PROGRESS IN BIOLOGICAL INQUIRIES, 192 7- 239 



directly comparable. There are four of these, situated close tooether 

 and of nearly the same size. Two of them were fertilized, and each 

 produced approximately the same weight in fish per unit area. Their 

 yield was about four times as great, per unit area, as that of one of 

 the unfertilized ponds and three times as great as that of the other. 



Experiments also were conducted in small concrete ponds that con- 

 tained no fish, to determine the effect of various fertilizers on plank- 

 ton production. The fertilizers tried were sheep manure, superphos- 

 phate, and soy-bean meal. The results show that soy-bean meal 

 increases production more than either sheep manure or superphos- 

 phate. The last two are about equally effective in increasing plankton, 

 production. 



A study of a number of factors that may influence plankton pro- 

 duction directly and fish production indirectly also was made. These 

 were dissolved oxygen, free carbon dioxide, half-bound and bound 

 carbon dioxide, dissolved phosphorus, organic phosphorus, hydrogen- 

 ion concentration, chlorides, four forms of nitrogen (nitrite, nitrate^ 

 ammonia, and organic nitrogen), dissolved silica, temperature, and 

 turbidity of water. In addition, a quantitative study of the total 

 plankton production of a number of ponds was continued during the 

 summer. The voluminous data acquired as a result of these inves- 

 tigations have not yet been studied sufficiently to justify drawing 

 any definite conclusions. 



PATHOLOGY OF FISHES 



Investigations on disease of the gills of trout were continued by 

 the pathologist- at Holden (Vt.) station. This disease frequently is 

 the cause of serious losses, especially among advanced fry and finger- 

 lings. As shown in the report for 1926, the disease is caused by a 

 bacterial growth on the surface of the gills. In the case of fry, and 

 probably young fingerlings, the bacteria may develop on the fins 

 and body as well, but appear to produce no noticeable effect other 

 than greater secretion of mucus. Infection of the gills is much more 

 serious and usually results in a noticeable thickening of the gill fila- 

 'ments, caused by proliferation of the epithelial cells. In large fin- 

 gerlings and yearlings this may be followed by infection with fungus. 

 Experience with the disease in 1926 led to the opinion that it would 

 not prove to be a serious menace to trout culture, but several serious 

 outbreaks in 1927 showed conclusively that this view was too optimis- 

 tic. It now appears that the gill disease may cause heavy losses 

 among both fingerling and yearling trout, and there can be little 

 doubt that it was the cause of several obscure epidemics that occurred 

 at various hatcheries in recent years. 



For a number of years heavy losses have been suffered annually 

 among the trout fry at the Holden Station at about the time when 

 they were beginning to feed. Until recentlj'^ it has been the practice at 

 this station to hatch the eggs in brook water or in a mixture of brook 

 and spring water. However, it was found that if the eggs were 

 held in spring water the fry did not suffer from the usual mortality, 

 which in some cases reached 100 per cent. During the spring of 1927 

 it was discovered that virtually all of the fry held in brook Avater 

 contracted the gill disease, whereas no bacteria could be found on the 



