caddisflies, amphipods) to "pollution-tolerant" forms (primarily oligo- 

 chaetes and midge larvae) and from greater to lesser species diversity. 

 The geographic areas affected by loss of intolerant organisms are being 

 extended further into the lakes from the pollution sources (primarily river 

 mouths). The effects of these changes on fish populations will remain 

 speculative until the interactions can be quantitatively assessed. 



Phytoplankton and zooplankton populations have also changed markedly in 

 many areas of the Great Lakes (Beeton, 1969). The changes in phytoplankton 

 have been from dominance by multispecies diatom communities to species of 

 green and blue-green algae more tolerant of eutrophic conditions. Zoo- 

 plankton communities have reacted similarly (Beeton, 1969), resulting in a 

 loss of species diversity and increases in species associated with 

 eutrophic environments. Again, the relation of these changes in fish 

 populations is incompletely understood. 



CONCLUSIONS 



It is obvious that the environment in many areas of the Great Lakes has 

 deteriorated. Assigning direct cause and effect to changes in specific 

 populations or species of Great Lakes fish is difficult and controversial. 

 Heavy exploitation of many stocks is undoubtedly a factor in the decline of 

 many species. Changes in water chemistry, plankton, bottom fauna, and un- 

 exploited fish species, however, clearly show that factors other than fish- 

 ing have drastically changed the characteristics of the Great Lakes. 



Now that we know our capabilities, how can we avoid past mistakes and 

 stop, or perhaps even reverse, the trend toward environmental chaos? One 

 possibility is to understand the forces that operated in the past to pro- 

 duce present conditions. Scientists and administrators with responsibility 

 for protecting the aquatic environment can learn much from the perturba- 

 tions foisted on the Great Lakes. For example, early recognition of the 

 effects of unmanaged commercial fishing could have prevented, or at least 

 delayed, the decimation of many fish populations. Wise management of the 

 uses of tributary streams would have saved many stocks of anadromous 

 species. It is difficult, however, to blame these errors of omission on 

 our predecessors, for they did not have the advantage of hindsight to im- 

 prove their foresight. Our generation has no such excuse. Opportunities 

 missed in the past to protect the aquatic communities are gone, but oppor- 

 tunities remain to save and rehabilitate our aquatic environment. 



Recognition of environmental degradation in the Great Lakes has led 

 Canada and the United States to a firm commitment to halt and reverse this 

 trend. Evidence of success in this endeavor is already apparent. Rehabil- 

 itation of many tributaries has permitted the establishment of spawning 

 runs by anadromous species. Levels of DDT in fish tissue have decreased as 

 much as 80% after the use of the insecticide was banned. More comprehen- 

 sive and better treatment of municipal and industrial waste has resulted in 

 noticeable improvements in the quality of receiving waters. 



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