BUREAU OF FISHERIES 103 



diseases ; but during the past year great emphasis was placed on field 

 studies since it is impossible to use the production of our hatcheries 

 to the best advantage until we learn more about the various factors 

 that affect fish in natural waters. Our lack of information of these 

 fundamental factors is the greatest obstacle at present in developing 

 a scientific policy of fish management. 



In pond-fish culture for bass and other warm-water fishes con- 

 ducted on the basis of natural food production in ponds, previous 

 experiments have determined the most satisfactory fertilizers to 

 maintain food production. During the past year the quantities of 

 fertilizers and the periods that reveal the limitations of productive 

 capacity of the ponds and the maximum fish production have been 

 determined. 



More elaborate studies have been conducted on trout culture. Feed- 

 ing experiments conducted at two of the Bureau's experimental 

 hatcheries have involved the testing of various artificially prepared 

 commercial diets and the substitution of cheaper foods for the stand- 

 ard diet of beef liver. Detailed researches conducted in cooperation 

 with the State of New York and Cornell University have been 

 developed along three lines: (1) continuation of the study of growth 

 of four species of trout fed similar diets and maintained under simi- 

 lar conditions to determine the efficiency of food utilization; (2) the 

 determination of the amount of food required per unit of weight to 

 obtain the most economical production; and (3) chemical balance 

 studies to determine the amount of phosphorus and calcium needed 

 by growing trout. 



Selective breeding studies have been conducted ; and in addition to 

 increased rate of growth and higher disease resistance previously 

 developed, a strain of trout has been selected with high egg produc- 

 tion and higher fertility of eggs. 



As in previous years, the study of fish diseases was largely confined 

 to investigations of epidemics in hatcheries and the development of 

 better methods of control. The establishment of a pathological 

 laboratory at Seattle, Wash., has made it possible to devote more 

 attention to the diseases prevalent in the Intermountain region and 

 the Pacific Coast States. Additional information regarding the 

 causes and control of two relatively new diseases has been obtained ; 

 and a free clinic or disease service has been established and greatly 

 expanded during the year for the examination of specimens of dis- 

 eased fish sent in by hatchery superintendents for diagnosis, and for 

 the furnishing of advice for treatment and prophylaxis. 



SHELLFISH INVESTIGATIONS 



The scope of shellfish investigations previously conducted by the 

 Bureau was greatly enlarged last year by the special appropriation 

 of $100,000 for the study of oyster pests and the development of prac- 

 tical methods for their control in the Atlantic and Gulf States. In 

 addition limited investigations of oyster and hard clam culture were 

 continued in the waters of New England, the South Atlantic States 

 and in Puget Sound. A special allotment of $20,000 was provided 

 by the Public Works Administration for studies of the failure of 

 oyster culture in York Eiver, Va., allegedly caused by pulp mill 

 wastes. All of this work has been carried out by the regular staff 



