874 BULLETIN OF THE BUREAU OF FISHERIES. 



value as an oxygenator, or as a breeding place for minute animal food for fry, 

 if such it be. A method of eliminating this growth is therefore a great desid- 

 eratum in fish culture. 



Moore and Kellerman," in work conducted with the object of cleansing 

 municipal water supplies of obnoxious algje, developed a method of treating the 

 water with copper sulphate, finding that this salt dissolved in the water is highly 

 toxic to algje at a dilution so weak that it may with safety be taken into the 

 human system. The small cost of such treatment moreover, by reason of the 

 cheapness of commercial copper sulphate and the simph means by which it may 

 be used, makes this remedy readily available for practical purposes, and it has 

 for several years now been successfully applied not only for the removal of 

 algae from reservoirs and ponds, but also in the process of filtration as well. Its 

 possibilities as a useful agent in fish culture have therefore invited investiga- 

 tion with, so far, the results set forth in this paper, concerning algge as a mechan- 

 ical annoyance to the fish culturist. Wider application is suggested in certain 

 experiments dealing with bacterial diseases of fish, in which the treatment aims 

 at physiological effect upon the fish themselves, but as yet no definite results 

 in this phase can be reported. 



ESSENTIAL PRINCIPLES OF THE TREATMENT. 



The efficiency of copper sulphate in the treatment of city water supplies 

 and fish-cultural ponds or streams depends, of course, fundamentally upon the 

 fact that it is by its nature a poison to algse. Its use for practical purposes 

 depends further upon the fact that it is poisonous in extremely dilute solutions, 

 which are not injurious to most of the higher forms of life and are moreover 

 available by reason of the cheapness of the substance. The first point of con- 

 sideration in fish culture, therefore, these facts being known, is the suscepti- 

 bility of the fish contained in the water that is to be treated. If, under given 

 water conditions, the fish are more susceptible than the algae, the remedy is 

 not applicable. Use can be made of it only where as the pioportion of copper 

 sulphate increases the death point to algae is reached before the death point to 

 the fish. The larger the margin the better, but the method may be used where 

 the margin is very small. The second and remaining consideration is an ade- 

 quate method of applying the remedy. 



SUSCEPTIBILITY OF FISHES. 



The chemical reactions by which copper sulphate kills fish are not known. 

 The poison acts through the medium of the water in which it is in solution and 

 in which the fish breathes. The water has other dissolved substances in solu- 



o A method of destroying or preventing the growth of alga; and certain pathogenic bacteria in 

 water supplies. Department of Agriculture, Bureau of Plant Industry, Bulletin No. 64, 1904. 



