MIXING TROUT IN WESTERN WATERS. 



By Aldo Leopold, 

 U. S. Forest Service, Albuquerque, New Mexico. 



If a stream, is stocked with 10,000 native trout, 10,000 eastern 

 brook trout, and 10,000 rainbow trout, and granting that the 

 conditions are suited to each of them, will that stream produce 

 more or less pounds of trout per year than if stocked with 30,000 

 of any one of the three ? In the west, at least, this is a live question 

 which does not seem to have received serious study. It is the 

 object of this paper to summarize such data as the United States 

 Forest Service has been able to collect with the object of arriving 

 at an answer. At least a tentative answer is needed as a guide to 

 practice. 



The question obviously involves variable local factors whose 

 reactions are not susceptible to generalization. These variable 

 factors may even preclude a general "yes" or "no" in answer to 

 the question. Then, too little is known about the actual relations 

 of the species to their environment and to each other to allow of 

 reaching an answer by inductive reasoning. However, the question 

 seems to involve the law of hybridization, from which important 

 conclusions bearing on rules of practice can easily be deduced, and 

 furthermore, the question can, to some degree, be illuminated 

 empirically from actual observations. 



The United States Bureau of Fisheries is authority for the 

 statement that the trout hybrids so far studied have been fertile, 

 but decreasingly so with successive generations. The Bureau 

 believes, however, that hybridization is rare, but states that 

 nobody knows exactly how rare. The law of hybrids would 

 indicate that any trout hybrids which do occur between definite 

 species are infertile, or at least less fertile than the pure stock. In 

 either case the existence of hybrids would reduce the productive 

 capacity of the water in which they occur. They must necessarily 

 consume food which might be used by more fertile fish. 



The actual observations of the writer, though meagre and 

 confined to the southwest, are as follows: 



1. Rainbow, eastern brook, cutthroat, and German brown 

 trout have been indiscriminately mixed with the native black- 

 spotted trout of our southwestern streams. 



2. Where so mixed, it is commonly believed by fishermen 

 that, (a), the rainbow and black-spotted trout have crossed 



IOI 



