Climate-driven range extensions are reorganising marine communities, yet mechanisms linking environmental variability to physiology and behaviour at leading edges remain unclear. We integrated acoustic telemetry movement data (n = 30) including accelerometer tags (n = 10) calibrated with swim-tunnel respirometry to examine how subadult and adult Australasian snapper (Chrysophrys auratus) persist at their poleward range extension front in southern Tasmania, Australia. During summer, snapper typically occupied small ranges in a warm embayment (where they were tagged), then rapidly migrated offshore (~50 km) as soon as inshore waters cooled and lost thermal advantage to milder offshore habitats. Snapper returned to the same summer habitats the following spring. This reflects a novel thermoregulatory behaviour as snapper subadults typically remain in shallow nurseries even at winter minimum temperatures. In fact, range-edge snapper offshore experienced warmer winter temperatures than those found at their range-optimum. Acceleration and proportional aerobic scope use peaked during migration at moderate temperatures, not at seasonal extremes, cautioning against acceleration as a thermal performance proxy. Outside of migration, swim speeds remained near energetic optima and aerobic scope use was modest, even at minimum winter temperatures. Thus, aerobic scope insufficiency does not appear to be a limiting mechanism here. Instead, behaviourally mediated integration of seasonally suitable habitats enables persistence. Rather than acute limitation arising due to critical minimum temperatures, we propose distribution limits may arise from the effects of milder but chronically suboptimal temperatures on bioenergetics, specific performances and/or life stages.