Fishes, whether in the freedom of nature or in artificial receptacles, show plainly enough the approach of spawning. The belly of the female becomes distended and yields readily to pressure. There is a fluctuation under the hand, which shows that the eggs are free from the ovary and easily displaced. This being the case, take up in your left hand a female fish, and hold it suspended by the head and thorax over a flat-bottomed vessel containing clear water. Then with the right hand passed from above downwards, squeeze the loosened eggs through the anal opening. A male fish is then taken, and the milt is expressed in the same way, though often it flows by the mere act of suspending. This substance, white and cream like, soon gives to the water the appearance of whey. To insure effectual fecundation, the mixture in this state should be gently stirred with the hand, or with a soft brush. It requires but 2 or 3 minutes to accomplish the fecundation.
The subsequent processes may be carried on upon the spot, or the impregnated eggs may, like those of the silkworm, be packed and transported to other places, there to be hatched.
In the first case, the water with the eggs in it is poured immediately into the hatching apparatus. This may be very simple. Mr. Coste tells us that he has often used a long and narrow wooden box lined with zinc or lead, with a fish box of earthenware. In the laboratory of the colleges of France, the troughs used are of potter's enamelled ware. The eggs are spread, upon a movable frame or grate composed of glass rods, about one-tenth of an inch apart. It seems to be a condition of Nature that this operation of hers, like the great water lily of the tropics, can go on well only in running-water. The water which supplies the hatching-trough must have a constant flow.
Double sieves of wire gauze set in floating frames, which keep them immersed, but near the surface, have been used for hatching fish in ponds and rivers; but the mud is apt to gather in them, incrusting the eggs and making it necessary to remove them for the purpose of cleaning. Such changes retard the process of incubation. Even after they are hatched, the young fish are apt to chafe the umbilical vesicle by coming in contact with the wire, an injury which generally proves fatal.
In preference to the above, another method recommends the use of a wooden box with hinged ends and cover, in all of which are openings for the water, protected by wire gauze, and containing also a fourfold frame of glass rods for the accommodation of the spawn.
In the course of a few hours after the process of fecundation, a change may be seen in the eggs. At first they become opaque, but soon resume their transparency. A small, round spot next appears, which gradually extends until one end takes the shape of a tail, and the other that of a spatula-shaped head. Two black points upon the sides presently turn into eyes. It is not long before the young animal gives sign of life by motion of the tail. As the eggs open the head and tail first emerge, and then the umbilical vesicle attached to the belly of the fish, and there retained for some time, as the only source of nutriment.
In case the eggs in the hatching-box become covered with film from the impurity of the water, they should be cleansed with a feather, or with a fine brush of badger's hair.
The eggs may be transferred from one vessel to another by means of a glass pipe, the stem of which is closed by the finger. The egg is made to enter the tube by removing the finger.
The young fish very soon displays differences of nature and instinct. Some, like the pike and perch, quickly free themselves from the umbilical vesicle and shoot about with great vivacity. Others, as the salmon and trout, retain their provision bags longer, seem more sluggish, and huddle together in dark corners. Some kinds are so bold and hardy that they require but little care. The pike, for instance, and the trout, may very soon be put into ponds and rivers, where they will look out for themselves. But others, more delicate and often more valuable, must be kept in artificial basins until they have acquired strength to resist the destructive agencies that await them in the ravenous waters.
In a box less than 2 feet long, 6 inches wide and 4 inches deep, Prof. Coste has sometimes reared to a sufficient size for removal, no less than 2000 salmon at a time.
The basin used at the College of France may serve as a model for the receptacles above named. It has different compartments for the fish of different ages. The wall is built waist-high, that the fish may be conveniently overlooked. Here and there, on the gravelly bed, are small heaps of rounded pebbles. Little shelters of earthenware are scattered about, that the fish may have dark places in which to hide and rest. A few aquatic plants are added to complete the conditions which would be found in nature.
The salmon, the trout and the eel, are fed upon boiled beef or horse-flesh, which is prepared for them by pounding in a mortar. These delicate morsels are eagerly seized by the young fish. After 8 or 10 days the boiled flesh is exchanged for raw, which is pounded and given in little pellets. At Hummingue, salmon and trout are fed with the flesh of other and cheaper fish, which is prepared for them by pounding. Small earthworms and the minute crustacea of stagnant waters are sought with avidity by these young fry.
For the proper acclimation of fishes, and for other reasons, it is often desirable to transport the eggs to a considerable distance. When the eggs are free and separate, with a tough covering as in the case with the salmon and the trout, pine boxes are used. These are filled with sand or moss, or fragments of sponge, or with some aquatic plant, in the moist folds of which the eggs are ranged in layers.
The eggs, which come in agglutinated clusters, with tender envelopes, such as the spawn of the carp, the roach, the perch, etc. cannot be conveyed so easily. The best method is to put them into jars three-quarters filled with water and containing some aquatic plant. There is another class of eggs which are deposited upon grass or small sticks. Let these, with the objects to which they adhere, be wrapped up in a wet cloth, and then be put into a box or basket.
The young fish also are often transported to great distances in bottles containing water and some living aquatic plants. The water must be renewed from time to time. To keep up the supply of air, which fishes must have, no less than animals which live in it, an ingenious apparatus has been devised by some fishermen of the Vosges. The vessel which holds the fish is swung at the back in the style of the ragpicker. A bellows, like that of the Scotch bagpipe, worked under the arm, sends at pleasure its current of air through the water that contains the fish. An accasional squeeze of the bellows keeps the fish in good breathing condition.
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