ons.

The alevin stage is the stage in which the least mortality should be
expected, and the little fish give but little trouble. There are,
however, several diseases besides fungus (of which I have spoken already
when dealing with the ova) from which the alevins may suffer.

I was, I believe, the first to describe (in the "Rainbow Trout") a
peculiar disease from which alevins suffered. 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. An effectual cure for this is to put the
affected alevins into still water for about thirty-six hours. I have
observed this affection in the alevins of the rainbow trout (_Salmo
irideus_), the common trout (_S. fario_) and the Quinnat or Californian
Salmon (_Onchorynchus conicha_).

"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. 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.

CHAPTER VII

TROUT. MANAGEMENT OF THE FRY

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. They will probably dart
at the minute pieces of food floating past and a little more may then be
given to them. If, however, they do not take any notice of little pieces
of food or any other matter which floats past them, they should not be
tried again till the next 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
in