J 6 The Fungus of Salmon Disease. 



until nearly the end of May (or two months later), and no 

 single instance of it was observed till then, when the water was 

 about 56 Fahr. The fact of my finding a fish with a healthy 

 young growth of fungus on it showed plainly that the fungus 

 existed in the river, and that a small scratch was enough to 

 ensure its developing, though so slowly that I feel sure I am 

 right in thinking with the gillie that the wound would have 

 healed in a few days ; and such scratches could not have been 

 unusual with the water at almost summer level, and fish running 

 regularly at the time. 



I knew from previous experience and study of the marked 

 stimulus of heat given to low plant life of many kinds besides 

 salmon disease, that it might be the cause of the disease break- 

 ing out in summer, and after my observation I was confident 

 that with heat and low water the disease would again become 

 prevalent. This belief was, unfortunately, confirmed ; and, 

 though the outbreak occurred fully a month later than usual 

 (as it was coincident with a very cold spring), it promptly fol- 

 lowed the arrival of the first two or three days of really warm 

 weather, and with such exactitude, that I was myself somewhat 

 surprised. {See Appendix, p. 82.) 



The question of possible stimulus imparted to the fungus by 

 the heat of summer weather does not appear a necessary factor* 

 as in most rivers the epidemic form of the disease occurs during 

 winter, when the water is at its coldest, and the fungus would 

 in such water (other conditions being equal) develop much more 

 slowly than in summer. 



But the condition of temperature variation is one to which 

 all organisms of the kind can adapt themselves quickly, though 

 of course only to a limited extent. In winter epidemics the 

 simplest explanation suggests that the fish themselves offer at 

 that time the best opportunity for the fungoid attack. During 

 the periods of spawning the fish are often in a very degraded 

 condition of health — reduced by their long stay in fresh water 

 (where, it is generally accepted, they do not feed) to the most 

 pitiable condition of weakness. They often seem to possess 



