REPORT OF THE COMMISSIONER OF FISHERIES. 23 



its cooperation with the Bureau of Entomology at Mound, La., where 

 experiments are conducted looking to the improvement of health 

 as regards malaria in rural communities and to the protection of farm 

 labor in the South. 



Experiments of like nature are being prosecuted in northern waters 

 where, though mosquito-borne diseases are less prevalent, neverthe- 

 less the economic losses attributable to the prevalence of mosquitoes 

 are of great significance. 



EXPERIMENTAL FISH CULTURE. 



Previous reports have dwelt upon the importance of fish-cultural 

 experiment work and the progress in this field which has been made 

 in connection with fisheries biological station at Fairport, Iowa. 

 The experiments and investigations nave continued to show favorable 

 progress, and at the close of the fiscal year the Bureau had taken steps 

 to apply some of the results hitherto gained at Fairport for the 

 improvement of pond fish-cultural operations at several of its regular 

 stations. The field of experimental work in fish culture is so broad, 

 so complex, and so little surveyed, that it is a matter of regret that 

 the means are wanting for the effective prosecution of studies of this 

 character under the very diverse conditions encountered in different 

 parts of the country and with reference to a greater number of species 

 of fish. 



SHELLFISH INVESTIGATIONS. 



Oysters. — The problems of oyster culture continue to be of such 

 critical importance that the Bureau has been impelled to devote to 

 their solution as great a measure of its resources as could legitimately 

 be applied to one object. Wliile the investigations have not been 

 localized more than was essential for the accomplishment of useful 

 results, the primary seat of activiti(>s has been in the vicinity of 

 Milford, Conn., where a temporary field laboratory is maintained for 

 studies of the oyster. 



Examination of the waters of this region revealed the fact that a 

 large number, perhaps more than half, of the leased grounds have 

 ceased to be worked because of the small yield of oysters thereon, 

 which is due, in turn, to the general failure of set. The strike of set 

 during the past two years has, in fact, been practically negligible. 

 Consequently, the investigation has been directed primarily toward 

 ascertaining the cause of the failure of the set. 



Following suggestions arising from earlier investigations, it was 

 endeavored in 1918 to work out suitable methods for following up 

 the movements of the oyster larva3 with the view of ascertaining 

 just where to place cultch immediately before the time for spatting 

 or setting to occur. The centrifugal machine used during the season 

 of 1917 for separating larvre from the water was largely superseded 

 this year by a series of screens or sieves of copper- wire cloth of suc- 

 cessive stages of fineness from 40 to 200 meshes to the inch. Passing 

 samples oi water through the sieves seems to yield as satisfactory 

 results as tlie centrifuge in separating the oyster larvje from other 

 objects of different j^izes, besides possessing the advantage of roughly 

 classifying the larva? according to size and age. 



