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Blue whale sightings have been monitored in the Gulf of St. Lawrence (GSL) by the Mingan Island Cetacean Study (MICS) since 1979. More than 400 individuals with a heterogeneous GSL distribution have been identified to date. This report aims to qualitatively describe the distribution, dispersal, and relative abundance of blue whales on temporal and spatial scales. Information gathered on humpback whales is also described. Results confirm the general nomadic behaviour of blue whales and suggest that the GSL represents only a portion of the broad summer feeding range. Results also indicate that individual behaviours on site fidelity vary considerably: occasional vs. regular visitors and cosmopolitan vs. exclusive animals. In addition, we were able to distinguish between several areas of significant blue whale sightings based on temporal trends in the sites frequentation. While the lower Estuary seems to be the area where blue whales are most often sighted, the Mingan region has seen its blue whale frequentation decrease to such an extent that observations are now rare. Conversely, an increasing number of humpback whales have been observed in the Mingan region each year. Because many questions on their general ecology remain unanswered, the aim of this study was to increase our knowledge on the habitat use and behaviour of these rorquals. Furthermore, since marine mammals have been proposed as ecosystem sentinels, long-term monitoring can detect spatial or temporal trends that could reflect changing processes in the environment
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