THE DESTRUCTION OF TROUT FRY BY HYDRA. 
By A. E. BEARDSLEY, 
Professor of Biology, Colorado Stale Normal School. 
The following observations were made during an investigation at the United 
States Fish Commission hatchery, Leadville, Colo., in August, 1902. On August 4, 
some eggs of the black-spotted trout in a number of the hatching- troughs were just 
hatching, while in others the young fish were several days old. Each trough was 
separated by screens into three divisions. The first division — that into which the 
water enters from the supply pipes — contained no eggs, these having all been removed 
several days before on account of the great mortality of the young fishes hatched in 
this division of the troughs. In the second, or middle, division, the newly hatched 
fry were dying in considerable numbers, some before leaving the egg trays. In the 
third division of these troughs, as well as in the troughs not directly fed from the 
supply pipes, the death rate was merely nominal. 
These facts clearly indicated that the cause of the mortality was directly connected 
with the water supply, which was found to be derived chiefly from two sources. 
The main supply pipes were fed from Rock Creek, and an auxiliary set of pipes led 
from a spring near the hatchery. Connected with the main pipes was a branch 
leading from the third or lowest of the Evergreen lakes. This was closed at the 
time, only a small quantity of water coming through leaks around the gate at the 
head of the pipe. 
The water from the main pipes was clear, containing very little sediment, and with 
a temperature of 48° F. ; that from the spring was very clear and pure, without 
sediment, its temperature being 43° F. There was very little sediment in the 
hatching-troughs. In this, however, microscopical examination disclosed the pres- 
ence of great numbers of a very transparent hydra, which had been discovered by 
the attendants at the hatchery a few days before, when the sun’s rays, just before 
sunset, had fallen obliquely into one of the troughs. In the dim light of the 
hatchery this hydra was quite invisible, but by placing a large mirror outside of 
the building so as to throw a beam of sunlight through the window, with a hand 
mirror reflecting this beam so as to throw it into the trough, the hydras could be 
plainly seen as slender, whitish threads, 1 to 2 centimeters in length and 0.15 to 0.30 
millimeter in diameter, fixed by one end to the bottom or to the side of the trough, 
and bearing a crown of 5 or 6 long tentacles around the mouth at the free end. The 
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