We investigated the abundance and distribution patterns of a range of seabird species in the Lower Estuary and Gulf of St Lawrence in the western North Atlantic Ocean using ship-based surveys during the summers of 2007, 2008 and 2009. This area is known to be of particular importance for several seabird and cetacean species. We analysed distribution and abundance of common seabird species in mid and late summer, and estimated total numbers for the Southern Gulf, which was most intensively surveyed. Northern Gannets Morus bassanus were overall most abundant and widespread. Our at-sea estimate of 150,000 birds for the Southern Gulf constitutes 64% of the North American breeding population, rendering the site one of the most important areas for this species worldwide during this period. Our at-sea estimates suggest that according to the 1% threshold of the Ramsar Convention considerable proportions of the Canadian breeding population of Razorbills Alca torda (5-11%), Common Guillemots Uria aalge (2-3%), Black-legged Kittiwakes Rissa tridactyla (2-4%) and Black Guillemots Cepphus grylle (1-2%) use the Southern Gulf. Relative to their biogeographic populations, at-sea totals were also considerable in American Herring Gulls Larus smithsonianus (3-4%), Great Black-backed Gulls L. marinus (1-4%) and Great Northern Divers Gavia immer (1-4%). Areas of high seabird densities and multispecies aggregations (hotspots) occurred around the Gaspé Peninsula (Northern Gannets, alcids, Black-legged Kittiwakes, Larus gulls), in the Northwestern Gulf, along the Lower North Shore (near St Mary's Islands), along the west coast of Newfoundland (Bay of Islands to St Georges Bay), in Cabot Strait, around Cape Breton Island and the Magdalen Islands, as well as west and east of Prince Edward Island.

Since the 1960s, considerable effort has been directed at surveying seabirds in the marine environment. Previous studies resulted in detailed publications on seabird distribution and estimated total numbers of seabirds at sea (e.g. Brown et al. 1975; Brown 1986; Skov et al. 1995; Sonntag et al. 2006; Garthe et al. 2007a). The methods for ship-based surveys started with qualitative approaches, collecting data on the relative abundance of seabirds as for PIROP (Programme Integré de Recherches sur les Oiseaux Pélagiques) in Canada (Brown 1986). Standardisation and improvement of survey methods (Tasker et al. 1984) has led to widely used and compatible protocols (e.g. Garthe et al. 2002; Camphuysen et al. 2004; Gjerdrum et al. 2012). Many current survey programmes apply distance sampling (Buckland et al. 2001) for clearly defined sampling units (e.g. adjusted line-transect) to account for imperfect detection of animals. These approaches enable the estimation of total numbers at sea for large marine areas (e.g. Sonntag et al. 2006; Garthe et al. 2007a; Fifield et al. 2009). We applied these methods to gain information on the current distribution patterns of the most common seabirds in the Lower Estuary and Gulf of St Lawrence.

The study area is a highly productive marine area (White & Johns 1997; Dufour & Ouellet 2007) holding important breeding colonies of Northern Gannets Morus bassanus, alcids, and Black-legged Kittiwakes Rissa tridactyla (Cotter & Rail 2007; Rail & Cotter 2007; Cotter et al. 2012; Chardine et al. 2013). The area has been subject to profound changes, such as the collapse of demersal fish stocks, which have altered the structure of the marine ecosystem, with fundamental effects on many fish species (e.g. Savenkoff et al. 2007a,b; Bundy et al. 2009) that are important prey for a multitude of seabirds. Moreover, seabirds were intensively surveyed at sea in this area during the 1970s and 1980s (Brown 1986), but only little effort was subsequently available between the 1980s and 2006 and current and easily available information is lacking. We identified areas of high seabird densities and multispecies aggregations (i.e. hotspots) in the entire Gulf and Lower Estuary and estimated total numbers at sea for common seabird species in the Southern Gulf to characterise its importance in a regional and international context.

