Concentrations of polycyclic aromatic hydrocarbons in U.S. coastal mussels and oysters sampled in 1995 and 1996 range 500-fold, from about 8 parts per billion (ppb) wet weight (ww) in mussels from a site in the Southern California to over 3,000 ppb ww in oysters from several Gulf Coast embayments; the U.S. national mean concentration for 258 sites was about 140 ppb ww or about 700 ppb dry weight (dw). Regions with the lowest background concentrations (less than 30 ppb ww) include Alaska, the open coast of California and western Louisiana. Sites with the highest background concentrations (greater than 1000 ppb ww) include many urban and rural harbors and regions such as Puget Sound. On a nationwide basis there are no longterm decreases or increases in shellfish polycyclic aromatic hydrocarbon (PAH) concentrations despite clearly decreasing concentrations for other chemical contaminants. These results are relevant for developing criteria for closing and opening fisheries during responses to oil spills.

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