American Fisheries Society. 75 



bar out the germs. It is hoped that cement ponds will be an 

 adequate barrier. The predisposition Avill remain. It is lioped 

 that the brook trout will thrive in spite of it. 



Until this year, as far as I know, the disease in question 

 affected only the brook trout. Now the Loch Leven has it. But 

 the rainbow in ponds alongside and under identical conditions, 

 has never acqnired the disease, and seems to be immune. This is 

 the only explanation I can offer, that of immunity. It is a mat- 

 ter of lack of susceptibility. This is a sufficient explanation for 

 there are plenty of similar cases among higher animals, in which 

 closely related species have very different susceptibility to the 

 same disease. The immunity may not be complete, and the dis- 

 ease may sooner or later estal)lish itself among rainbow trout. 



The fact that many places where trout are bred and held in 

 wooden ponds have not experienced the disease is only an appar- 

 ent objection to the explanation of its cause as here given. It 

 merely means that the germ is not present in the vicinity or has 

 not gained a foothold in the ponds and in the trout. If one were 

 permitted to inoculate these ponds or trout with material from 

 diseased trout it would be surprising if the result were not the 

 establishment of the disease there. 



In the light of the facts at present known about the brook 

 trout disease, its infectious nature, the exjDerience of the United 

 States Fish Commission with it, and the general principles that 

 o])taiii in analagous diseases of the human family, it is submitted 

 that the theory of pond infection is the most probable of the 

 possible explanations of its regular recurrence; and that while 

 impervious ponds are not proven to l)e the best and only way of 

 dealing with the trouble — experience alone finally settling that — 

 they are worth installing on an experimental 1)asis. 



DISCUSSION OF TEOF. MARSEl's PAPER. 



The President: We are greatly indcl)ted to Mr. Marsh for 

 that paper. 



Mr. Clark : I think that the people here wlio have had any 

 experience in the way of fish diseases should relate them and that 

 there should be full discussion of the paper, any questions that 

 may occur to us on this subject being directed to Prof. Marsh. 



Mr. Bower: Prof. Marsh rather intimates that where the 



