MANUAL OF FISH-CULTURE. 65 



The conditions at Craig Pond, where some of the worst cases recov- 

 ered, were these: An area of 231 acres 5 a maximum depth of 69 feet; 

 exceedingly pure and transparent water, like that of the inclosure. 



At two of the other inclosures tried that summer, where there was 

 no attack of fungus, the water was brown and dark, like that of ordi- 

 nary brooks and ponds, and in the remaining one it was intermediate 

 in character. 



These facts point strongly to the character of the water as the cause 

 of the fatality of the disease, and especially to its pellucid character, 

 which exposed the salmon to an extraordinary glare of light, whereby 

 the growth of the pest was greatly encouraged. The recovery in the 

 transparent water of Craig Pond was rendered i)ossible by the great 

 depth of the water, through which but a small fraction of the light of 

 day could penetrate. No doubt the salmon liberated there at once took 

 refuge in the deeper parts. The suggestion naturally arises that arti- 

 ficial shade might be useful in the treatment of such diseases, whether 

 the attacking fungus be identical with that observed in the above in- 

 stances or a related one. 



It is certain, from the promirtness with which dead animal matter 

 becomes the prey of saprophytic growths, that the spores of these 

 water-molds are well disseminated throughout fresh waters, everywhere 

 ready to seize uj)on an opportunity for germination and growth, and 

 that as a general rule these spores are quite unable to seize upon any 

 animal substance which is not already dead or in a diseased condition. 



A growth of Saprolegnia ferax once established on the body of a 

 salmon is able to extend itself upon and into the living tissues around 

 it, which it seizes upon and destroys. Growing upon a dead egg, it not 

 only ensnares the neighboring living eggs, but sometimes pierces their 

 shells and establishes itself on the internal parts. In one instance the 

 fungus had gone so far as to attach itself to a living embryo, which, on 

 rebioval from the shell, was found to support on the sac quite a tuft of 

 growing fungus, though neither on the sac nor any other part of the 

 fish was a trace of dead substance discernible. 



It has been ascertained that the Saprolegnia which attacked the 

 living salmon can be communicated by contact to dead flies, and that 

 Saprolegnia found growing in the ordinary way on dead flies in water 

 can be communicated in its turn to living and healthy dace and may 

 so flourish on them as to cause their death. 



The impression has prevailed that the Sa/prolegnia which infests the 

 eggs in hatching-troughs originates in or is encouraged by bare wood 

 exx)osed to water, and that special effort is necessary to prevent its 

 forming; but exjierieuce at this station does not show that attacks of 

 fungus on either eggs or fish could be traced to bare wood, and, on the 

 other hand, eggs and fish in troughs carefully varnished with asphaltum 

 are no freer from fungoid or other disease than those in neighboring 

 troughs from which long use had worn almost the last vestige of varnish. 



F. C. R. 1897 5 



