OCTOMITTJS SALMONIS, A PARASITIC FLAGELLATE OF TROUT 23 
CONTROL MEASURES 
Since Octomitus salmonis is so widely distributed and occurs in fish of all ages, 
it is believed that it will be impracticable to eliminate the parasite from the hatcheries. 
It is probable that many, if not all, adult fish in infected hatcheries harbor small 
numbers of the parasite and are thus carriers of the disease. Even when no flagellates 
can be found in the intestine a few intracellular forms can usually be discovered if 
the search is sufficiently prolonged. It is very doubtful if these parasites can be 
reached by any drug administered in the food, so that in order to free a hatchery 
from infection it would be necessary to remove all the fish and throughly disinfect 
the entire plant. Even were this done, it would probably not be practicable to 
restock with uninfected fish. 
It is even doubtful if drugs could be successfully used in getting rid of the 
flagellates. While no experiments have been conducted along this line, the fact 
that attempts to get rid of Giardia (a closely related parasite in man) by this means 
has so far resulted in failure affords little encouragement in that direction, especially 
when we consider the many practical difficulties in attempting to administer drugs 
to trout 1 to 2 inches long. 
In view of these facts it is felt that efforts to control the disease must be based 
almost entirely on prophylactic measures. Fortunately there is good reason to 
believe that by the adoption of such methods the ravages of the disease can be 
reduced to a point where it will no longer be a serious menace to trout culture. In 
tbis connection the fact that Octomitus is apparently not injurious to trout under 
natural conditions is especially significant. All the evidence at hand points clearly 
to the conclusion that the ravages of octomitiasis are in inverse ratio to the suit- 
ability of the environment, and that if the fish can be kept in a strong, vigorous 
condition they will be able to resist the attacks of the parasite. 
The factors that appear to be most conducive to the development of Octo- 
mitus are overcrowding, an unsuitable water supply, especially a deficiency of oxygen 
and improper food. Owing to the inherent defects of artificial propagation, it is 
almost impossible to entirely avoid the unfavorable effects of these factors, but 
certainly in most cases much can be done to improve present conditions. There are, 
for instance, probably few hatcheries where the fish are not overcrowded at certain 
seasons. It is a natural tendency to attempt to increase the output by overloading 
the equipment. In some cases this may be successful for a time, but in others it is 
attended with disaster. That such a practice is ever advisable in the long run is 
more than questionable, since, even if not attended by heavy mortality, the fish 
reared under such conditions are likely to be weak and underdeveloped. 
It is very probable that part of the injurious effects of overcrowding are due to 
an insufficient supply of oxygen, and this is, of course, especially noticeable when 
the water is not properly aerated before entering the troughs. With rare exceptions 
the water from springs is deficient in oxygen and should always be made to flow over 
an efficient aerating device before entering the troughs or ponds. 
The third factor, that of improper food, is less easy to correct. Undoubtedly 
no substitute can equal natural food, but it will probably never be practicable 
to supply this in sufficient quantities to meet the demands of the hatcheries. To 
