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.