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Beached Seabird Surveys on Sable Island(January 2004) |
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During summer Common and Arctic Terns nest on Sable Island and forage for food for themselves and their chicks in waters around the island. |
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Seabirds are an important component of the ecosystem on and around Sable Island. Large numbers of seabirds which breed in eastern Canada overwinter in Scotian Shelf waters, and are joined by overwintering birds from Europe. Also, some species which breed in the Southern Hemisphere come into the region during the northern summer. Thus, there is a large seabird fauna using the waters of the Scotian Shelf and the shelf break at all times of the year. These birds are vulnerable to oil pollution in their marine environment. Seabird mortality due to chronic oiling can, in some cases, exceed the more publicized results of large catastrophic oil spills such as the Exxon Valdez. Some researchers estimate that about 300,000 seabirds die every year off Canada's east coast (i.e. the Grand Banks and Scotian Shelf). Sable Island's location at the outer edge of the continental shelf off Nova Scotia, and proximity to areas of heavy marine traffic and offshore energy development and production make it useful as a platform for monitoring trends in oiled seabird species and numbers in the Scotian Shelf region. Beached bird surveys have been carried out on the island, by ZL, from January 1993 to the present. This report summarizes some results of the first ten years of the study, January 1993 to December 2002 (full publication of the results for 1993-2003 is expected in 2004). The continuing program - part of the Environmental Effects Monitoring Program for the Sable Island Bank region - is supported by ExxonMobil and EnCana. |
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Click on thumbnails for more information |
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Species Composition: During 1993-2002, >7000 seabird corpses, representing 6 families and 29 species, were recovered. Most corpses were of three families, shearwaters (Procellariidae), gulls (Laridae), and auks (Alcidae). There were strong species-specific trends in occurrence, with most auks (>85%) found in late winter, and most shearwaters (>85%) found in mid-summer. Corpse Density: Corpse numbers, primarily auks, peaked in winter, and the average auk corpse density was 1.69/km. A lesser peak occurred in June-July when Sooty and Greater sheawaters beached, with an average of 0.61/km. Oil Contamination: Of the 29 seabird species recorded, oiling was found in 14. The 10-year average oiling rate for all species combined was 40%. Auks were most frequently found oiled (>60%); and shearwaters least frequently (<3%). Gas chromatograph analysis of oil specimens collected from bird feathers indicated that most of the contamination was weathered crude and heavy fuel oil, mixed with varying amounts of lubricants and diesel. Numbers of beached birds and species composition recorded on Sable Island during 1993-2002 were subject to large fluctuations reflecting both weather and seasonal distribution of species. In winter, auks comprised the majority of corpses found, and this reflects the predominance of Arctic-breeding birds, such as Thick-billed Murres and Dovekies, overwintering in the area. Most shearwater corpses were found in June through August, which is consistent with the pelagic distribution of shearwaters on the Scotian Shelf during the summer months. In winter, when prevailing winds are northerly and northwesterly, a proportion of the many oiled auks was likely birds oiled by ship discharges along the main shipping routes to and from the Gulf of St. Lawrence. Overall the results from the 1993-2002 beached bird surveys on Sable Island are consistent with those of studies in southern Newfoundland (e.g. Wiesel & Ryan 1999) which found that most seabird species showed a marked seasonality in occurrence, and in species-specific oiling rates. High rates of oiling were typical of diving species such as auks, and low rates were typical for less vulnerable aerial birds such as shearwaters and gulls. The composition of oils found on bird corpses on Sable Island indicated that contaminants were largely derived from cargo tank washings and bilge discharges, and that on some dates beached birds had all encountered the same contamination source, and on other dates birds had come into contact with several different sources. This suggests that most contamination resulted from chronic pollution comprising various types of oil that occurs as a consequence of day-to-day operations of shipping and oil industries, particularly large ocean-going vessels. This is consistent with findings elsewhere (e.g. Furness & Camphuysen 1997). |
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Beached bird surveys have been criticized as yielding biased and unreliable measures of actual mortality rates, one reason being that oiled bird corpses found on shorelines represent only a fraction of overall mortality at sea. Many factors affect the proportion of oiled birds eventually found on beaches, for example: at sea: distribution and density of birds, sea, wind, ocean currents, sinking of corpses; and on the beach: scavengers, burial by sand, frequency of surveys, topography. However, if done systematically and over a long period of time, beach surveys can provide useful information on prevalence, severity and trends of oiling, in addition to data on species composition and seasonality, and species-specific oiling rates. References:
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