90 



to the consideration as to what circumstances are favourable and what are antag- 

 onistic to their growth. The conditions under which the zoospores germinate 

 most readily are the presence of animal and sometimes vegetable matter in a 

 moist state of decay, or even of living animals, especially if the skin has been in- 

 jured in any way. Warm weather, deficient oxygenation of the water, as from 

 absence of rainfall, form additional causes of its rapid growth ; and thus a mild 

 winter and spring encourages it. Pollution of rivers by sewage, refuse from mill 

 factories, infusions of agricultural manures, sheep washing, and other causes to 

 which the disease has been often attributed, is clearly exonerated from the charge 

 of producing it for the very obvious reasons of the disease having been most viru- 

 lent in rivers particularly free from pollution, such as the river Eden, the river 

 Kent, the river Leven (which flows out of Lake Windermere), and some rivers in 

 California, although of course it is not denied that the greater the amount of 

 decayed animal and vegetable matter present in the water, the more the pro- 

 duction of the dijiease must be encouraged, seeing that it might convert a disease 

 which was simply sporadic into an epidemic. In the epidemic this year in the 

 Wye, the gravel beds were observed by the fishermen to be covered in some 

 places — for instance, at the mouth of the Lugg river — with a layer, several inches 

 thick, of a very filthy deposit. 



Any injury that the salmon may meet with certainly renders them very liable 

 to be attacked by Saprolegnia. Mr. Wilmot, engaged in the cultivation of fish 

 in Ontario in 1867 to 1868, found that the fish transferred from one stream to an- 

 other suffered so much from disease when seized by the head and back, that he 

 introduced the use of indiai-ubber gloves, and directed that the fish should always 

 be seized by the tail. Although the mortality was diminished by this means, 

 nevertheless, in a year or two many fish died with the fungoid growth appearing 

 first round the tail. 



The injuries salmon inflict upon each other, and their low vitality when re- 

 turning from their spawning beds, render them almost certain to be attacked 

 when the fungus is prevalent. 



The causes which tend to prevent the growth of the fungus are numerous. 

 In the first place, cold is as antagonistic to the Saprolegnia as it is to all fungoid 

 growth. Thus Mr. A. D. Berrington, in his last report on the Usk fishery, 

 observes that "the only year in which there was no evidence of disease at all in 

 the Usk river (although carefully looked for) was in the year 1881, following the 

 very severe winter of 1880 " ; and many other similar instances might be given. 



Sea water, or common salt in water when of a sufficiently strong solution, ih 

 fatal to the growth of the fungus. Mr. Silk (pisciculturist to the Marquis cf 

 Exeter,) has in two days cured black bass fish infected with the disease by plac- 

 ing them in salt water. Again, Mr. List (Chief Constable of Berwick,) has, at 

 the instigation of Professor Huxley, kept several diseased salmon in cages at the 

 estuary of the Tweed with the result of so complete a resovery, that upon a 

 microscopic examination no trace of tho disease could be discovered by Professor 

 Huxley. 



The most antagonistic cause however to the increase of the disease is, beyond 



