Five fish (no fewer than three) are pooled to make up a sample and no 

 sample may exceed 113.4 kg (25 lb). Fish are rinsed in tap water and care 

 is taken to insure that they do not come into contact with potential con- 

 taminating surfaces such as plastics, printed paper, metal, or mud. Each 

 fish is weighed and measured (total length) and the age of each fish is 

 determined whenever possible. Fish are then wrapped individually in clean 

 aluminum foil and labeled, after which the specimens making up each pooled 

 sample are placed into a heavy bag and frozen immediately in dry ice. The 

 samples are then transported frozen, by air freight, to CNFRL for analysis, 



Fish samples are kept frozen until the time of analysis. The five 

 specimens are then thawed, homogenized and appropriate subsamples are re- 

 moved for analysis of metals or chlorinated organic contaminant residues. 

 Metals are analyzed by atomic absorption spectrometry, and organochlorine 

 compounds are measured by gas-liquid chromatography; organic residues in 

 some samples are confirmed using mass spectrometry. Selected samples are 

 sent to an independent laboratory for analysis as a means of confirming 

 results . 



SELECTED TEMPORAL AND GEOGRAPHIC TRENDS IN CONTAMINANT RESIDUES 



Residues of DDT and it metabolites in fishes from the nation's major 

 rivers and lakes have shown a continuing downward trend. The steady de- 

 crease in total DDT, as reflected in summed p,p'-homologues (Figure 2) il- 

 lustrates the effectiveness of the 1972 ban on the use of DDT in the United 

 States. Although DDT residues remain high in some areas where it was used 

 extensively in the past, the overall trend has been downward. Even in those 

 areas where total DDT residues remain high, the p-p'-homologue, DDE, is pre- 

 sent in much greater proportion than in the past (Table 4), indicating sub- 

 stantial degradation of DDT and DDD in the environment. 



The number of collection sites where DDT has been observed in at least 

 one samples has also decreased somewhat since 1970 (Table 5). Although the 

 present occurrence of p,p'-DDT appears to have increased in recent years 

 (1976-77), this change can probably be attributed to improved analytical 

 techniques that enable better resolution and higher sensitivity for organo- 

 chlorine contaminants. 



PCBs have become virtually ubiquitous, reflecting the former widespread 

 use of these persistent industrial compounds as hydraulic fluids and as heat 

 transfer agents in capacitors and other electrical equipment. Fish contain- 

 ing residues of 0.5 ijg/g (wet weight, whole fish), the criterion established 

 for the protection of piscivorous fishes and wildlife, are collected re- 

 gularly from all NPMP stations near urban and/or industrial areas, and trace 

 levels are present in fish from the major watershed of all 50 states. 



Definite trends in the overall magnitude of PCB residues are more diffi- 

 cult to discern due to the evolution of analytical methods between 1970 and 

 1974 (Tables 3 and 4). While there appears to be a slight downward trend 

 nationwide, especially in Aroclor 1254 residues, more data produced by 



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