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Water birds
State of Colonial Nesting Aquatic Bird Populations
Background
Throughout the 1960s and into the early 1970s, populations of aquatic birds such as Herring Gulls and Double-crested Cormorants in the Great Lakes and Northern Gannets in the Gulf of St. Lawrence, experienced major declines. Studies had then shown the link between high levels of organochlorine compounds and eggshell thinning as well as reproductive failure. These conditions prompted governments to ban or regulate the use of PCBs and many organochlorine pesticides such as DDT (dichlorodiphenyltrichloroethane). Now, more than 30 years later, the situation is much improved; most of the old chemicals considered problematic are still present, although at much reduced concentrations, and most bird populations have increased. However, new toxic and persistent substances, such as the polybrominated diphenyl ethers (PBDEs, flame retardants), are detected in the environment and accumulate in tissues of birds.
Colonial aquatic birds are good indicators of contamination and ecosystem health because they are near the top of the food web and they are generally abundant and easy to sample. For these reasons, many species have been selected as sentinel species in monitoring programs, as is the case in the Great Lakes-St. Lawrence basin.
This fact sheet presents a synthesis of the state of the populations of two sentinel species: the Herring Gull and the Double-crested Cormorant.
Figure 1 |
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| An adult Herring Gull | and its nest and clutch of eggs. | |
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Photo by: Bruce Szczechowski |
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Photo by: Bruce Szczechowski |
Figure 2 |
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An adult Double-crested Cormorant |
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and its nest and clutch of eggs. |
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Photo by: John Mitchell |
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Photo by: Bruce Szczechowski |
It also presents spatial and temporal trends in their contamination by five toxic substances: mercury, polychlorinated biphenyl (PCB), polybrominated diphenyl ethers (PBDEs) and two organochlorine pesticides: dieldrin and DDE, a persistent degradation product of the pesticide DDT, in the Great Lakes-St. Lawrence basin.
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For the spatial analysis, data on contaminants in Herring Gull eggs were available from 34 different sites. To put the data into more manageable units, sites have been grouped by lake (Superior, Michigan, Huron, Erie and Ontario) and section (Upper St. Lawrence, fluvial estuary, upper estuary, lower estuary, North Shore and Gaspé Peninsula). In this way, 11 collection areas were compared spatially for the period 1990-1993, the most recent years for which comparable data were available.
- For cormorant eggs, data on contaminants were available from 37 sites for the period 1991-1995. The collection sites were grouped into upper (lakes Superior , Michigan and Huron) and lower (lakes Erie and Ontario) Great Lakes, and four areas: upper estuary, lower estuary, North Shore and Gaspé Peninsula.
- For the temporal analysis, the Herring Gull data were again grouped by lake (1974-2003) while for the cormorants, only data from 1972, 1989 and 1995 from three lakes were sufficient to allow analysis. For the fluvial area, the only data come from the upper St. Lawrence and one site on the North Shore for the Herring Gull (1972-2002) and one colony from the lower estuary for the cormorants (1972-2000).
Date modified: 2008/05/01 – Important Notices
