FISH CULTURE. 301 
deposited in a natural stream, under circumstances as 
closely as possible resembling those chosen by the fish, 
and left to themselves; or, as is far better, they may 
be subjected to artificial hatching. By this they may be 
guarded from various mishaps, the supply of water can 
be so regulated that it will be uniform, and the eggs can 
be examined from time to time, and dead and diseased 
ones be removed before they can injure their neighbors. 
It is essential that the incubation be conducted under 
circumstances like those under which it naturally takes 
place. The temperature, quality, and state of the water 
are the main conditions. Some species spawn in fresh 
water, and some in salt; some in rapid streams, and some 
in lakes and ponds ; some in winter, and some in summer. 
The temperature required by trout is about forty-one 
degrees Fahrenheit, ranging, however, from several de- 
grees below this, to gious fifty degrees ; ; while some 
Species of summer-spawning fish require a temperature 
higher than sixty degrees. - The time required for de- 
velopment. varies with different species, and is much 
affected by temperature. Some species hatch in five 
days, while the trout is rarely less than fifty days, and 
at thirty-seven degrees of heat requires one hundred and 
thirty-six days. 
The apparatus employed in artificial incubation is of 
various kinds. A metal box, with many holes to admit a 
free circulation of water, was one of the first employed ; 
this is immersed in the water. Troughs of stone, vessels 
of earthenware, willow baskets, and wooden boxes have 
all been used with success in the incubation of salmon 
and trout. — : 
A favorite form of hatching-box for trout is a long 
wooden trough, its bottom inclined sufficiently to cause a 
