the second stage of sampling, subsamples of fillet from each fish could 

 be selected randomly for chemical analyses. Multistage sampling is an 

 extension of two-stage sampling. 



Cluster sampling involves choosing groups of individual organisms at 

 random, then measuring contaminant concentrations in all individuals 

 within each cluster. Cluster sampling is sometimes used to estimate 

 means if clusters of sampling units (e.g., individual organisms in a 

 clump) can be selected randomly more easily than can individual units. 



Systematic sampling consists of sampling at locations and/or times 

 according to a pattern. For example, samples may be collected at 

 equidistant points on a spatial grid or at equsilly spaced time intervals. 

 Systematic sampling is generally preferred for mapping patterns of 

 contamination. As such, it is more appropriate for soil or sediment 

 sampling than for bioaccumulation studies. The random-sampling- 

 within-blocks strategy shown in Figure 5 combines systematic and 

 random sampling. Such procedures produce more uniform coverage 

 than does simple random sampling. 



Gilbert (1987) describes systematic sampling approaches for locating 

 "hot spots" or highly contaminated local areas. He addresses the fol- 

 lowing questions: 



• "What grid spacing is needed to hit a hot spot with specified 

 confidence?" > 



• "For a given grid spacing, what is the probability of hitting a hot 

 spot of specified size?" 



• "What is the probability that a hot spot exists when no hot spots 

 were found by sampling on a grid?" 



If grid sampling is to be applied to a bioaccumulation study, the target 

 species must exhibit limited mobility. Grid sampling can also be ap- 

 plied to collection of aquatic sediment samples. Gilbert (1987) 

 provides guidance on spacing of grid samples. 



Grid sampling is especially appropriate for identifying environmental 

 contamination associated with discrete sources of pollution such as 

 industrial discharges, storm drains, and combined sewer overflows. 

 The use of caged mussels is a promising approach for identifying 

 sources through chemical residue analysis. As part of the Long Island 

 Sound Estuary Program, EPA Region I is using caged mussels to 

 monitor chemical contaminants entering the Sound from tributaries. 

 The California mussel watch program (e.g., Ladd et al. 1984), the U.S 

 mussel watch (Goldberg et al. 1978, 1983; Farrington et al. 1983), and 

 the NOAA status and trends program (Boehm 1984) illustrate the use 

 of both resident and caged transplant mussels to monitor bioaccumula- 

 tion of toxic chemicals over space and time. Toxic chemical residues 

 in mussels are excellent indicators of point source discharges as well 

 as pollution gradients (Phillips 1976; Popham et al. 1980; Phelps et al. 

 1981). U.S. EPA (1982) described recommended protocols for caged 

 mussel studies. 



A combination of two-stage and stratified-random (or stratified-grid) 

 sampling is recommended here for most studies of fisheries contamina- 



42 



