FRESH WATER FISHES AND THEIR ECOLOGY* 

 BY STEPHEN A. FORBES 



When we watch a summer thunder storm, which covers the earth 

 with a sudden flood and makes rivulets by the road-side, each carrying 

 down to the smaller streams its load of leaves and other organic debris, 

 together with the lighter parts of the soil, and when we see these silt- 

 laden streams unite in rivers turbid with the rich spoil of the land, we 

 are inclined to lament the enormous and oft-repeated waste, seeing no 

 way in which it can be recovered in any considerable measure to the use 

 of man ; but if we follow it to the lake bottom and the river bed we shall 

 see much of it arrested there, to become an aquatic soil, partly muddy 

 water and partly wet mud, more fertile even than the richest fields, and 

 sustaining a new population of plants and animals, of many grades and 

 classes, one climbing upward, as we may say, upon the shoulders of 

 another, to reach a level which makes them accessible again to our use. 



Since the waters which wash the surface of the earth fall virtually 

 lifeless and sterile from the sky, whatever population they eventually 

 contain must evidently be supplied from the contributions made to them 

 by the earth, including, of course, the organic and inorganic substances 

 dissolved out of the earth by surface wash and underground nitration. 

 The aquatic population of a lake or stream is thus sustained by the wastes 

 of the land materials which would otherwise be carried down prac- 

 tically unaltered to the sea ; and our rivers and lakes may be looked upon 

 as a huge apparatus for the arrest, appropriation, digestion, and assimila- 

 tion of certain raw materials about to pass from our control, valueless 

 and sometimes deleterious as they leave us, but capable of being worked 

 over, renovated, and returned to us in new and valuable forms, mainly 

 as fishes available for food. 



The raw materials thus contributed by the land vary according to 

 their origin. In uncivilized nature they were mainly the washings and 

 sweepings of the primitive prairie and forest, rich in carbon but with a 

 minimum amount of nitrogen. With the occupation of the country, the 

 cultivation of its lands, and the building of towns and cities, the animal 

 wastes are increased, with their larger increment of nitrogen, and larger 

 quantities of the soil itself are swept into the streams all alike available 



* Read at the University of Chicago, August 20, 1913. 



