Multiple Residues 



The difficulty of assessing the effects of chemical residues on wild 

 populations of fish is compounded by the simultaneous occurrence of several 

 contaminants. Potentially harmful levels of DDT, dieldrin, PCB, and other 

 toxic chemicals are present in the Great Lakes ecosystem. These multiple 

 residues present analytical uncertainty as well as potential additive 

 effects on aquatic populations. 



Prior to 1970 PCB residues in Great Lakes fish were not identified and 

 in many cases were probably mistakenly included in the results given for 

 pesticide residues. The use of gas chromatography-mass spectrometry has 

 improved our ability to identify contaminants, but the procedure remains 

 difficult and expensive. A much more complex problem is the interpretation 

 of the ecological significance or hazards associated with exposure to 

 multiple residues. 



Some research indicates a "synergistic" or more-than-additive toxic 

 action between PCB and certain pesticides. Joint action of PCB and DDT was 

 found in chronic exposure tests with Daphnia magna (Maki and Johnson, 1975). 

 Toxicity tests with insects have also shown joint action between PCB and 

 some carbonate and organochlorine insecticides (Lichtenstein et aj_. , 1969; 

 Plapp, 1973). 



Thus, we find that some compounds occur in the environment at concentra- 

 tions that are known to have adverse effects in laboratory tests. Where 

 these levels are not directly lethal, effects on growth or reproduction may 

 be expressed as slow changes in the size and abundance of fish populations. 

 Therefore, although it appears likely that adverse effects are occurring, 

 it is very difficult to show this conclusively. 



Effects on Higher Trophic Levels 



Toxic organic chemical residues present a serious hazard to consumer 

 organisms at the higher trophic levels, including man. The biological 

 accumulation of residues and their trophic levels are especially hazardous 

 to animals that utilize fish as a major food source. 



Piscivorous birds in the Great Lakes region have suffered unnaturally 

 high mortalities in recent years, and depressed reproduction of some popula- 

 tions has been correlated with chemical residues (Hesse, 1975). The high 

 residue concentrations of PCB in gulls in some regions of the Great Lakes 

 may seriously threaten these bird populations, but the full extent of the 

 problem is unknown. 



Even before PCB and pesticide contamination in the Great Lakes was re- 

 cognized, fur farmers in the region reported reduced reproduction of mink 

 that were fed with Great Lakes fish. Surplus coho salmon or salmon by- 

 products caused death or reproductive failure of mink when these products 

 formed 30% of their diet (Aulerich, Ringer, and Iwamoto, 1973). The rela- 

 tively high concentrations of DDT, dieldrin, and PCB in the fish were sus- 



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