covered with a layer of material which would tend to prevent ready- 

 interchange of gases. Since almost all of the commercial and game 

 fishes of Western Lake Erie lay their eggs on the bottom, and many of 

 them feed wholly or partially on bottom organisms, it follows that a 

 study of the bottom and its associated organisms is of great im- 

 portance in the present investigation. 



The accumulation of masses of decaying organic matter on the 

 bottom is one of the most striking results of pollution by domestic 

 and certain types of industrial wastes. The amount of organic matter 

 present can be determined by two methods: directly by physical and 

 chemical analysis, or indirectly by a determination of the kinds and 

 abundance of organisms living in the deposits. The first of these 

 methods is somewhat outside the sphere of the biologists, and also 

 requires time and special equipment. The second method lies within the 

 field of the biologist, is less time-consuming, and calls for the same 

 equipment used in the study of non-polluted bottoms. This method has 

 been used in the present study and has been supplemented by general ob- 

 servations on the consistency, appearance, and odor of the bottom de- 

 posits. 



The indirect method of study is .based on the well-established 

 fact that certain organisms thrive in the presence of an abundance of 

 decaying organic debris, while certain others cannot exist in such a 

 situation. It has been found that the number of certain tolerant organisms 

 is roughly proportional to the amount of organic matter, while the number 

 of certain, intolerant organisms is inversely proportional to the amount 

 of organic matter. These highly tolerant and highly intolerant forms 

 are said to have index value, that is, they are good indicators of the 

 degree of pollution. Certain other forms, while showing a distinct 

 preference for polluted or non-polluted situations, have little or no 

 index value because of their irregular occurrence. 



A number of workers have developed systems of classification of 

 bottom organisms on the basis of their tolerance to pollution. The 

 classification which is most often quoted in this country and which has 

 been found most useful in this study is the one developed by 

 Stephen A. Forbes and his associates in their investigations of Illinois 

 River. The most recent and complete presentation of the classification 

 is found in a paper by Richardson (1928) . Richardson was careful to 

 point out the dangers involved in a rigid application of a set of rules 

 in pollution studies. 



In Western Lake Erie it was soon found that only two kinds of 

 animals could be used as index organisms with any degree of confidence 

 and one of these could be used only on mud bottom. A study of numerous 



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