20 
BULLETIN OF THE BUREAU OF FISHERIES 
influence on the abundance of the flagellates. We have found in a recent feeding 
experiment that rainbow trout on a sheep-liver diet contained, on the average, a 
smaller number of flagellates than trout of the same age kept under similar condi- 
tions but fed beef heart instead of liver. This is shown in the accompanying table, 
which indicates the comparative numbers of flagellates in the intestines of 40 
rainbow fingerlings at the White Sulphur Springs hatchery. These fish were ex- 
amined on August 7, 1924, and the relative abundance of flagellates indicated as 
correctly as possible by the familiar plus-sign method. All of the fish examined 
were in a fairly vigorous condition at the time, and the mortality was not excessive. 
Total 
Kind 
+ 
++ 
+++ 
++++ 
+++++ 
number 
of fish 
Liver-fed fish 
4 
7 
7 
2 
0 
20 
Heart-fed fish 
3 
3 
3 
4 
7 
20 
While, as previously stated, the flagellates are ordinarily not a serious problem 
in the yearling fish, a notable exception to this rule has come to the writer’s attention. 
The Manchester (Iowa) station has for a number of years experienced considerable 
difficulty with both brook and rainbow trout during the second summer when the 
fish are years old. These fish are stunted, grow slowly, and may develop a 
“pot-belly,” while the mortality is considerably higher than is usually the case 
with fish of this age. It was found that, almost without exception, the yearlings 
at this station are heavily infested with flagellates; so much so, in fact, that the 
intestinal contents is often a squirming, seething mass of these parasites. 
Remarkably enough, the fingerlings of both rainbow and brook trout are not in- 
fected, and little difficulty is experienced with these fish, which are healthy and 
vigorous and show a rapid growth with a comparatively low mortality. Just why 
the fingerlings at this station do not become infected is not entirely clear. Possibly 
it may be due to the fact that the water supply is derived from a covered spring 
and there is no possibility of its becoming contaminated by fish before reaching 
the hatchery. The greater susceptibility of the yearling fish to the effects of 
Octomitus is probably due to the fact that they have had no opportunity to acquire 
an immunity. When the fingerlings are infected, those that survive doubtless 
develop a more or less complete immunity by the end of the first summer, which 
protects them from serious injury the following year. 
While the chronic, wasting disease just described is undoubtedly the most 
common result of infection by Octomitus salmonis and is prevalent to a greater 
or less extent at nearly all trout hatcheries, there is good reason to believe that 
under certain conditions the disease may manifest itself in a quite different manner. 
The writer’s attention was first called to this infection in the spring of 1922 by 
reports that the rainbow fingerlings from the White Sulphur Springs (W. Va.) 
hatchery did not stand up well during shipment. The loss during the first 24 hours 
was not excessive, but if the fish were in transit for a much longer period the mortal- 
ity was exceptionally heavy, sometimes reaching 75 per cent, or even more. In an 
attempt to discover the cause of the trouble, a number of the fish were shipped to 
