BUREAU OF FISHERIES 87 



Xufr/f/'i'c value of p's/u ry productx. — It has boon found that a diet 

 of oystors and milk not only permits normal blood formaticm but 

 also o:ood <2:rowth. reproduction, and lactation in laboratory animals. 

 Experiments in which white rats have received diets for a period of 

 12 montiis which are many times richer in copper than any oysters 

 found on the mai'kot reveal that when the element is fed in conjunc- 

 tion with 03'sters a smaller quantity of the metal is stored in the 

 liver than when fed with the stock diet alone. The toxicity of the 

 copper contained in market oysters should, therefore, give very little 

 concern. 



Other nutrition studies have revealed the relatively high vitamin 

 content of various fish oils, such as swordfish-liver oil, oils from 

 salmon cannery trimmings, salmon eggs, salmon livers, and other 

 miscellaneous fish oils. 



Development and improvements of fishing gear. — The mesh size 

 of nets determines the kinds and numbers of undersized and imma- 

 ture fish which ■will be permitted to escape from the commercial fish- 

 ermen in the interests of conservation. Technologists of this Bureau 

 and of the Bureau of Standards have made a study of devices to 

 enable the conservation authorities of the States to establish and 

 apply uniform enforcement of regulations pertaining to the mesh 

 sizes of nets. 



For many years methods have been studied for chemically treating 

 nets in order to prolong their useful life. In addition to recom- 

 mendations for treating these nets with toxic dyes as suggested in 

 previous annual reports, it has been found, during the past year, 

 that chrome tanning of the cotton netting gives excellent results and 

 that, where bacterial action on nets is not serious, an improved 

 method of cutching twine produces good service. In all cases, better 

 results are obtained by covering the treated nets, in addition to one 

 of the above treatments, with a good grade of tar. properly applied. 

 Mercury compounds are valuable in checking weed and other marine 

 grow^ths on nets exposed in waters for varying lengths of time, 



BIOLOGICAL FISHERY INVESTIGATIONS 



Reduced appropriations made it necessary to curtail drastically 

 scientific investigations on the main problems of the national fisher- 

 ies. In spite of a smaller staff, diminished laboratory facilities, and 

 lowered operating funds, a reorganization made it possible to carry 

 on the most essential lines of research. Funds furnished by the 

 Public AVorks Administration enabled the undertaking of important 

 lines of investigation which had previously received little attention. 



Investigations of the comm(>rcia] fisheries ai'o concerned with the 

 changes in abundance of the food fishes of the North and Middle 

 Atlantic areas and with the correction of abuses in the commercial 

 fisheries of the Great Lakes. The shrimp fishery of the South At- 

 lantic and Gulf has also been studied with the aim of discovering 

 and preventing depletion of the su])i)ly; and the salmon and herring 

 fisheries of Alaska are undergoing scientific analysis as a basis for 

 their regulation. Aquicultural investigations include studies on the 

 improvement of hatchery technique for both cold- and warm-water 

 fishes and the planning of rational stocking policies in interior 



