Fisli Culture in Inland Waters 263 



that of spliagnum bogs, floods after a prolonged dry period, and the 

 last snow freshets of spring are often more or less deleterious to 

 fish. In an instance of hea\y flood following a drought, it was 

 found (Richmond, '21) that brown trout were considerably 

 aifected, the young fish being most seriously so. This flood water 

 was found to be nearly four times as acid as the normal water which 

 ran during the drought, and was of an acidity intense enough to 

 coagulate the mucus on the bodies of some of the younger fish. 



This is only one of the many natural factors which may affect 

 the habitableness of certain waters by fish, but it is one which may 

 be ai:g"mented perhaps b}' deforestation, with attendant droughts 

 and freshets, and may have some significance in selecting " head- 

 waters " for planting trout, even though the water is apparently cold 

 enough. 



An important practical study of New York stream pollution has 

 been made b}^ Suter and Moore ('22). In this they have shown, by 

 a series of plates, representative plants and animals from clean water 

 to a severe degree of pollution. They have also compiled a table 

 showing the limits of toleration of fish to various concentrations of 

 pollution. For a general account of the pollution of New York 

 streams see Ward ('19). 



Forbes and Richardson ('13. '19), and Richardson ('21a) have 

 made very valuable investigations of " index animals " in their 

 studies of the pollution of the Illinois River, and very recently 

 F. C. Baker ('22) has given special study to mollusks as indicators 

 of pollution. Because of the sedentary habits of these animals they 

 are of special value as an index. These pollution studies have thus 

 given a new interest to the associations of animals found living 

 together in their minor habitats. 



Physical Obstructions. The earliest and most .decided artificial 

 modifications of area and capacity of lakes and streams were pro- 

 duced by dams. There are but few if any water systems in the 

 United States, however remote, that have not been more or less 

 affected by dams erected for one purpose or another. 



By the erection of many recent dams constantly higher stages of 

 water than formerly have been produced, accompanied by some 

 advantageous conditions, perhaps, such as that of enlarged physical 

 or biological capacity. But even in instances where constantly higher 

 levels are maintained there are usually fluctuations due to utilization 

 of the water. There are other instances where the higher level is 

 only temporary and where it may be reduced to below the original 

 or normal level, so that areas above the dam may become practically 

 dry at times. The fluctuating level caused by drawing off the water 

 tends to destroy the marginal vegetation and the fish food organisms 

 living in this shallow water. (Cf. Moore, '23; Osburn, '21.) 



The effects of dams may be not only restrictive upon the limit and 

 approximate equilibrium of life, but injurious by more or less obscure 

 interference with the habits of fishes. High stages of water may 

 aft'ord new feeding areas for some kinds of fish, but this signifies 



