518 A NATIONAL PLAN FOR AMERICAN FORESTRY 



to get the greatest possible benefit from the utilization of our waters 

 we must have recourse to the principle of stream improvement or 

 control. Although it is evident that the idea of environmental control 

 is sound in theory, there has been very little practical application of 

 the principle in this country. In Great Britain the importance of 

 stream conditioning has long been recognized and notable success 

 has attended its practical application to trout streams. 



The purpose of stream improvement is to make the stream a 

 better place for fish to live. Less attention has been paid to lakes, 

 but it is evident that the principle is also applicable to them although 

 its practical application will be more difficult. The basic idea of stream 

 improvement is to ascertain what factors are limiting the abundance of 

 catchable fish and then proceed to overcome or remedy the natural 

 deficiencies. There are probably very few lakes ^or streams which can- 

 not be improved to some extent, and in many instances it is possible 

 materially to increase the production of fish at comparatively small 

 cost. 



While different species of fish frequently differ widely in their 

 requirements in respect to certain features of their environment, 

 there are other fundamental requirements in which most species are 

 in essential agreement. These factors are a stable environment, pure 

 water, adequate shelter for young and old, sufficient food for fish of 

 all ages, and adequate spawning areas. A deficiency in any one of 

 these requirements may result in its becoming a limiting factor even 

 though in all other respects the stream or lake may be able to support 

 a much larger population. If we regard a body of water as a complex 

 biological unit, it is apparent that the correct balancing of conditions 

 is of the greatest importance. The overdevelopment as well as the 

 underdevelopment of any one factor should be avoided if we wish to 

 obtain maximum production. For instance an unlimited increase in 

 spawning facilities or in stocking would throw the fish population 

 out of balance in relation to the food supply. The greatest annual 

 production will be obtained when there are just enough and not too 

 many individuals in relation to the food. 



The importance of a stable environment can scarcely be overem- 

 phasized. As previously pointed out, streams or lakes with great 

 fluctuations in the water level are much less productive than those in 

 which the level is fairly constant. Anything which will tend to check 

 rapid fluctuations such as the construction of dams or other obstruc- 

 tions which will impound the water or impede its flow will obviously 

 be beneficial to the fish. The importance of beaver dams, for instance, 

 in improving conditions for trout has frequently been emphasized. 

 Obviously this phase of stream improvement is closely linked with 

 the problem of flood control since, in general, any measures which 

 will tend to impede the run-off will be of direct benefit to the fish. 

 Measures which will reduce the volume of flow will also tend to reduce 

 erosion and the deposition of large quantities of sediment, the evil 

 effects of which have already been referred to. The beneficial effects 

 of reforestation in this connection can scarcely be overestimated. 

 Any cover which will reduce the run-off and erosion cannot fail to 

 have a beneficial effect on the streams and increase their capacity for 

 carrying fish. 



Needless to say, one of the prime requirements for the production 

 of fish is an adequate food supply, and it is probably true that in the 



