ARTIFICIAL PROPAGATION OF WHITEFISH, ETC. 29 



absorb the yolk sac in 30 days at a mean temperature of 39°. These 

 stages of development are all subject to considerable variation, being 

 shortened or lengthened as the mean water temperature is decreased 

 or increased. As the time of hatching approaches the distinct outline 

 of the fish is plainly visible in the dark hue of the egg, and the con- 

 vulsive movements of the embryo fish continue to increase until 

 hatching occurs, when the fish breaks forth from the shell with its 

 tail. The bright warm days of sj^ring and the slightly warmer water 

 temperatures usually bring out 50 per cent of the fry between the 

 hours of 6 and 10 p. m. 



Just prior to the hatching period the eggs are removed from 

 the trays on which they have been developing and transferred to 

 trays covered with wire cloth of smaller mesh — 16 to the inch. 

 The larger mesh cloth gives a freer circulation of water during 

 the development of the eggs, but the smaller mesh is required for 

 the fry, as the yolk sacs of many of them become enmeshed if a 

 more closely woven cloth is not provided. 



Considerable attention is required of the fish-culturist at this 

 time to guard against smothering of the fry. The shells from 

 hatching fish must be removed to prevent clogging of the -trays 

 and stoppage of the water circulation. To provide against it, 

 when about 50 per cent of the eggs have completed development, 

 one stack of 12 trays is emptied into a pan of water and the eggs 

 and fry stirred with a feather. This causes the shells to rise to 

 the top, and they can be poured off easily by gently tipping the 

 pan. By repeating the operation several times the hatching fish 

 are entirely freed from refuse. In returning the fish and eggs to 

 the trays they should be divided as equally as possible among the 

 12 trays. To accomplish this the trays are placed in the picking 

 trough and the fish-culturist starts at the upper end, pouring an 

 equal quantity on each of the trays. Since handling the eggs at 

 this time hastens hatching, it is seldom necessary to give this treat- 

 ment more than two or three times. 



From the hatching period until just prior to the complete absorp- 

 tion of the food sac, at which time the fry are usually distributed, 

 the treatment given them is somewhat similar to that prescribed for 

 the eggs. Monstrosities, " blue-sacs ", and dead fry are picked 

 out as soon as discovered. The fry grow very fast at this period; 

 therefore it is not advisable to place more than 3,500 on a hatching 

 tray, 14 by 16 inches in dimensions, which number can be held until 

 they are ready to plant. 



DISTRIBUTION OF FRY. 



In view of all the considerations involved, perhaps the most satis- 

 factory results are to be obtained from planting lake-trout fry just 

 prior to the complete absorption of the yolk sac. Liberated at this 

 stage of development, they are provided with sufficient nourishment 

 to sustain life until they become somewhat acclimated to their new 

 surroundings and the impulse to forage for their natural food has 

 not been lessened by having artificial food placed conveniently before 

 them. At none of its stations where lake trout are propagated is the 

 bureau equipped for rearing to the fingerling size the large numbers 

 of fry produced. In view of the importance of the industry de- 



