char responses may intensify, however, when future heatwaves exceed physiological or growth-performance thresholds.