The sampling strategy appropriate for Example 5 is complicated by the 

 occurrence of discrete harvest areas. Replicate composite samples of 

 a given species would generally be required for each season and area 

 in which the species is harvested. Because the characteristics of the 

 exposed population may differ among harvest areas, it may be ap- 

 propriate to divide the exposed population into segments correspond- 

 ing to geographic areas, ethnic groups, age classes or other factors. 

 The seasonal and total annual exposure for each segment of the 

 exposed population would be calculated for each species as in Example 

 4 above. 



Influence of Environmental and Population Factors-The four ex- 

 amples just given illustrate the variety of general study designs that may 

 be needed to meet diverse objectives. The specific design of a chemical 

 residue study will depend on the interplay between dietary patterns of 

 the exposed population and environmental factors that influence con- 

 centrations of contaminants in tissues of aquatic organisms. Some of 

 the important environmental factors are: 



• Conventional water quality (i.e., hardness, salinity, temper- 

 ature, suspended solids) 



• Habitat location, depth, proximity to contaminant sources 



• Contaminant concentrations in water 



• Contaminant concentrations in sediments 



• Species available for harvest, as influenced by habitat, 

 migratory cycles, and fisheries management practices 



• Organism activity pattern, food habits, and habitat 



• Seasonal biological cycles (e.g., stage of sexual cycle) in rela- 

 tion to the frequency and seasonality of contaminant inputs 

 (e.g., industrial discharges, waste dumps, dredging) 



• Organism size (or weight), age, and sex 



• Lipid content of tissue analyzed (where lipophilic organic 

 contaminants are of concern). 



Examples of the interaction between these factors and parameters of 

 the exposed population are given in Figure 3. 



Seasonal variation in environmental factors or activities of the exposed 

 population may correlate with contaminant concentrations in con- 

 sumed fish and shellfish. Therefore, at least general knowledge of 

 seasonal changes in contaminant concentrations and human consump- 

 tion patterns may be needed to design an appropriate sampling ap- 

 proach for estimating long-term exposure. Two extreme examples of 

 contamination and diet patterns are provided below: 



Homogeneous Diet and Contamination: 



• Each of the species is present in the harvest area all year 



• There is no seasonal variation in contaminant concentrations 



• Contaminant concentrations do not vary among species 



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