432 
BULLETIN OF THE BUREAU OF FISHERIES. 
essential fish cultural factors are among the most pronounced of all the ponds. The 
immunity of this species is apparently not quite impregnable. One case of supposed 
small incipient tumor was lost by accident before a proper examination of it could be 
made. Several fish with definite red floors have been observed, but these are cases of 
colloid goiter rather than the hyperplastic stages comparable to those of the susceptible 
species. 
At Craig Brook station many unpainted wooden troughs established out of doors 
are used for holding and rearing young fish until they are large enough to demand 
transfer to ponds. They are each about 3.1 meters long, by 32 centimeters wide and 
23 centimeters deep. (Fig. 79.) They are supplied by brook water, in part from the 
hatchery building with w'ater w^hich had flowed through the hatchery troughs, and in part 
freshly from Craig Brook. The water supply of these outside troughs flows from one 
trough to the other among some of the series of troughs. Troughs 93 to 104, however, 
shown in table v, receive water immediately from the hatchery. Fish are not usually 
held in these wooden troughs beyond the first year, but in those of table v they have 
been continued beyond the third year for experimental results wdth thyroid disease in 
these surroundings. The fish were fed the regular artificial food, chiefly liver, and were 
in general subjected to the usual artificial conditions, the space available and the water 
supply being less than the earth ponds of table iv would have afforded. This series of 
troughs was chosen because they were the containers of the young fish which later 
showed the most visible tumors, and from one of them (no. 92) was obtained the 5-months- 
old brook trout with a tumor of considerable size, the youngest fish yet found with a 
visible thyroid tumor. Entirely uniform conditions obtained in these troughs. They 
were of the same size and shape, supplied by the same water, subject to the same regime, 
and afford the opportunity for comparable experiments. 
It is seen that during these three years the brook trout, landlocked salmon, and 
various hybrids show progressively increasing thyroid growth from year to year. Fish 
with visibly affected thyroids not reaching to tumor formation are much more numerous 
from the beginning than those with visible tumors. The latter often appear among 
the yearlings, but the chief incidence is among 2 and 3 year olds. The Scotch sea trout 
are exceptional and show a pronounced resistance here as in the earth ponds. No 
visible tumors appear during the three years. Out of some 300 fish the second year, 2 
only show the incipient hyperemia of the floor of the mouth, and in the third year i 
more. These trout were plainly exposed to exactly the same conditions which result 
in large percentages of tumor involvement in other species. Their resistance amounts 
to practical immunity. 
In neither the earth ponds nor the wooden troughs are the observations sufficient 
to establish anything but the general conditions of fish culture or domestication, under 
which thyroid disease progresses. None of the specific factors of which domestication 
consists have been or can be isolated in this way. It is plain, nevertheless, that earth 
ponds are unnecessary to the disease. The wooden troughs are easily kept clean and 
do not become fouled for long by the accumulation of the products of the fish and 
