Green Bay (Lake Michigan) and the Bay of Quinte (Lake Ontario). Low DO 

 levels in the open waters of the other lakes have not been reported. The 

 virtual extinction of the sauger and blue pike and the decline of the 

 walleye population in Lake Erie are thought to be partially caused by the 

 low DO conditions (Smith, 1974). 



Toxic Substances in Fish 



Chemical contaminants in Great Lakes fish have been measured with in- 

 creasing frequency in the past decade (1965-75). Measurements were first 

 made in 1965 of the residues of the insecticides DDT and dieldrin in Great 

 Lakes fish. All 28 species for which DDT and dieldrin analysis has been 

 made contained measurable levels. Several species (chubs, lake trout, lake 

 herring) from Lake Michigan exceeded the U.S. Food and Drug Administra- 

 tion's (FDA) tolerance level of 5 yg/g in fish used for human consumption 

 (Reinert, 1970). Since the use of DDT was banned in 1972, the level in 

 Lake Michigan fish has decreased rapidly, from an average of 10 yg/g in 

 bloater chubs before 1972 to less than 3 yg/g in 1974. 



During the same period in which DDT levels were decreasing in Great 

 Lakes fish, polychlorinated biphenol (PCB) levels were increasing. Again, 

 the species containing the highest levels were lake trout and bloater chubs 

 in Lake Michigan. The average concentration of PCB in Lake Michigan lake 

 trout above 24 inches exceeded 20 yg/g in 1974. Concentrations above the 

 FDA's 5 yg/g tolerance level have been reported in fish from Lake Ontario 

 and Lake Huron, as well as Lake Michigan. 



In 1969 mercury levels in excess of the FDA's tolerance level of 0.5 

 yg/g were discovered in several species of fish (including walleye and 

 white bass) from Lakes St. Clair and Erie. Mercury levels above 0.5 yg/g 

 were also reported from Lakes Superior and Ontario. Two years following 

 curtailment of the source of mercury pollution to Lake St. Clair, the 

 levels in fish began to decrease. In two instances (DDT and mercury) stop- 

 ping the sources of chemical contaminants resulted in the rapid decline of 

 the toxicants in the environment. This success should give support to con- 

 tinued efforts to solve problems by eliminating the direct cause. 



The contamination of Great Lakes fish with levels of DDT, PCB, and mer- 

 cury exceeding the FDA tolerance level has resulted in great financial hard- 

 ship to the commercial fishing industry. Direct or even indirect adverse 

 effects on the fish populations of the Great Lakes have not been detected. 

 Apparently, DDT and PCB in the low nannogram-per-liter levels in the open 

 lake waters have, through biomagnif ication, reached the microgram-per-gram 

 level in fish tissue. 



Changes in Benthos and Plankton 



Changes in bottom-dwelling organisms have been documented by several 

 investigators within the past 20 years: Britt (1955) and Carr and Hiltunen 

 (1965) for Lake Erie; Schneider, Hooper, and Beeton (1969) for Saginaw Bay; 

 Hiltunen (19b7) for Lake Michigan. In all of these studies the changes 

 have been from the more "pollution-intolerant" organisms (mayflies, 



215 