We are sincerely grateful to Alain Gagné from the Maurice Lamontagne Institute (IML) in Mont-Joli, Tom Hurlbut from the Gulf Fisheries Centre in Moncton, François Grégoire (IML), Charline Lévesque (IML) and their colleagues for the opportunity to join their research cruises and for their kind support throughout these ventures. We thank the crews of the 'Teleost' and other vessels we worked on for their support and cooperation during the surveys. We also thank Jean-François Rail and Carina Gjerdrum from the Canadian Wildlife Service in Québec City and Dartmouth very much for providing data on seabird colonies and population estimates. Andy Webb, Iain Stenhouse and Martin Heubeck made valuable comments on earlier drafts of our manuscript, which greatly improved the quality of our paper.

Bächler, E. & Liechti, F. 2007. On the importance of g(0) for estimating bird population densities with standard distance-sampling: implications from a telemetry study and a literature review. Ibis 149: 693-700. [Crossref]

Bellebaum, J., Diederichs, A., Kube, J., Schulz, A. & Nehls, G. 2006. Flucht- und Meidedistanzen überwinternder Seetaucher und Meeresenten gegenüber Schiffen auf See. Ornithologischer Rundbrief Mecklenburg-Vorpommern 45: 86-90.

Bolduc, F. & Desbiens, A. 2011. Delineating Distance Intervals for Ship-based Seabird Surveys. Waterbirds 34: 253-257. [Crossref]

Brown, R. G. 1986. Revised atlas of eastern Canadian seabirds. Canadian Wildlife Service, Ottawa.

Brown, R. G., Nettleship, D. N., Germain, P., Tull, C. E. & Davis, T. 1975. Atlas of eastern Canadian seabirds. Canadian Wildlife Service, Ottawa.

Buckland, S. T., Anderson, D. R., Burnham, K. P., Laake, J. L., Borchers, D. L. & Thomas, L. 2001. Introduction to distance sampling: estimating abundance of wildlife populations. Oxford University Press, Oxford.

Bundy, A., Heymans, J. J., Morissette, L. & Savenkoff, C. 2009. Seals, Cod and forage fish: a comparative exploration of variations in the theme of stock collapse and ecosystem change in four Northwest Atlantic ecosystems. Progress in Oceanography 81: 188-206. [Crossref]

Cairns, D. K., Chapdelaine, G. & Montevecchi, W. A. 1991. Prey exploitation by seabirds in the Gulf of St Lawrence. In: Therriault, J.-C. (ed.) The Gulf of St Lawrence: small ocean or big estuary? Canadian Special Publication of Fisheries and Aquatic Sciences 113: 277-291.

Camphuysen, C. J. 2011. Northern Gannets in the North Sea: foraging distribution and feeding techniques around the Bass Rock. British Birds 104: 60-75.

Camphuysen, C. J., Calvo, B., Durinck, J., Ensor, K., Follestad, A., Furness, R. W., Garthe, S., Leaper, G., Skov, H., Tasker, M. L. & Winter, C. J. N. 1995. Consumption of discards by seabirds in the North Sea. Final report EC DG XIV research contract BIOECO/93/10.NIOZ Rapport 1995 - 5. Netherlands Institute for Sea Research, Texel.

Camphuysen, C. J., Fox, A. D., Leopold, M. F. & Petersen, I. K. 2004. Towards standardised seabirds at sea census techniques in connection with environmental impact assessments for offshore wind farms in the U.K. COWRIE-Report BAM-02-2002, Texel.

Chardine, J. W., Rail J.-F. & Wilhelm, S. 2013. Population dynamics of Northern Gannets in North America, 1984-2009. Journal of Field Ornithology 84: 187-192. [Crossref]

Cotter, R. & Rail, J.-F. 2007. Third census of seabird populations of the Gaspé Peninsula, Québec, 2002. Canadian Field-Naturalist 121: 274-286. [Crossref]

Cotter, R., Rail, J.-F., Boyne, A. W., Robertson, G. J., Weseloh, D. V. C. & Chaulk, K. G. 2012. Population status, distribution, and trends of gulls and kittiwakes breeding in eastern Canada, 1998-2007. Canadian Wildlife Service Occasional Paper 120.

Dufour, R., & Ouellet, P. 2007. Estuary and Gulf of St Lawrence marine ecosystem overview and assessment report. Canadian Technical Report of Fisheries and Aquatic Sciences 2744E.

Fifield, D. A., Lewis, K. P., Gjerdrum, C., Robertson, G. J. & Wells, R. 2009. Offshore seabird monitoring program. Environmental Studies Research Funds 183, St John's.

