shore of the lake and in Detroit River has become commer8iaJ.ly extinct 

 through overfishing. Or this race may have changed its migration 

 behavior for some reason other than pollution. The whitefish has shown 

 marked changes in migration from time to time (Michigan Fisherman, 1928), 

 and it is known that a related fish ( Leucichthys artedi) became almost 

 commercially extinct as an indirect result of a change in its 

 habits of migration (Van Oosten, 1930) . Another possible factor in the 

 decline in the whitefish is that of competition for spawning grounds 

 with the introduced carp ( Cyprinus carpio) . 



The brief discussion given above is sufficient to show that 

 pollution of the spawning grounds is not the only possible factor in the 

 depletion of the whitefish. It is worthy of note that the whitefish was 

 on the decline in early times, when pollution of the bottom must have 

 been very light. Thus Milner (187U) reported depletion of the supply. 

 in St. Clair River prior to 1870, and, as mentioned before, the fishery 

 of Detroit River was abandoned before 1897. In I89O the population of 

 Detroit was only 205,876, and the total population contributing sewage 

 to the river probably was not more than one fourth of the number in 1930. 

 In the light of our knowledge of the extent and degree of pollution in 

 1930, it seems highly improbable that pollution was sufficiently intense 

 before I9OO to account for the observed decline. 



It is impossible to determine how large an area of bottom once 

 suitable for spawning has been made unsuitable by the deposition of sludge. 

 No one will object to the statement that parts of the area now polluted 

 never were used as spawning ground. The deposition of silt carried by 

 the rivers would make parts of the lake unfit for such purposes. This 

 would be true particularly in Maumee Bay, because of the protection from 

 strong winds and currents. Near River Raisin and Detroit River littoral 

 currents and waves would tend to keep the bottom scoured clean. Yet, 

 even in these sections, the bottom near the outer limit of the zone of 

 light pollution was heavily silted in places. On the whole it seems 

 probable that not more than 60 square miles of the total of 101.5 (Table 

 100) included in the three sections were formerly suitable for spawning. 



Moreover, not all of the 60 square miles can be considered unsuit- 

 able now. In the River Raisin and Detroit River Sections certain stations 

 arbitrarily enclosed by the outer limit of the zone of light pollution 

 showed no evidence of pollution. The area of firm, clean bottom within 

 this zone in the two sections probably was not less than l5 square miles. 

 According to these estimates the total area rendered unfit for spawning, 

 by pollution, was not more than U5 square miles, or 3.U per cent of the 



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