Cr 



5.6 3.2 1.8 1.0 1.8 3.2 5.6 10 18 32 56 100 



mussel > oyster oyster > mussel 



Figure 2. Factors by which concentrations in mussels differ from those in oysters at a single 

 site. 



times higher in oysters than in mussels, 

 while the concentrations for chromium 

 and lead are threefold higher in mussels. 

 These large differences mean that con- 

 centrations of silver, copper, chromium, 

 lead, and zinc in mussels cannot be 

 compared with those in oysters. For the 

 remaining chemicals, the differences are 

 deemed small enough to ignore, and 

 sites are compared regardless of species 

 in this report. At a site near the mouth of 

 the Columbia River, two species of mus- 

 sels, M. edulis and M. calif ornicinus, 

 have been sampled. Concentration dif- 

 ferences were small for all chemicals, 



and the two species can be considered 

 equivalent. 



In this regard, it is important to note that 

 the primary reason for collecting and 

 analyzing mollusks on a yearly basis is 

 to track temporal trends in chemical 

 concentrations. Annual data on concen- 

 trations of any chemical at a single site 

 can be compared without consideration 

 of the species, as long as the same spe- 

 cies is collected every year. Conse- 

 quently, while species differences do 

 affect the analysis of national distribu- 

 tions of chemical concentrations, they 



11 



