38 NORTHERN FISHES 



current, the absence of great depths, and the absence of comphcations 

 caused by the stratification present in the deeper lakes. Lakes depend 

 primarily on vegetation for their oxygen supply, but streams secure 

 much of their oxygen from the atmosphere, through the action of their 

 currents. Streams also differ from lakes in that they are not more or less 

 closed bodies of water but have a continual supply of nutritive salts and 

 other elements washing in from land sources. 



Many streams in the northern states have deteriorated in recent years 

 through manufacturing, agricultural, and lumbering operations and 

 they are no longer supporting the fish populations they once carried. 

 In numy cases it is possible to rehabilitate fishable streams. Because of 

 the numerous lakes available, streams have been neglected in the north- 

 ern states, and only in the last few years have attempts been made 

 to restore good fishing in the streams in order to relieve the fishing 

 pressure on the lakes. 



Scientific surveys of streams are essential before any major improve- 

 ment can be attempted. Data on the source, volume, and chemical 

 conditions of the water and on the temperature, shade, food organisms, 

 depth, bottom types, pools and rapids, and the fishes present should be 

 secured first in order to determine the conditions requiring correction. 



Streams may be divided into a number of types. Small streams are 

 usually swift; large streams are usually slower and have more deep, 

 pondlike areas. Small streams may have cold water or warm water, 

 depending on the source and the local conditions of shade, depth, and 

 current. Cold streams are usually suitable for various species of trout. 

 Warm streams are not suitable for trout, but if large enough are better 

 for largemouth and smallmouth bass, sunfishes. channel catfish, bull- 

 heads, and various species of suckers. In the largest streams many 

 additional species may be found, particularly those that prefer quiet 

 waters and muddy bottoms, such as the big catfishes, the buffalofishes, 

 gars, carp, sheepshead, and the paddlefish. 



Pollution, loss of cover, erosion, and extreme alternations of high and 

 low water levels caused by extensive drainage and by the cutting of 

 moisture-retaining forests have all contributed to rendering streams 

 unsuitable for the support of large fish populations. In many streams 

 unrestricted seining of minnows destroys the food, the spawning and 

 feeding grounds, and even the game fishes themselves. All these con- 

 ditions must be remedied in order to restore many streams to their 

 former productivity. 



Trout streams have been surveyed and improved more than any other 

 type of streams. Because of their smaller size most of them are easier 

 to improve than are the larger rivers. Detailed instructions for survey- 

 ing and improving have been described by many investigators: Hubbs, 

 Greeley, and Tarzwell in 1932, Davis in 1938, and Needham in 1938. 