Garthe, S. 1993. Quantifizierung von Abfall und Beifang der Fischerei in der südöstlichen Nordsee und deren Nutzung durch Seevögel. Hamburger Avifaunistische Beiträge 25: 125-237.

Garthe, S. & Hüppop, O. 2004. Scaling possible adverse effects of marine wind farms on seabirds: developing and applying a vulnerability index. Journal of Applied Ecology 41: 724-734. [Crossref]

Garthe, S., Hüppop, O. & Weichler, T. 2002. Anleitung zur Erfassung von Seevögeln auf See von Schiffen. Seevögel 23: 47-55.

Garthe, S., Markones, N., Hüppop, O. & Adler, S. 2009. Effects of hydrographic and meteorological factors on seasonal seabird abundance in the southern North Sea. Marine Ecology Progress Series 391: 243-255. [Crossref]

Garthe, S., Sonntag, N., Schwemmer, P. & Dierschke, V. 2007a. Estimation of seabird numbers in the German North Sea throughout the annual cycle and their biogeographic importance. Vogelwelt 128: 163-178.

Garthe, S., Montevecchi, W. A., Chapdelaine, G., Rail, J.-F. & Hedd, A. 2007b.. Contrasting foraging tactics by Northern Gannets (Sula bassana) breeding in different oceanographic domains with different prey fields. Marine Biology 151: 687-694. [Crossref]

Gjerdrum, C., Fifield, D. A. & Wilhelm, S. I. 2012. Eastern Canada Seabirds at Sea (ECSAS) standardized protocol for pelagic seabird surveys from moving and stationary platforms. Canadian Wildlife Service Technical Report Series 515, Atlantic Region.

Gjerdrum, C., Head, E. J. H. & Fifield, D. A. 2008. Monitoring seabirds at sea in Eastern Canada. AZMP Bulletin PMZA 7: 52-58.

Guse, N. 2013. 'Habitat selection and foraging ecology of seabirds in the Gulf of St Lawrence'. PhD thesis, University of Kiel, Kiel. http://eldiss.uni-kiel.de/macau/receive/dissertation_diss_00012079

Government of Canada 1991. The State of Canada's Environment Minister of Supply and Services, Ottawa.

Hamer, K. C., Humphreys, E. M., Garthe, S., Hennicke, J., Peters, G., Grémillet, D., Phillips, R. A., Harris, M. P. & Wanless, S. 2007. Annual variation in diets, feeding locations and foraging behaviour of Gannets in the North Sea: flexibility, consistence and constraint. Marine Ecology Progress Series 338: 295-305. [Crossref]

Haney, J. C. 1990. Winter habitat of Common Loons on the continental shelf of the southeastern United States. Wilson Bulletin 102: 253-263.

Head, E. J. H. & Pepin, P. 2007. Variations in overwintering depth distributions of Calanus finmarchicus in the slope waters of the NW Atlantic continental shelf and the Labrador Sea. Journal of Northwest Atlantic Fishery Science 39: 49-69. [Crossref]

Heinemann, D. 1981. A range finder for pelagic bird censusing. Journal of Wildlife Management 45: 489-493. [Crossref]

Hyrenbach, K. D., Henry, M. F., Morgan, K. H., Welch, D. W. & Sydeman, W. J. 2007. Optimizing the width of strip transects for seabird surveys from vessels of opportunity. Marine Ornithology 35: 29-37.

Hudson, A. C. & Furness, R. W. 1988. Utilization of discarded fish by scavenging seabirds behind whitefish trawlers in Shetland. Journal of Zoology 215: 151-166.

Kober, K., Webb, A., Win, I., Lewis, M., O'Brien, S., Wilson, L. J. & Reid, J. B. 2010. An analysis of the numbers and distribution of seabirds within the British Fishery Limit aimed at identifying areas that qualify as possible marine SPAs. JNCC report No. 431, Peterborough.

Koutitonsky, V. G. & Bugden, G. L. 1991. The physical oceanography of the Gulf of St Lawrence: a review with emphasis on the synoptic variability of the motion. In: Therriault, J.- C. (ed.) The Gulf of St Lawrence: small ocean or big estuary? Canadian Special Publication of Fisheries and Aquatic Sciences 113: 57-90.

