THE DESTRUCTION OF TROUT FRY BY HYDRA. 
159 
supervened. The five trout in the beaker without hydras were kept in the beaker 
until the next day and were then found to be all alive and in good health. 
No other cause of injury having been discovered after the most careful search, 
the destructive effects of the hydras upon the fishes in the foregoing experiment were 
taken as conclusive evidence that these were the cause of the unusual mortality of the 
trout fry. This fact being demonstrated, a careful examination of all the sources of 
water supply to the hatchery was made. The lower of the three lakes was first 
visited. This lake is quite shallow, being about 12 feet (1 meters) in depth in the 
deepest part; along the borders there is considerable aquatic vegetation, consisting 
of sedges and cat-tails; here the hydras were found in immense numbers, clinging to 
the submerged stems and leaves as well as to the green filamentous alga which was 
growing abundantly on the bowlders which are scattered over the bottom. The other 
lakes, and Rock Creek for a distance of about half a mile above the head of the supply 
pipe, as well as the spring, were examined in turn, but although very careful search 
was made, no hydras were found in either of these waters. 
The temperature of the water in each of the three lakes at 1 foot below the 
surface was taken August 8, about 2 p. m., and was found to be as follows: Upper 
Evergreen Lake, 60° E. ; Middle Evergreen Lake, 64°; Lower Evergreen Lake, 65°. 
The leaks about the head of the pipe leading from the lower lake were immediately 
stopped and no water from the lake is now entering the hatchery. 
The natural causes which control the development of the different species of hydra, 
favoring or retarding it, are as yet but little understood. At one period hydras may 
be very abundant at a given point, and soon afterwards entirely disappear without any 
apparent cause. They have been found in the vicinity of Greeley, Colo., during all 
months of the }'ear, sometimes in great abundance; sometimes, however, a whole year 
has passed without a single one being seen, although searched for most diligently. 
They occur in lakes, ponds, and marshes, usually in clear water. Warm water (60° 
to 80° F.) appears to favor their rapid multiplication, since they are usually most 
abundant in summer and earl}' autumn; cold water does not apparently injure them, 
however, as the writer has frequently taken vigorous individuals in the winter, 
through holes in the ice. Hydras reproduce at certain times by eggs, which settle 
to the bottom and probably remain dormant through the winter, but the usual and 
most rapid mode of multiplication is by budding. Little buds arise from the side of 
the parent, soon acquire a mouth and tentacles like the parent, and after a time break 
loose and lead an independent existence. In the lake most of the hydras examined 
were bearing from two to six buds, showing that the conditions there were favorable 
to their rapid multiplication. In the hatchery troughs, on the contrary, very few 
were found bearing buds, and these were probably recent arrivals. The conditions 
within the hatchery do not, therefore, appear favorable for their increase, and it 
only remains to rid the troughs of them in the most practicable manner. 
As the hydra is very tenacious of life and may even be cut into several pieces 
without serious injury, each piece developing the lost parts and becoming, in a few 
days, a complete hydra, it is not probable that it can be destroyed in the troughs 
without injury to the fish eggs or young fry. By removing all eggs and fry, briskly 
scrubbing the bottom and sides of the trough with a stiff brush so as to cause the 
hydras to loosen their hold, then quickly flushing the trough into the waste-pipe, 
most of them can be removed. 
