Selection of Target Species and 

 Size Classes 



• Species are equally represented in the diet. 

 Heterogeneous Diet and Contamination: 



• Some species are absent from the harvest area during one or 

 more seasons 



• Contaminant concentrations vary among species and among 

 seasons 



• Some species are eaten more than others, and diet composition 

 varies seasonally. 



In the first case above (homogeneous diet and contamination), the 

 study design could be relatively simple. Mean contaminant concen- 

 trations could be estimated from analyses of a single composite sample 

 of one of the species collected at one time of year from each harvest 

 area. If previous data were available to verify the lack of variation in 

 chemical concentrations among species and among seasons, it would 

 be appropriate to extrapolate the results from a single composite 

 sample to the entire diet composed of several species. However, this 

 is an unrealistic case. It is more likely that both contaminant 

 concentration and diet composition will vary seasonally, and that 

 contaminant concentrations will vary among species. Analyses of 

 contaminant concentrations in each species during different seasons is 

 generally recommended here to meet the diverse objectives of a typical 

 exposure assessment. 



Ideally, the set of species selected for contaminant analysis would 

 include all harvested species. Because available data and funds for 

 collecting new data are often limited, only one or a few target species 

 may be used for human health risk assessment. The particular species 

 selected for an exposure assessment will depend on the study objec- 

 tives. Examples of approaches and guidance on selection of target 

 species are given below. 



Four alternative objectives that affect the choice of target species are: 



• Perform a comprehensive analysis of harvested species 



• Characterize the typical exposure case represented by the 

 dominant harvested species 



• Characterize exposure for the worst-case species (e.g., heavily 

 consumed species expected to be highly contaminated) 



• Characterize the spatial distribution of contamination using an 

 indicator species. 



The criteria for selecting species for chemical analyses to meet each of 

 these objectives are shown in Table 4. For the first objective (compre- 

 hensive species analysis), all of the harvested species do not necessarily 

 need to be analyzed, but some criterion is required to select species for 

 analysis (e.g., the most important species in the harvest that together 

 comprise greater than 95 percent of the catch by weight). For the 

 second objective (typical exposure), a few of the dominant species (by 



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