CAUSES OF DISEASE IN YOUNG SALMONOIDS. 915 



These fry were fed regularly every day, from the fourth day after hatching, 

 with pulp of beef spleen, first crushed in a mortar, then passed through a horse- 

 hair sieve. Somewhat later I contented myself with rubbing the spleen pulp, 

 which was somewhat diluted, in the trough and between my hands, the fry 

 eating this very well. On April 20, i. e., about thirty-five days after hatching, 

 the trout, now in a pond, were eating pulp which I did not crush, but placed 

 in a small basket suspended between two currents in the pond. 



The pond was supplied with river water, of a temperature of i2X°- It 

 was 7 meters in length, 2.5 meters in width, and 1.8 meters in depth. The 

 flow of water was 10 liters per minute from the town supply. There were 

 9,000 fish. Time passed and these trout, having in only one and one-half 

 months reached a size varying between 0.032 and 0.035 meter long, were well 

 and healthy. 



We had reached April 24 when two things happened to cause me appre- 

 hension. The allowance of food given to these trout had been gradually dimin- 

 ished, and on the other hand the water flowing into the basin, at the rate of 10 

 liters per minute, passed through a gravel filter, which did not inspire any 

 confidence in me. 



Before reaching this filter the water passed through three decantation 

 basins, each containing about 10 cubic meters, then it rose between two walls, 

 where it met a filter composed of medium pebbles, a layer of 0.25 meter, then 

 a similar layer of gravel, then another layer 0.3 meter thick of finer gravel, 

 then coarse sand 0.15 meter thick, and finally a layer of 0.15 meter of fine sand. 

 These layers of gravel and sand were separated from each other by suitable 

 sheets of metal. 



Being able to see from the side into the interior of this filter, through glass 

 panes 0.027 meter thick and i meter high, I ascertained that the filter was in 

 reality dirty, the dirt obstructing the passage of the water through the gravel, 

 so that the latter was kept back and rose in the basins of decantation. (It is 

 well known that a filter made of gravel and sand does not operate well until 

 it collects a surface layer of dirt, but on the other hand, as is not so well known, 

 it is not necessary to wait until the surface is dirty to have the under layers 

 cleaned.) I saw then that at hundreds of places organic matter formed with the 

 ■ gravel a more or less compact mass, and mold was to be found everywhere. This 

 organic matter, this mold, consumed the oxygen contained by the water to the 

 detriment of the welfare of the rainbow trout. 



I foresaw danger in this growth of Saprolegnia of all kinds, and the multi- 

 tudes of small animal life, some of it almost invisible to the naked eye. 



The water was cut off and the upper part of the filter was partly cleaned; 

 not thoroughly, however, as both time and space are needed for the washing 



