86 REPORT OF THE COMMISSIONER OF FISHERIES. 



At the Yuma (Ariz.) project the fish are excluded from the canals, 

 because of an arrangement whereby the water enters the canals from 

 a settling pool through a siphon that is fish tight. Seven miles below 

 Yuma the maintenance of a dam on the Colorado River for diversion 

 of water in Imperial Valley, Cal., causes the river bed below the dam 

 to be left dry at times so that quantities of fish are stranded. The 

 Salt River project involves a large system, but as the canals and 

 ditches always contain water there is little reason to suppose that 

 nriuch damage to fish occurs. The reservoir formed by the construc- 

 tion of the Roosevelt Dam in Arizona has been well stocked with 

 bass. Fish are reported to be lost in the spring freshets, when the 

 water rises to a height of 10 feet or more above the top of the spill- 

 ways, carrying fish over the dam and through a fall of 225 feet. The 

 prevention of such losses by the use of screens to hold the fish back 

 has been given consideration, but the difficulties are very great and 

 possibly msurmountable. 



In California, and especially in the Sacramento River basin, where 

 large areas of land are farmed by irrigation, large losses of fish would 

 occur but for the effective work of the State authorities in requiring 

 all ditches and intakes to be provided with screens and aU dams 

 with fishways. A recent act of the legislature requires the owners 

 of dams that are too high for a useful fish ladder to build and main- 

 tain hatcheries. In that State the ' ' squirrel-cage ' ' type of revolving 

 screen is generally recommended for its simple design and cheapness 

 of construction, but for canals wider than 25 or 30 feet the parallel- 

 bar type of screen is considered the only practical means of Keeping 

 fish out. In Nevada a new law effective September, 1917, requires 

 the screening of intakes and ditches as weU as the use of fishways. 

 Irrigation is extensively practiced in the northern half of the State, 

 and heretofore countless numbers of trout fry and other fishes have 

 been poured into the fields. 



SERVICE OF THE BIOLOGICAL LABORATORIES. 



The various investigations in progress at the several biological 

 laboratories at the close of the preceding fiscal year were continued 

 during the early part of the present year. 'Before the close of the 

 year, however, it was found desirable to adopt temporarily a now 

 policy with regard to the laboratories because of the necessity of 

 concentrating all efforts, as far as possible, upon the immediate 

 increase of the aquatic food supply. 



The laboratory at Woods Hole was not opened for general investi- 

 gations but a special staff was stationed at that laboratory for work 

 relating to the improvement of methods of preserving fish. One 

 investigator was employed for observation of the occurrence of nema- 

 tode parasites in the flesh of marine fishes, a question which has 

 been found to have a direct bearing upon the marketing of fish. 



At the Beaufort laboratory the scientific staff consisted only of the 

 director and one investigator who was enabled to continue the 

 important and timely investigation of the protection of wood against 

 marine borers. The director devoted himself to experiments in the 

 curing of fish by methods of salting and of salting and smoking. It 

 had been generally beheved that the curing of local fishes during the 

 summer was not practicable, but, largely as a result of the Bureau's 



