The effects of the steady recession of the Arctic ice cap and the associated lengthening of the ice-free season on the underwater soundscape and the presence of marine mammal species was estimated from the analysis of multiyear time-series of broadband acoustic recordings at 50-m depths in eastern Beaufort Sea and southeastern Hudson Bay in 2005-2007. An ambient noise extraction algorithm was developed to obtain the annual time-series of the natural soundscape over a 0-4 kHz bandwidth, exclusive of transient sounds due to biological or physical sources. During winter, the ice cover lowers by ~10 dB the envelope of the ambient noise spectra below 1 kHz, by isolating the water mass from noise generated by wind and precipitation during the open water season. The expected rapid shrinking of the ice cover in coming decades will alter the exceptionally silent ambient soundscape of the Arctic Ocean and Subarctic seas, to which many arctic species are adapted, through an larger exposition to natural surface sources, followed eventually by increased destination and intercontinental shipping. The same acoustic time-series were used to monitor the timing of the frequentation by vocal marine mammal species in relation with the timing of the ice cover, which provides a seasonal solid platform to pagophilic species, such as seals, or acts as a moving barrier limiting the free access to the habitat for whales. The results show a close association of beluga, bowhead and bearded seal call densities with the fi rst signs of ice breakup in spring, evidencing the action of the seasonal ice cover on the utilisation of the habitat by these species. Such PAM (passive acoustic monitoring) observatory appears as an effi cient means for systematically tracking the expected changes in the utilisation of the Arctic and Subarctic ecosystems due to the lengthening of the open water season.