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Trout Management Of The Ova And Alevins








Everything should now be ready for the reception of the ova. The rearing
boxes are resting upon stones placed at the bottom of the ponds, with
the edges some six inches above the level of the water, and moored to
the sides to prevent their being moved by the current. The hatching
trays are suspended in the rearing boxes, or placed upon movable rests
in the boxes, with their edges just above the level of the water.

Notice is usually sent a day or two before the ova are despatched from
fish cultural establishments, so the amateur has no excuse for not being
absolutely ready for their reception. They are packed in various ways,
and nowadays suffer but little in the transit. The ova should always be
carefully washed before they are placed in the hatching trays. Mr.
Armistead, in _A Handy Guide to Fish Culture_, says:--"If just turned
out of a packing case there may be small pieces of moss or other
material amongst them. In any case a wash will do them no harm, and the
process is a very simple one. Take a pail, half-filled with ova, and
then fill up with water, and with a small lading-can lift some of the
water out, and pour it back again, so as to cause a downward current,
which will agitate the ova. Their specific gravity being greater than
that of water, they immediately retire again to the bottom of the pail,
and by at once pouring off as much water as is practicable, any floating
particles of moss, etc., may be carried off. Should any be left, the
process should be repeated, and it may even be necessary to repeat it
several times. When all is right take a ladle, or small vessel of some
kind, say a good-sized tea-cup, and gently ladle out the eggs, and place
them roughly on the grills, where they may be roughly spread by means of
a feather."

To these instructions I would add some for the amateur, who will
probably deal with a comparatively small number of ova. The ova should
be washed in some large vessel full of water in the manner above
described. When the water is quite clear, and the ova clean, they may be
caught in mid-water as they are sinking either in the hatching trays or
in a cup. If caught in a cup they should be transferred with great care
to the hatching trays, and spread out in a single and somewhat spare
layer. They must on no account be poured into the trays from a height.
While under water well-eyed ova will stand a good deal of gentle
tumbling about, but if dropped into the water from even a little height
the concussion is likely to kill them.

Mr. Armistead recommends glass grills rather than trays such as I have
described, but I have found the trays work very well, and they are very
simple and clean. Glass grills are, however, very excellent, though they
necessitate a somewhat greater initial outlay than do the perforated
zinc trays.

A German fish culturist has recently recommended keeping a stock of
fresh-water shrimps (_Gammarus pulex_) in the hatching trays and rearing
boxes. He says that the shrimps eat only the dead ova, and never touch
the living ones. They also eat any vegetable or animal _debris_. I have
never tried the experiment myself, and so cannot speak from experience.

Dead ova should be always removed at once, and the hatching trays should
be gone over carefully once or twice a day to see if any are present in
them. Dead ova are easily recognized from the fact that they become
opaque and white. They are best removed with a glass tube. The thumb is
placed over one end of the tube, and the other end brought directly over
the dead ovum. When the thumb is removed from the end of the tube held
in the hand the water will rush up into the tube, carrying with it the
dead ovum. The thumb is then replaced over the end of the tube, which is
lifted from the water with the ovum retained in it. This tube may also
be used for removing any extraneous bodies which may get into the trays
or boxes.

A form of fungus known as _Byssus_ grows upon dead ova, and it is
principally for this reason that they must be removed. Livingstone Stone
says of _Byssus_:--"With trout eggs in water at 40 deg. or 50 deg. Fahrenheit,
it generally appears within forty-eight hours after the egg turns white,
and often sooner, and the warmer the water the quicker it comes. It is
never quite safe to leave the dead eggs over twenty-four hours in the
hatching boxes. The peculiarity of _Byssus_ is that it stretches out its
long, slender arms, which grow rapidly over everything within its reach.
This makes it peculiarly mischievous, for it will sometimes clasp a
dozen or even twenty eggs in its Briarean grasp before it is discovered,
and any egg that it has seized has received its death warrant." Mr.
Armistead has known it appear within twenty-four hours. _Byssus_
develops only on dead ova.

_Saprolegnia_, known to fish culturists as "fungus," attacks both living
or dead ova. If the woodwork is properly varnished or charred, and the
ova managed thoroughly, there should, however, be but little risk of
fungus. Light is favourable to the growth of fungus, and, therefore,
wooden lids should be placed over the rearing boxes. These should be
kept partially on after the young fish have hatched out, and be replaced
by covers of fine wire netting spread on closely-fitting frames, when
the fry have begun to feed. These obviate the necessity of covering up
the ponds during the first stages.

Many small creatures such as caddis-worms will eat the ova, and
therefore a careful watch should be kept upon the hatching trays as it
is marvellous how such creatures find their way in, in spite of all
precautions. Birds of several kinds are also likely to cause great
damage unless the ova and young fish are carefully guarded from their
depredations.

In a short time, probably within a few days of receiving the ova, the
amateur will find that the young fish are beginning to hatch out. They
generally come out tail first, and in wriggling this about in their
attempts to get further out, they propel the ovum about the bottom of
the tray. When the little fish attempts to come out head first, he
sometimes gets into difficulties and if this is observed, he may be
helped by a gentle touch with a feather or a camel's hair brush.

When first hatched out the young fish have a large translucent
protuberance on the under-surface. 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. The rearing boxes should be kept partly covered, and the
alevins will crowd into a pack in the darker parts at the bottom of the
hatching tray.

The shells of the ova must be removed from the hatching trays. As they
are lighter than the alevins, the current will generally carry them to
the lower end of the tray, whence they may be removed with a piece of
gauze spread on a wire ring, or by raising and lowering the tray gently
in the water in alternately slanting directions.

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.





Next: Trout Management Of The Fry

Previous: Trout Rearing Ponds Boxes And Hatching Trays



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