PROGRESS IN BIOLOGICAL INQUIRIES, 1937 63 



than in the riffles in both streams, but in pools of the South Branch 

 there was about twice as much food as in pools of the Shenandoah 

 Rivers. The \vei<2;ht of oroanisms per square foot in the riffles was 

 considerably greater in the Shenandoah than in the South Branch. 

 IVIoreover, the former river has more extensive riffles in which bass 

 can feed than has the latter. 



Pcn'GJiifes; in fingerJhig has!^. — It was found that fino-erlino; bass 

 from both the South Branch and Shenandoah Rivers were infested 

 with parasitic trematodes that were encysted in the liver. These 

 parasites were much more numerous in fish from the South Branch 

 than in those from the Shenandoah. Approximately 75 percent of 

 fingerlino; bass from the former stream were parasitized as compared 

 with about 12 percent from the latter. Bass from tlie South Branch 

 also contained many more cysts, on the average, than those from the 

 Shenandoah River. Considering the abundance of cysts in many 

 instances, it is remarkable that there is no evidence that the para- 

 sites affected the growth since the average size of infested fish was 

 very nearly the same as that of the uninfested. 



Forage minnows in boss ponds. — Three ponds at the Leetown Sta- 

 tion were stocked very heavily with blackhead minnows before the 

 bass fry were introduced. Each pond when drained in the fall con- 

 tained only a relatively small number of bass, although they were 

 all of large size, averaging 6.5 inches in length. The obvious expla- 

 nation of the disappearance of the bass fry is that they were eaten 

 by the minnows although direct observations are lacking. In view 

 of this possibility, it seems advisable to rear bass fry to a larger size 

 before transferring them to ponds containing forage fish, or to defer 

 the stocking of brood minnows until the fry have attained a length 

 of three-fourths of an inch or more. 



FISH DISEASES 



Studies on fish diseases were continued by Dr. Frederic F. Fish 

 at the pathological laboratory in Seattle, Wash. An investigation of 

 furunculosis completed at Lake Madison. j\Iont., showed that a small 

 percentage of loch leven trout were infected with the disease. Spawn- 

 takers, however, reported the incidence of infected fish to be much 

 lower than in 1934 and 1935. They also reported that the number 

 of fish running into the traps showed a marked progressive clecline 

 during those years. It appears probable that furunculosis contributed 

 materially to this decrease in the loch leven trout population in Lake 

 Madison.' If this supposition is correct cyclic increases and decreases 

 in the numbers of these fish are to be expected in the future. 



A program of disease control studies was instituted at the field lab- 

 oratory recently constructed with the cooperation and financial assist- 

 ance of the Division of Fish Culture at the Bureau's hatchery located 

 at Quilcene, Wash. Both preventive and therapeutic measures for 

 combatting the common hatch.ery diseases were investigated under con- 

 trolled conditions. These experiments showed conclusively that fin- 

 gerling trout can be subjected to routine preventive treatments at 

 weekly intervals without incurring any increase in mortality. 



In two series of experimental infections the common protozoan 

 parasite Cyclochaeta was allowed to increase to epidemic proportions 



76622—38 5 



