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Updated: May 19, 2025
An effectual cure for this is to put the affected alevins into still water for about thirty-six hours. "Blue Swelling" of the yolk-sac is another disease from which alevins sometimes suffer, but I have never heard of any cure for this. Another, "paralysis," may be caused by lack of sufficient current and by insufficient aeration of the water.
As soon as the yolk-sacs of the alevins are absorbed the little fish cease to be alevins, and are called "fry." The alevin stage was that in which the fish give least trouble, the stage I am now describing is that in which they give most. They must be fed frequently at least four times a day.
In a few days from the first of the alevins beginning to feed, all of them will be working up with their heads to the current, darting at any particles floating in the water. The tray should now be lowered so that its edge is some three or four inches below the surface and the little fish allowed to swim out into the box.
This is the umbilical or yolk-sac, and contains the nourishment upon which the little fish lives during the first stage of its life after it is hatched. This sac is gradually absorbed but until it is absorbed the young fish are called "alevins." At first the little fish do not require any food, but they generally begin to feed in about six weeks, and before the yolk-sac is completely absorbed.
A greatly varying period of time having elapsed and the yolk-sacs of the alevins being nearly absorbed, the fish culturist will see that some of the little fish begin to leave the pack at the bottom of the tray, and to swim up against the current. When this is observed some very finely divided food should be offered to these alevins.
This is best done in the same way that the alevins were allowed to escape from the hatching tray into the box by lowering the level of the box so that its upper edges are some two or three inches below the surface of the water. The food should now be thrown into the pond higher up, so that the little fish may be induced to swim up and station themselves as near the inlet as possible.
Buckland observed that the alevins of the char very frequently hatch out head first, and consequently that many of them die before they can work themselves free from the eggs.
Sickly alevins will, as a rule, drop out of the pack, and lie on the bottom or against the end of the hatching tray, where they are carried by the current. Dead alevins should be removed at once, and for this reason it is necessary that the hatching trays should be examined at least once a day.
When hatched out and kept in water containing a very large quantity of air in solution, I found that sometimes alevins developed an air bubble in the yolk-sac. On developing this bubble they are unable to stay at the bottom as they usually do, but swim about on their backs at the surface, with part of the yolk-sac out of the water.
This peculiarity in the hatching out of the char has also been observed by Mr. J. J. Armistead, and I have been able to verify it personally. The mortality which occurs in the actual hatching out of the alevins does not, however, by any means end the trouble which the fish culturist has to encounter in the rearing of char.
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