example, when contaminant concentrations are positively correlated 

 with fish (or shellfish) size, frequent consumption of the smaller in- 

 dividuals may be acceptable even though consumption of larger in- 

 dividuals should be severely limited. 



Two general approaches to field sampling are possible. First, the 

 investigator can obtain samples directly from harvesters. This ap- 

 proach has the advantage that the sampled population is the popula- 

 tion of direct interest for the exposure and risk assessments. However, 

 one drawback of this approach is the potential for contamination or 

 degradation of samples due to handhng of the samples by the har- 

 vesters. Moreover, the precise sampling locations may be unknown if 

 samples are collected at dockside from recreational or commercial 

 fishing boats. The second approach is to obtain samples independent 

 of the normal harvesting efforts, allowing standard sample handling 

 practices to be implemented. Independent sampling also facilitates the 

 collection of adequate samples for stratification by organism size, 

 habitat, or some other variable. The remainder of this section address- 

 es a sampling effort that is independent of normal harvesting activities. 



Sampling stations should generally be located in known harvest areas. 

 However, additional stations in relatively uncontaminated reference or 

 control areas should also be sampled. By comparing results among 

 harvest areas and between each harvest area and the reference station, 

 one can establish not only the degree of spatial heterogeneity but also 

 the magnitude of elevation above reference of contaminant concentra- 

 tions (and corresponding health risks) at each harvest area. Because 

 sampling depth or vertical position on the shore may influence con- 

 taminant concentration in aquatic organisms, reference station char- 

 acteristics should be closely matched to those for the harvest areas. 



Sampling stations may be located within a study area according to one 

 of several probability sampling designs (Figure 5). Gilbert (1987) 

 provides a concise summary of conditions under which each sampling 

 design is preferred. 



Simple random sampling implies that each individual organism of a 

 species within a specified area has an equal chance of being selected 

 for measurement and that selection of one individual does not in- 

 Huence selection of others. A simple random sampling strategy is 

 appropriate if there are no major trends or patterns of contamination 

 in the study area. Note that sampling of fish or shellfish with sampling 

 gear (e.g., hook and line, nets) will often be nonrandom with respect 

 to species and size classes because of the selective nature of the gear. 



Stratified random sampling involves random sampling within nonover- 

 lapping strata of a population (e.g., subareas where recreational fishing 

 effort is concentrated or where contamination is greatest). This sam- 

 pling approach is appropriate when localized geographic areas within 

 a harvest region are heterogeneous relative to the kind or degree of 

 contamination. 



Two-stage sampling involves random or systematic subsampling of 

 primary units selected by a random sampling technique. For example, 

 fish could initially be collected randomly from a given stream reach. In 



Sampling Station Locations 



41 



