Non-Breeding Distribution of Herring Gull (Larus argentatus) and Great Black-Backed Gull (Larus marinus) in Eastern Canada from Ship-Based Surveys

Comprehensive understanding of the potential threats faced by seabirds requires information on their distribution and abundance in the marine environment where they spend a significant part of their life cycle. Data collected from shipboard seabird surveys from 2006-2014 were examined to identify high density marine areas in eastern Canada for Herring (Larus argentatus) and Great Black-backed (L. marinus) gulls during the non-breeding season. The relative importance of these offshore areas was also compared to those used by Herring and Great Black-backed gulls in previous decades based on data collected from 1965-1992. Results show that both Herring and Great Black-backed gulls were broadly distributed over shelf waters throughout Atlantic Canada during the non-breeding season, as well as in the southern Gulf of St. Lawrence and the maritime St. Lawrence Estuary. These two species of gulls were also found to use the deeper slope waters, beyond the shelf break. The relative importance of different offshore areas for both species has changed over time, with decreases observed around the island of Newfoundland and on parts of the Scotian Shelf. An assessment of the overall vulnerability of Herring and Great Black-backed gull populations outside the breeding season will require continued monitoring at sea, integration of multiple datasets, and a comprehensive analysis of human activities offshore.