7 
considerable doubt, but, theoretically, we can make some assumptions. To re- 
turn again to Van Oosten (1949), we can quote his beliefs: 
Much of the reduced abundance in modern fishery must 
be attributed to overfishing or unwise fishing (cisco, whitefish, 
lake trout, chubs). Part of it we believe was caused by an infec- 
tious disease as was true for the smelt; part of it by the para- 
sitic predator, the sea lamprey. Perhaps increased competition 
for space or food such as might have been brought about by the 
smelt in Lakes Huron and Michigan or the alewives in Lake On- 
tario may have played a role. Pollution, too, may have taken its 
toll. Often we have no better explanation to offer than to state 
that some unknown change in the environment was responsible. 
Overexploitation has been listed as one cause of decimation in the Great 
Lakes. It would be difficult to explain the reduced yield in the face of increased 
advantage in any other way, unless there were other negative factors at work at 
the same time. However, the reproductive potential of most fishes is so great 
that they should hold their populations despite a good annual catch unless their 
reproductive activities are highly specialized and easily upset. 
Overexploitation may be but one possible factor. With the progress of 
the times, the waters of the Great Lakes are continuously being manipulated, 
the bottoms changed, and the shore lines modified. These changes are bound to 
have some effect on the fishery. Changes in the physical environment are ap- 
parent and are continuing. We cannot have 10 per cent of our population in 
counties fronting the Lakes, and great industries developing on their shores, 
without changing in some way the character of the waters. 
CHANGES IN THE PHYSICAL ENVIRONMENT 
We may say that, due to the great expanse of the waters themselves, 
shore areas are of little importance. But we are wrong, for shores and shal- 
lows are of importance far beyond their proportionate area, In these shallower 
waters, temperatures rise to heights which allow the rapid growth of plankton, 
which we have indicated in the discussion of the food chain is an alimentary an- 
cestor of fishes. In these waters aquatic plants can thrive, furnishing food and 
shelter. Fry and fingerlings of many species are at a much greater advantage 
in these situations than they would be in the open, deeper areas. And all catch- 
able fishes must go through these juvenile states before they are netted, dressed, 
sold, and eaten by humans. Therefore, changes brought about by human en- 
croachment on the shore lines may be of great importance to the fishery. 
Dumpage of inert by-products of our civilization occurs in the Great 
Lakes. Dredging of materials from the bottoms goes on. Pollution of three 
kinds exists: (1) Domestic sewage is released into the Lakes, adding to the nu- 
trients and subtracting from the dissolved oxygen. (2) Industrial sewage is re- 
leased into the Lakes, causing injury or destruction to life in the waters. (3) The 
heavy utilization of the watershed for agricultural purposes has released silt in- 
to tributary streams, and this has changed the character of the bottom. 
