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23 some 7 years old in 1979, and criteria were never estab- lished for residues of toxic metals. Conversely, the "action levels" for residues in edible portions, by which potential threats to human health are evaluated, are subject to intense scrutiny, and have been reviewed frequently and revised. Furthermore, significant improvements in residue analysis have been made since the NAS-NAE criteria were published, and many studies of the effects of organochlorine residues in the diets of predatory fish and wildlife have been conducted. Recent findings need to be considered and the NAS-NAE criteria reviewed for possible revision. Summary and Conclusions Results for 1976-79 continued to illustrate the effective- ness of the 1972 ban on the use of DDT in the United States. Residues of the most persistent DDT homolog (p,p'-DDE) declined after 1974, but appeared to stabilize by 1976-77. However, some of this apparent decrease in rate of disap- pearance may have been due to the counteracting effect of increases recorded for a few stations. Of the p,p'-DDT homologs, p,p'-DDE represented about 70% of the total between 1974 and 1978-79. Significant amounts of p,p'-DDT were found at only a few stations-which is further evidence of the effectiveness of the 1972 ban; more widespread use of DDT would have shown up as propor- tionately higher p,p'-DDT residues in fish from a greater number of stations. However, the ubiquity of DDT and its metabolites highlights the persistence and mobility of these toxic compounds. Furthermore, elevated DDT residues in fish attributable to past manufacturing and formulating opera- tions may represent more of a threat to piscivorous fish and wildlife than previously envisioned. In addition to well- documented cases at Wheeler National Wildlife Refuge (O'Shea et al. 1980) and in the California Bight (Young et al. 1979; Fry and Toone 1981), our data also suggest indus- trial DDT contamination in the Tombigbee and Arkansas rivers. Elevated residues in migratory forage fish from the Lower Colorado River suggest that continued DDT use in Mexico could adversely affect wildlife in the Southwest. Like DDT, PCB's have become widespread; residues were detected in fish from most of the stations sampled in 1978-79. Atmospheric transport and the movement of con- taminated sediments offer reasonable explanations for the apparent dispersal of PCB's. Although residues at some of the most heavily contaminated stations appeared to be declining slightly and Aroclor 1242 was no longer found, the national mean total PCB level did not decrease substan- tially between 1974 and 1978-79. Furthermore, the con- tinuing presence of residues resembling Aroclor 1248 sug- gested that relatively unaltered PCB's continued to enter the environment. Mean dieldrin residues declined slightly after 1974, and detectable residues were found at fewer stations in 1978-79 than in 1976-77. Additionally, no measurable aldrin residues were found in 1978-79. Collectively, these results suggest an overall decline in the abundance of dieldrin in the environment after its use was curtailed in 1975 (Schnoor 1981). However, as of 1978-79, dieldrin residues had not declined in the Great Lakes, where they remained espe- cially high in fish from Lake Michigan, and they were also high in fish from Hawaii. Chlordane has become the most widespread cyclodiene insecticide. Chlordane residues in fish were highest in the Midwest and in Hawaii, where the insecticide was used most extensively. A growing body of data indicates that toxaphene con- tamination, like that we reported for the Upper Great Lakes, is becoming widespread (Zell and Ballschmitter 1980). Toxaphene is highly toxic to most fishes (Johnson and Finley 1980) and may persist for years in some aquatic environments (Terriere et al. 1966). Analytical resolution for identifying and quantifying organochlorine residues in fish has improved greatly in the last decade. Although comparatively few of the potentially dangerous substances in the environment can be so docu- mented, several of the ones that have been successfully monitored are displaying similar behavior. Residues of per- sistent compounds no longer in use-DDT, PCB's, dieldrin, etc. -have declined in areas where they were most heavily used, but atmospheric transport and the movement of con- taminated sediments have apparently made them more widespread. These compounds also appear to be accumu- lating in natural sinks, such as the Upper Great Lakes, which receive contaminants not only from within their own watersheds but from the atmosphere as well (Eisenreich et al. 1981). This is also true for toxaphene, which was used heavily in some areas of the United States more recently than were the other compounds. The percentage of NPMP stations at which residues in whole fish exceeded NAS-NAE (1972) criteria for the pro- tection of piscivorous fish and wildlife did not change between 1974 (Schmitt et al. 1981) and 1978-79. Acknowledgments We appreciate the technical assistance of D. Stalling, G. Dubay, and B. Seabolt (Columbia National Fisheries Re- search Laboratory) and M. Ellersieck (University of Mis- souri, Columbia); and the administrative assistance of B. Berger (FWS, Washington, D.C.). We are especially grate- ful for the assistance provided by the biologists in the FWS Regional Offices (D. Lenhart, Region 1; H. Kennedy, Region 2; J. Elder, Region 3; W. Johnson, Region 4; A. Julin, Region 5; G. Hansmann, Region 6; and H. Metsker, Region 7), and their cooperators, who have coordinated the collection and shipment of NPMP samples since the pro- gram's inception in 1967.