By anthropogenic cause, even the most optimistic climate models (i.e. SSP1–RCP2.6) predict the Arctic system to heat up by more than 4°C until the year 2100, relative to the present. For ectothermic fishes, energy demand is fundamentally determined by temperature. As energy is physiologically limiting, their means to cope with climate change are limited. Therefore, understanding the impact of environmental changes on bioenergetics is imperative for the management of marine ecosystems. In recent years, the species-specific relevance of heart rate (ƒH) as a proxy for energy expenditure has been highlighted by the scientific community. The advent of bio-logging sciences has enabled ƒH observation in free swimming individuals. For Arctic fishes, however, harsh environmental conditions have restricted the pursue of ƒH bio- logging so far. To bridge this knowledge gap, we partnered with Star-Oddi, who developed a novel, internal ƒH and temperature bio-logger, calibrated for temperatures down to –5°C. In the present study, this bio-logger was implanted in the cold-adapted Arctic specialist polar cod (Boreogadus saida) and the ƒH bio-logging methodology was progressed in simulation of the ecologically relevant temperature range (i.e. 0 to 8°C) and free-roaming exercise (i.e. critical swimming speed (Ucrit) tests). Bio-logger positioning with exterior-facing electrodes and increase in sampling frequency from 100 Hz to 125 Hz improved electrocardiogram (ECG) quality significantly (p < 0.0001 and p = 0.02, respectively), due to decreased electromyogram (EMG) noise penetration and more distinct mapping of processed ECG characteristics. Under these settings, in the range of 0 to 4°C, in relation to 1180 manually calculated ECG traces, 80 ± 1.5% of on-board processed ƒH measurements displayed highest quality (i.e. QI = 0) with a confidence of ∆ƒH = 0.45 ± 0.56 bpm. Furthermore, 53 ± 5.5% of measurements displayed highest quality homogenously across swimming velocities up to Ucrit. Hence, present ƒH bio-logging methodology was validated to be highly robust in response to simulated Arctic conditions. Species-specifically for polar cod, standard metabolic rate (SMR) of bio-logged individuals at 0 ± 0.5°C amounted to 0.38 μmol/g/h. Therewith, it was lower than the previously determined 0.44 μmol/g/h for untagged conspecifics at 2.5 ± 1°C, indicating that present measurements were representative, especially given expected deviation from Q10 rules due to oxygen demands of cold adaptations. Polar cod ƒH was highly sensitive to, and consequently significantly impacted by, both temperature and swimming velocity (each with p < 0.0001). Remarkably, ƒH at Ucrit mirrored ƒHmax values previously obtained at the same temperatures by humoral injections, supporting causal relationship of ƒHmax and consequent performance limitations. Further, incremental ƒH Q10 values decreased from 2.54 ± 0.76 at 0–4°C to 2.00 ± 0.50 at 6°C and 1.73 ± 0.74 at 8°C. Hence, polar cod ƒH started failing to scale with temperatures past 4– 6°C, which in accordance with previously described temperature ranges and susceptibilities, potentially indicated the transition to passive thermal tolerance. Overall, oxygen consumption was significantly correlated to ƒH with a spearman rank correlation coefficient rho = 0.42. Lastly, the interaction of swimming velocity and temperature did not significantly impact MO2 (p = 0.71) and the relationship’s slopes displayed high similarity between 0, 2, and 8°C. In conclusion, the contribution of ƒH in regulation, and ultimately limitation, of oxygen supply in response to temperature- and performance-related energy demand, was determined as highly probable. Therefore, the potential of ƒH as a proxy for energy expenditure in polar cod was highlighted over the course of the present research.