376 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
vacant ponds which are free from predaceous fish, it is nevertheless an advantage to 
insure proper care and attention to the young fish or crawfish, which in the beginning- 
are very helpless (the umbilical period, during which the young fish take no outside 
food, lasts with the salmonoids, e. g., four to six weeks), and to bring them sooner to 
a state of independence by placing them in the large basin covered with sand and 
gravel and amply supplied with aquatic plants, giving them plenty of good food 
(live crustaceans, maggots, worms, etc., to the fish; fish, frogs, meat, crustaceans, etc., 
chopped fine, to the crawfish). 
The limited space at my disposal does not permit me to describe here an estab- 
lishment for raising shrimps (which in the exclusive culture of salmonoids may take 
the place of the basin), by means of which the young fish will grow strong very 
rapidly, through the continuous nutritious natural food which is furnished to them. 
I would refer the reader to my pamphlet on the subject, entitled, Neues Fisch- und 
Krebszuchtverfahren mit Weidenkultur verbuuden und auf die natiirliche Fiitterung 
basirend (new method of raising fish and crawfish in connection with the cultivation 
of willows, and based on the system of natural food). 
It will be found an advantage to stock the waters with large and strong fish or 
crawfish, because fewer will be lost during raising, and because wider gratings may 
be used for shutting off the ditches. If the basin can be properly secured, it may, 
during autumn and winter, when there is no other use for it, be employed as a reser- 
voir for fish or crawfish. 
The number of fish or crawfish to be placed in the water of the inclosures depends 
entirely on the food conditions of the water. It may be said, however, that 20 to 30 
young fish, or 30 to 40 young crawfish, to the cubic meter [35.3 cubic feet] will not be 
too many if the conditions are favorable for constantly supplying ample food as the 
fish or crawfish grow up and need more food. A cubic meter of water in a trout 
basin may, with ample and good food, hold 40 to 50 trout, each weighing 200 to 250 
grams [6f to 8^ ounces] and reaching even a heavier weight. I would warn against 
overstocking a sheet of water, because the food would then hardly be sufficient to 
proper ly support the fish. 
As regards the feeding of fish, etc., nature furnishes an exceedingly welcome aid. 
The large river shrimp ( Gammarus fossarum) is always found in enormous quantities 
in clear running water. In the kind of establishment described by me, the bottom 
current of the water caused by the holes in the damming boards proves exceedingly 
favorable to these shrimps, which form the most desirable food for trout and crawfish. 
They increase very rapidly, prefer the gravelly, sandy beds of our trout brooks, and 
love to hide in dark places among aquatic plants, roots, branches, etc. Wherever the 
bottom does not consist of sand or gravel this should only be introduced in small 
quantities, because the trout and crawfish like such a bottom as well as the shrimp. 
While the large trout in streams where it prevails will not suffer any other fish 
(whitings, minnows, gudgeon, bullheads, etc.) or crawfish to exist for any length of 
time, it will not succeed in exterminating the shrimp. But, on the other hand, the 
small tront will lose a great deal of its principal food if the above-mentioned fish 
or crawfish increase very rapidly. It will therefore be best to banish such fish or 
crawfish entirely from the trout waters. 
In carp-raising ponds predaceous fish (pike, eel, perch, etc.,) are placed in order 
that they may destroy the too prolific offspring of the carp, as well as whitefish, frogs, 
etc., which would deprive the carp of a good deal of their food. As the shrimp is an 