Lafontaine, Y. de, Demers, S. & Runge, J. 1991. Pelagic food web interactions and productivity in the Gulf of St Lawrence: a perspective. In: Therriault, J.-C. (ed.) The Gulf of St Lawrence: small ocean or big estuary? Canadian Special Publication of Fisheries and Aquatic Sciences 113: 99-123.

McIntyre, J. W. 1988. The Common Loon: Spirit of the Northern Lakes. University of Minnesota Press, Minneapolis.

Markones, N. & Garthe, S. 2009. Erprobung eines Bund/Länder-Fachvorschlags für das Deutsche Meeresmonitoring von Seevögeln und Schweinswalen als Grundlage für die Erfüllung der Natura 2000-Berichtspflichten mit einem Schwerpunkt in der deutschen AWZ von Nord-und Ostsee (FFH-Berichtsperiode 2007-2012). Teilvorhaben Seevögel. Endbericht für das Bundesamt für Naturschutz, Vilm.

Markones, N. & Garthe, S. 2011. Monitoring von Seevögeln im Offshore-Bereich der schleswig-holsteinischen Nordsee im Rahmen von NATURA 2000 in den Jahren 2004 bis 2009. Corax 22: 11-50.

Markones, N. & Garthe, S. 2012. Ermittlung von artspezifischen Korrekturfaktoren für fluggestützte Seevogelerfassungen als Grundlage für Bestandsberechnungen von Seevögeln im Rahmen des Monitorings in der deutschen Ausschließlichen Wirtschaftszone von Nord- und Ostsee. Endbericht für das Bundesamt für Naturschutz, Vilm.

Markones, N., Garthe, S., Dierschke, V. & Adler, S. 2008. Small-scale temporal variability of seabird distribution patterns in the south-eastern North Sea. In: Wollny-Goerke, K. & Eskildsen, K. (eds.) Marine mammals and seabirds in front of offshore wind energy: 115-140. Teubner, Wiesbaden.

Markones, N., Schwemmer, H. & Garthe, S. 2012. Seevogel-Monitoring 2011/2012 in der deutschen Nord- und Ostsee. Bericht für das Bundesamt für Naturschutz, Vilm.

Rail, J.-F. & Cotter, R. 2007. Sixteenth census of seabird populations in the sanctuaries of the North Shore of the Gulf of St Lawrence, 2005. Canadian Field-Naturalist 121: 287-294. [Crossref]

Savenkoff, C., Castonguay, M., Chabot, D., Hammill, M. O., Bourdages, H. & Morissette, L. 2007a. Changes in the Northern Gulf of St Lawrence ecosystem estimated by inverse modelling: Evidence of a fishery-induced regime shift? Estuarine, Coastal and Shelf Science 73: 711-724. [Crossref]

Savenkoff, C., Swain, D. P., Hanson, J. M., Castonguay, M., Hammill, M. O., Bourdages, H., Morissette, L. & Chabot, D. 2007b. Effects of fishing and predation in a heavily exploited ecosystem: comparing periods before and after the collapse of groundfish in the Southern Gulf of St Lawrence (Canada). Ecological Modelling 204: 115-128. [Crossref]

Skov, H., Durinck, J., Leopold, M. F. & Tasker, M. L. 1995. Important bird areas for seabirds in the North Sea including the Channel and the Kattegat BirdLife International, Cambridge.

Sonntag, N., Mendel, B. & Garthe, S. 2006. Die Verbreitung von See- und Wasservögeln in der deutschen Ostsee im Jahresverlauf. Vogelwarte 44: 81-112.

Tasker, M. L., Hope Jones, P., Dixon, T. & Blake, B. F. 1984. Counting seabirds at sea from ships: a review of methods employed and a suggestion for a standardized approach. Auk 101: 567-577. [Crossref]

Wetlands International 2012. Waterbird Population Estimates. Retrieved from wpe.wetlands.org on 4 December 2012.

White, L. & Johns, F. 1997. Marine environmental assessment of the Estuary and Gulf of St Lawrence. Fisheries and Oceans Canada, Dartmouth and Mont-Joli.