MANUAL OF FISH-CULTURE. 53 



HATCHING. 



As the time for hatching draws near, the egga are i>laced on trays 

 provided with legs or some other support to keep them up from the 

 b(»ttom of the trough. Brass nails driven into the under sides of the 

 tray rims are good temporary legs, and after the hatchiug is over they 

 are readily removed and the necessity of a si^ecial set of trays for 

 hatching is avoided. When there are plenty of troughs, these trays 

 stand singly on the bottom of the trough, Vjut when it is necessary to 

 economize room two or even three are disjjosed one above another. 

 When no necessity exists for economy of space, 4,000 eggs are allowed 

 a whole trough, which is at the rate of 400 to the square foot; 2,000 

 or even 5,000 to the square foot may be carried through hatching and 

 the entire sac stage, but the latter number involves risky crowding. 



The hatching is sometimes expedited by giving eggs that are just at 

 the hatching point a decided shock, similar to that given at an earlier 

 date to kill the unimpregnated ones; also by the temporary stoppage 

 of the water supply. But at Craig Brook it is the custom to lay the eggs 

 out in good season and allow them unlimited time in which to hatch, 

 sometimes a week, sometimes two weeks. The earliest lots commonly 

 hatch the latter part of March, and it is not often that any remain 

 unhatched after April 20. The mean duration of the egg stage is 

 therefore about 157 days, during which the mean temperature of the 

 water has been about 37° V. While hatching is progressing, the outlet 

 screens are closely watched to keep the empty shells from clogging 

 them up; for when a considerable part of the screen is clogged the 

 force of the current through the open spaces is greatly increased, and 

 the soft and yielding sacs of the fish are liable to be drawn through 

 the meshes. 



THE SAC STAGE. 



When the shell breaks, though it has been coiled up in a space less 

 than ^ inch in/liameter, the trunk of the newly hatched salmon at once 

 straightens out to a length of about f inch. The yolk, scarcely dimin- 

 ished from its original size, hangs beneath and constitutes the greater 

 part of the bulk of the fish. The young salmon is for a while more 

 un wieldly than a tadpole. When frightened he sculls about with great 

 energy, but makes slow progress and is fain to lie on his side on the 

 bottom of the trough or crowd with his companions into a corner. The 

 sac is a store of nutriment, which is gradually absorbed into the other 

 parts of the fish; and so long as it lasts the young salmon will not eat. 

 The interval between hatching and total absorption of the sac varies 

 with the temperature, the mean at Craig Brook in April and May being 

 about six weeks. 



As time passes the embryo fish grows more and more to resemble the 

 adult, his body acquires strength, and his fins assume form and become 

 more efi'ective as organs of propulsion. At last his digestive system 



