water area of Western Lake Erie. The total area of Western Lake Erie 

 formerly available for spawning is not known exactly, so it is not 

 possible to determine what proportion has been made unavailable to the 

 fish. In view of the large area of firm bottom in the island Section, 

 it seems unlikely that the removal of U5 square miles at the west end 

 would cause a serious scarcity of grounds for spawning. 



It may be argued that pollution in Detroit River prevents the 

 whitefish from migrating to Lake St. Clair, so that the area of spawning 

 ground made unavailable is much greater than U5 square miles. This 

 seems improbable, for the chemical evidence indicates that pollution in 

 the river in mid-stream is not a barrier to migration. 



The evidence, then, points toward the conclusion that pollution 

 of the spawning grounds or their unavailability for others reasons, has 

 not been the controlling factor in the depletion of the whitefish. The 

 evidence applies with equal force to the cisco. Van Oosten (1930) showed 

 that this species was depleted by intensive fishing when it was concen- 

 trated in a small area. The whitefish and cisco supply has been reduced 

 to a greater extent than that of other commercial species in Lake Erie. 

 Those species which commonly enter streams such as Maumee River and 

 Raisin River to spawn may have had their spawning grounds reduced to a 

 point which limits the production of young. However, many of these 

 fishes, for example, the yellow pike-perch, will spawn in lakes if 

 prevented from entering streams (Adams and Hankinson, 1928, page U[t2) , 

 and it is quite possible that they have suffered little from the inter- 

 ference with their normal spawning migration. 



On the whole, pollution of the bottom in Western Lake Erie and 

 in some of its tributaries must be considered as undesirable, and very 

 likely has been positively harmful, but it seems highly improbable that 

 it has been the primary factor in depletion of the fishery. 



The plankton 



In the preceding sections only the undesirable or harmful 

 effects of pollution have been considered. There is little doubt that 

 pollution has had a t least one helpful effect, namely, that of in- 

 creasing the abundance of plankton organisms, which serve as food for 

 all young fishes and the adults of certain species. 



It was found that there were marked differences in the 

 abundance of phytoplankton in different sections of the area studied. 

 The sections, listed in descending order with respect to abundance, were: 

 (1) Maumee Bay, (2) River Raisin, (3) Portage River, (U) Island, (5) 

 Detroit River. This is precisely the same order in which the sections 

 arrange themselves with respect to intensity of pollution as indicated 



309 



