370 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
If the culture of fish or crawfish is to succeed in streams it is necessary — 
(a) That there should be a possibility of laying these streams entirely dry at 
times and removing all fish. 
(b) Fish of the same kind which are to be cultivated should be all of the same 
age. If several kinds of fish are cultivated in one and the same water, they must 
moreover be equally developed. Fish and crawfish, even if they are of the same age, 
must under no condition be raised in one and the same water. Trout, e. g., will, under 
suitable conditions, weigh 300 to 400 grams [about 10 to 134 ounces] after the com- 
pleted second year, and at that age fetch the best price in the market. The crawfish, 
however, needs from five to six years in order to find buyers at a weight of 50 grams 
[2£ ounces]. The objection to stocking water with young trout and crawfish of the 
same age is not so much the inconvenience caused by the circumstance that fishing 
will have to be carried on at two different times, as the vast difference in the growth 
of these two animals. If the yearling trout weighing about 100 to 150 grams [34 to 
5 ounces] do not entirely destroy the little crawfish of the same age, but weighing- 
only 1 to 2 grams during the first year, the 2-year-old trout, now weighing about 
400 grams [134 ounces] and having become more voracious, will certainly succeed in 
destroying all the young crawfish which now have only reached the weight of 4 
grams. 
( c ) Finally, care should be taken to furnish a constant supply of nutritious 
natural food. 
As it will be far easier to introduce a well regulated remunerative culture of fish 
and crawfish in small streams which are generally fed from springs, and consequently 
have pure and healthy water at all times, than in rivers whose water has been polluted 
by refuse from factories, more attention should be given to this matter. 
In order to derive the greatest possible benefit from the water of a stream and 
to gain as many separate inclosures as possible for the proper rotation necessary in 
well-regulated fish-culture, ditches of varying breadth must be dug on both sides of 
the stream 70 to 90 centimeters [24 to 34 inches] deep and 50 to 100 meters [16J to 324 
feet] in length. With these ditches (fig. 7) puddles, and in fact larger or smaller 
depressions of the ground, maybe advantageously united, and wherever the ground is 
favorable small independent ponds may be constructed and fed from the water of the 
stream. 
In order that the water of the stream within the limits of the establishment may 
at any time be withdrawn from the different ditches (the smaller ponds, as well as 
from the channels through which the water Hows in and out) so that all these water- 
courses or ponds may be laid dry and all the fish removed therefrom, they are arranged 
in such a manner that the bottoms form geometrically inclined planes (every valley 
of a stream will have sufficient fall), or at least do not deviate very much from such 
planes. The bottoms of the smaller ponds need not always have the same inclination 
as the bottoms of the ditches, etc., but may be dug out all the deeper, in proportion 
as their outflow channels are dug out deeper, supposing, of course, that there is 
sufficient fall. It is self-evident that the bottoms of the smaller ponds must always 
be a little higher than the upper part of the bottoms of their drains, in order that 
these small basins of water can be laid thoroughly dry at any time. If the water is 
led into these basins from a considerable height, and consequently with a smaller 
fall, they may be dug out deeper and their area may be likewise enlarged. All this 
work, however — which will be more fully described farther on — must be preceded by 
