74 The Fungus of Salmon Disease. 



salmon would be killed in half the time, and probably in far 

 greater numbers. 



But the stimulus of heat is not sufficient to account for an 

 outbreak of the disease, and it must be coupled with the fact 

 that in summer the water is low as well as warm — so low, in- 

 deed, that in some cases the fish do not find an open passage 

 when trying to enter from the sea, but have to struggle over 

 shoal water, and are often even unable to enter for want of 

 water until a flood gives them a chance ; but if there should be 

 water enough to enable them to struggle over a long bank, it 

 is only fair to assume that in a river, where they are to some 

 extent strangers, they are very liable to injure themselves on 

 stones in their efforts to ascend, or on weirs or such natural 

 obstructior s as are to be found in some rivers when the water 

 is very low, and no open passages exist. 



By injuring themselves, I do not mean necessarily that fish 

 must become actually wounded, but that they may thus injure the 

 delicate epidermis or slimy coating that covers their skin, and 

 which in a perfect fish not only answers as a complete and im- 

 pervious coating of protection, but by its continually exfoliating 

 on the surface, could also displace any accidental fungus spores 

 that were planted on it, before they could germinate. The 

 rate at which the exfoliation process goes on over the skin of a 

 salmon in good health is probably pretty regular. We have no 

 definite reason to believe, in the event of a scratch or injury 

 to the epidermis, that in that particular spot the rate of growth 

 would at once be increased ; and the question is one of obvious 

 importance, if we consider that, while the rate of growth with 

 the fungus is immensely increased during summer, the protect- 

 ing epidermis of the salmon, at a time when it is more liable to 

 injury* during the time the water is low and warm, grows at 

 the same rate. All growths of salmon fungus can easily start 

 on a broken epidermis. It seems to be the most favourable 

 nidus for them to take, and one on which they to all appear- 

 ance develop most readily. 



I may here mention that in cultivation practice it is usual to 



