American Fisheries Society. 107 
or able to put the money in to try them. The fish disease that 
has prevailed among United States stations, and particularly at 
Northville, for some years, is a bacterial disease that is caused by 
a vegetable microorganism, and much can be learned from the 
study of these bacteria. But that is not the case with the para- 
site at Cold Spring Harbor, for it cannot be grown artificially ; 
but the parasite, without much doubt, arises in the water, and 
if you put in a filtration plant of sufficient size, and let all the 
water go through it, you could take it out. But that would not 
pay on a commercial basis, and perhaps it would not be advisable 
for a state or the United States to put it in. 
Another remedy that Mr. Clark has in use, which is a partial 
remedy, that is, it permits a number of trout to be raised, but not 
so many as the same area would accommodate if the trout were 
not diseased, consists in putting the fish in a large pond, instead 
of small, narrow, restricted ponds such as are ordinarily used. 
But it does its work and prevents disease in this way: it merely 
increases the space that each trout can occupy, so that when the 
disease starts it does not transfer from one to the other as readily 
as when the ponds are crowded. 
Mr. Clark: How about vegetation ? 
Dr. Marsh: The vegetation gives opportunity for natural 
food, and indirectly in that way is beneficial. Otherwise I do 
not suppose vegetation enters into the matter very much. 
I have just been to the Bayfield hatchery of the Wisconsin 
State Commission, and been experimenting with an entirely new 
remedy in this connection: that is, copper sulphate, which has 
recently been used very largely in municipal reservoirs, both to 
destroy the alge, and still more recently for killing typhoid. Its 
use for the latter purpose is very much more restricted than for 
the alge. The typhoid germ in general is very much hke the 
trout organism, and if this copper sulphate will kill the typhoid 
germ, one is led to suspect that it would kill the trout organism ; 
and it will do so, but it is very much more fatal to fish than it is 
to people. You can add a good deal of copper sulphate to water 
for people to drink, and do no harm; but the trout are exceed- 
ingly susceptible to it, and the susceptibility varies greatly in 
different stations and in different waters. The disease is now 
