today's methods are needed to substantiate this trend. However, residues at 

 the most heavily contaminated sites appear to be declining more noticeably. 



PCBs occur in fish tissues most frequently and at the highest concentra- 

 tions in the industrial northeastern and midwestern sections of the United 

 States (Figure 3). Though no longer manufactured in the United States, PCBs 

 are still used and continue to contaminate the environment as a result of 

 spills and improper disposal of waste hydraulic fluids and discarded elec- 

 trical components. 



Mean toxaphene residues are increasing in freshwater fishes of the 

 United States (Table 4). The national geometric average has increased from 

 0.13 yg/g in 1972 to 0.36 yg/g in 1976-77, and residues exceeding 1.0 yg/g 

 are not uncommon. Studies by CNFRL have shown that toxaphene residues of 

 1 yg/g may be associated with impaired growth and developmental abnormal- 

 ities in young fish. 



Toxaphene also occurs much more widely now than it did in past years 

 (Table 5). Formerly found only in fish from the cotton growing regions of 

 the Southeast and Southwest, it now occurs in fish throughout the United 

 States (Figure 4). Its growing ubiquity may be explained by the increased 

 use of toxaphene in agriculture, largely as a substitute for DDT and other 

 compounds that have been banned. However, this interpretation is compli- 

 cated by findings indicating the possible occurrence of chlorinated cam- 

 phenes that behave like certain toxaphene components during gas chromato- 

 graphic analysis. Particularly high residues of this compound have been 

 found in fishes from the Upper Great Lakes. Despite extensive investiga- 

 tion by gas-liquid chromatography and mass spectrometry, neither the iden- 

 tity nor the source of this compound has yet been satisfactorily determined. 



Nationally, average residue of dieldrin and endrin in fish tissues have 

 remained essentially unchanged from 1970 through 1977 (Table 4). Dieldrin 

 residues remained widespread (Table 5), reflecting the extensive use of this 

 compound (and aldrin) before 1974. The apparent variation in endrin occur- 

 rence (Table 5), however, may merely indicate changing analytical resolu- 

 tion; endrin residues have remained generally low (Table 4). 



Using newly developed capabilities to measure trace metals, we at CNFRL 

 analyzed the fish samples collected in 1977 (representing 54 stations) for 

 residues of Cd, Pb, Hg, As, and Se. 'Background' levels for the five metals 

 in whole fish samples was determined, as well as geographic areas where 

 these levels are exceeded. As examples, we found As levels ^.5 yg/g in fish 

 from Texas, Oklahoma, and the Upper Great Lakes; Se of >] .0 yg/g at many 

 stations in the Upper Missouri River system, and at both stations in 

 Pennsylvania; Pb >] .0 yg/g at a group of stations in the central Missouri 

 River system; Hg 20-25 yg/g in the Great Lakes and in some Gulf Coast 

 rivers; and Cd ^0.15 yg/g at two Upper Missouri stations. 



Discerning geographic and temporal trends in contaminant residues is not 

 the only result of NPMP monitoring activities. More importantly, the re- 

 sults of these efforts are reflected in the planning of research at CNFRL. 

 For example, unknown gas chromatograph peaks are resolved using mass spec- 



107 



