THE SPEY 161 



tests ; the 4 per cent, tanks may be regarded as probably 

 similar to the condition found in a small stream below a dis- 

 tillery discharge after treatment by the bacterial method. A 

 full account of the experiment was published in the Sixth 

 Report (1909) of the Royal Commission on Sewage Disposal, 

 in which details respecting the rate of development of the ova, 

 temperatures of the various tanks, feeding and growth of the 

 parr and smolts, etc., are given, along with a description of the 

 hatchery arrangements and full analysis of the effluent taken 

 weekly throughout the time of the experiment. The accom- 

 panying photograph shows, in the foreground, the small 

 hatchery, and just behind the group of figures the bacterial 

 filter, with above that the mixing and precipitation tanks. 



It is sufficient to state here that the hatching of the ova 

 followed a perfectly natural course, the rate of hatching being 

 in proportion to the variation of temperature, and that no 

 unusual mortality occurred either amongst eggs or fry. The 

 open nature of the percolating filter resulted in a very low 

 temperature in the effluent during the winter months, and, 

 according as the mixture of water was small or great the 

 water temperature being relatively higher so the rate of 

 development was slow or rapid. But there was practically 

 nothing to choose between any of the tanks, the ova even in the 

 undiluted effluent hatching out with as good percentage as the 

 ova in the control tank of water only. 



The young fish introduced were of two ages, eight months 

 and twenty months. These fed and throve well. Not a 

 single death occurred amongst the older fish, which were 

 reared to the smolt stage and acquired the silvery scale. A 

 few of the younger fry became pugnacious and died through 

 fighting. A curious result was that the fry in the tank contain- 

 ing the undiluted effluent grew more rapidly than the others. 

 This, however, was probably in no way due to nourishing 

 properties of the effluent, but to the fact that as spring advanced 

 great numbers of fly larvae were hatched out in the filter and 

 from it were carried to the hatchery, the greatest numbers 

 being carried to the tank which received the effluent alone. 



From the successful results of this experiment, I think we 

 are justified in deciding that a solution has been found for the 

 worst kind of river pollution problem in Scotland. 



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