An underwater acoustic observatory, operated between 2007 and 2017 in the eastern part of the Saguenay–St. Lawrence Marine Park (Québec, Canada), monitored use of the area by blue (Balaenoptera musculus) and fin (Baleanoptera physalus) whales, and the concentration of their food source, krill. This 10-year time series provided new information showing that over the study period: a) the presence of these rorqual was relatively stable, with both species visiting the area annually, particularly during the second half of the year, until ice formation; b) the start and end dates varied and frequentation was more intense during late fall than summer, when whale-watching ecotourism reaches a peak; and c) the krill concentration was high (> 176 t/km2 for 50% of the time), but fluctuated over short periods, with no apparent link to the physical or biological factors longer than 24 hours from rhythmic analysis (e.g., the fortnightly tidal period). The krill concentration tended to reach a maximum between mid-December and mid-February. If global warming shortens the ice cover period, these rorquals may spend more time in the area during winter.