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DATA SOURCES
Wildlife toxicologists in the regional offices of Environment Canada:
Pacific and Yukon Region
Delta, B.C.
contact: Laurie Wilson
Ontario Region
Downsview, Ontario
contact: D.V. Weseloh
Atlantic Region
Sackville, N.B.
contact: Neil Burgess
National coordination of data integration by Glen Fox, Wildlife Toxicology Division, Canadian Wildlife Service (CWS), Environment Canada, Hull, Quebec.
DATA COLLECTED
a) DDE concentrations in Double-crested Cormorant eggs, measured in milligrams per kilogram (mg/kg) wet weight.
b) PCB concentrations in Double-crested Cormorant eggs, measured as PCB 1260, PCB 1254/1260 (1:1), or sum of PCB congeners in mg/kg wet weight.
GEOGRAPHICAL COVERAGE
Canada, as represented by Double-crested Cormorant colonies in four regions:
These colonies were selected for monitoring by CWS in order to provide national coverage and because historical data on contaminant levels were available.
DATA COLLECTION PERIOD
Discontinuous time series from 1970 to 1996.
SAMPLING METHODS
Eggs from Mandarte Island, Île aux Pommes, and Manawagonish Island are collected at variable intervals as part of the CWS seabird program. This program involves several bird species and contaminants, including those presented in this bulletin. The CWS Great Lakes program uses the same standardized methods of collection and analysis.
Freshly laid eggs are collected early in the season to encourage re-laying. Only one egg per clutch is taken, and eggs are refrigerated as soon after collection as possible. Portions of each sample are frozen and stored in the National Specimen Bank at the National Wildlife Research Centre in Hull, Quebec. Thus, retrospective analyses are possible if new techniques allow more sensitive measurements.
Concentrations of contaminants are measured by chemically analyzing a known amount of homogenized egg and determining the proportion by weight of each individual chemical. High-resolution capillary gas chromatography coupled with electron capture detection are the techniques used currently.
DATA PRESENTATION
The data for Figure 1, which are listed in Table 1, show the concentrations of DDE and total PCBs measured in Double-crested Cormorant eggs at four sites across Canada between 1970 and 1996.
In some years, eggs from individual colonies were pooled before contaminant analysis (i.e., equal volumes from each egg were combined to make one sample for analysis). In other years, individual eggs were analyzed. In this latter case, arithmetic means, rather than the more commonly used geometric means, were calculated to provide one data point per year (Turle and Collins 1992).
The technology for analyzing contaminants has been refined over the period these data were collected. In order to compare early concentrations with later values, conversion factors specific to each region were applied to concentrations of PCBs measured as PCB 1260 (early 1970s) and PCB 1254/1260 (1973–87) to convert them to sum of PCB congeners (after 1987). There are 209 possible congeners (chemical forms) of PCBs present in commercial PCB mixtures. Total PCBs refers to the summed concentrations of those congeners present in Double-crested Cormorant eggs.
Turle, R. and B. Collins. 1992. Validation of the use of pooled samples for monitoring of contaminants in wildlife. Chemosphere 25:463–469.
RELIABILITY OF DATA
All samples for all regions were collected and analyzed using standardized techniques (Peakall et al. 1986; Won and Turle 1987). The methods and results are used and accepted both nationally and internationally (International Joint Commission, Canada and the United States).
The cost of developing equipment and specialized training for technicians is a consideration that could affect reliability. One option to reduce costs (approximately $300 per sample) is to pool the eggs from one colony in order to measure average contamination. Pooled samples do not provide a measure of the range of values found in wild populations, and they are more difficult to analyze statistically. If there is reason to doubt an analysis, individual samples can be retrieved from the specimen bank to be reanalyzed separately.
Peakall, D.B., R.J. Norstrom, A.D. Rahimtula, and R.D. Butler. 1986. Characterization of mixed-function oxidase systems of the nestling Herring Gull and its implications for bioeffects monitoring. Environmental Toxicology and Chemistry 5(4):379–385.
Won, H. and R. Turle. 1987. 1. Method of analysis, 2. Internal quality assurance protocol. Unpublished internal report CWS-87-00. Canadian Wildlife Service, Wildlife Toxicology Division, contact: B. Wakeford.
EXPLANATION OF TERMS
DDE (dichlorodiphenyldichloroethylene) is a metabolite of DDT (dichlorodiphenyltrichloroethane). DDE is produced in most animals when the body attempts to rid itself of DDT.
The 209 possible congeners of PCBs (polychlorinated biphenyls) differ from one another in the number and relative position of the chlorine atoms on the biphenyl frame. A small number have toxic properties similar to those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), the most toxic dioxin congener, and are thought to account for most of the PCB-induced toxicity in animals.
Table 1 Concentrations of DDE and PCBs in Double-crested Cormorant eggs
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