Essays in Natural History and Agriculture. Garnett Thomas

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СКАЧАТЬ and having witnessed the results, I unhesitatingly say that there is little risk of failure, if due care be taken.

      The experiments of Shaw and Agassiz, my own also included, have proved that fish can be bred artificially. The experiments of Boccius I have not yet tried, although he proposes to arrive at the same result in another manner, and acting in the manner recommended by them, Trout and Salmon have been bred by thousands during the last ten years.

      As the season for making the experiment will shortly be here, I hope that those who intend to try the plan will lose no time in looking after their supply of breeding fish.

      To begin with Trout:—Catch as many as you can conveniently obtain upon the spawning beds, [6] and examine them carefully one by one, to see that the spawn and milt are in a fit state for exclusion; and also to enable you to separate the males from the females. If they are in a fit state to be operated upon (which may be known by the facility with which the milt and the roe run from them on a slight pressure), squeeze the milt of the males into a little water, and when you have obtained all the milt you can get, add so much water that the mixture remains slightly opalescent—say about equal in colour to a tablespoonful of milk mixed in a quart of water; pour this into a deep dish or bowl, large enough to hold the largest of your female Trouts; take one of these and put it into the water so prepared, and gently squeeze the roe from it whilst the vent is immersed in the water. [7] Do this as quickly as possible, and return the fish into fresh water, and then pour off the water containing the impregnated roe, through a strainer, carefully preserving it for the remaining fish, and immediately return the roe into fresh spring or brook water. Repeat the operation for every female Trout, and you will then have a quantity of impregnated roe, which if properly managed will hatch with great certainty. Have ready as many boxes as you are able to stock with spawn (three feet long, two feet broad, and six inches deep). Fill them to the depth of two inches of river sand, which ought to be previously so well washed that there is not a particle of mud left in it, and upon that put two inches of river gravel, also exceedingly well washed, the pebbles varying in size from a hazel nut to a pigeon or pullet's egg. These boxes must be so placed that the water from a spring will flow into the first, and from the surface of that into the second, and below the whole nest of boxes there ought to be a small reservoir made—say three yards by two and eighteen inches deep, and well gravelled at the bottom. All these matters having been previously arranged, and the water flowing nicely over the gravel, sprinkle the impregnated roe equally over the surface of the gravel, say a quarter of a pint to each box, and it will roll down into the interstices of the gravel and find a bed in which it will remain snugly until the spring, when, about March, if all has been properly managed, you will find, on a careful examination, that the young Trout are coming to life by hundreds. I am very particular in recommending spring rather than brook water, for several reasons. In the first place, brooks are liable to be flooded, and are sometimes so overcharged with sand and mud that the gravel in the spawning-boxes is completely choked with it and the spawn is lost, as I know to my great and frequent disappointments. At other times all is washed away together. In the second place, the gravel of brooks swarms with water-lice (shrimps) and the larvae of aquatic insects, as well as bull-heads and loaches, all of which prey upon the spawn of the Trout and Salmon. In the third place, if you put your spawning-boxes in a brook, you will find it difficult to prevent the escape of the fry when hatched, and you are left in doubt as to the success of your experiment. With spring water all these inconveniences are avoided. But if your watercourse should contain water-lice or aquatic larvae, it is a very easy matter to destroy them before putting in your boxes, with a little salt or quicklime. It is also desirable to cover your spawning-boxes with a wire grating, to exclude the light, and to protect them in severe weather from the chance of being frozen.

      When they begin to hatch, open a communication between the boxes and the little reservoir below, and if this communicates with a watercourse in which aquatic plants are growing, so much the better. The fry, as soon as they are strong enough, will make their way into this ditch, and will find abundance of food among the water plants; thence they ought to be able to make their way into the brook, river, or lake which it is intended to store with them. All ducks, wild and tame, should be driven from this ditch, or few of the Trout will be allowed to find their way to their final place of destination.

      These rules, with some modification, are applicable to the breeding of Salmon as well as Trout; the only difference being in the mode of placing the female fish, when obtaining the roe, and the size of the gravel in which the spawn is deposited in the boxes. The Salmon is too large a fish to put into the vessels in which the diluted milt is placed, but I think that she should be held by an assistant, in such a manner that the tail and lower part of the body up to the vent are immersed in the water containing the milt. And it is also very necessary to hold her firmly, otherwise a large fish, in the struggles which it makes to get free, is apt to upset the vessel containing the milt, and then the experiment is at an end, at least for the time. Being held firmly by the assistant, as above stated, the belly of the fish must be gently pressed by the hands to promote the exclusion of the spawn, which on exclusion must be gently stirred in the diluted milt, to bring every grain into contact with it; but the roe ought not to remain in contact with the milt a minute, if it can sooner be got out, as I have found that if the diluted milt be too strong, or if the ova remain too long in contact with it, they become opaque, and never hatch at all, apparently because they are over-impregnated. In the ordinary way in which Salmon and Trout are bred, the milt must be largely diluted with water, and the contact between the milt and ova can only be momentary, for the streams in which these fish spawn (particularly the Salmon) are so rapid, that the milt on exclusion must be carried away immediately.

      There is another method, which is preferred by Ramsbottom, to the one I have been describing, and it is certainly less troublesome. This is to take the ova from the female fish in the first place (taking care to exclude the air from it, by immersing the fish into water up to the vent), and when all the roe has been collected into a large bowl or basin, then mix the milt with it, the same diluted in the proportion which has been before described, namely, until the water which covers the roe becomes lightly opalescent.

      I am quite aware that there is another theory which assumes that impregnation takes place twelve months before the exclusion of the ova. [8] But a very careful and long continued examination of the spawning of minnows and lampreys (I have never been able closely to examine the spawning of Salmon), convinces me that it is not a correct one. Besides, did any one ever succeed in hatching the ova of a fish which had not been allowed to come in contact with milt after exclusion? If they have, when, where, and how has this been accomplished, and where is it recorded? I know that I could never succeed, although I have often tried the experiment. On the other hand, it is the easiest thing imaginable, with due care and a suitable situation, to hatch those which have been properly impregnated after exclusion. But if, to avoid argument, I admit that this theory is correct, it will not at all interfere with artificial breeding of Trout and Salmon; on the contrary, it will materially facilitate it. It will only be necessary to catch female fish with the ova ready for exclusion, and place these ova in clean gravel in a box, as before described, but there will be no occasion for males. But supposing Trout and Salmon can be bred in this manner, which I by no means believe, there would be no means of breeding hybrids, which I consider a far more important achievement, and to which I will now refer.

      Ever since my attention was turned to the artificial breeding of fish, it has always appeared to me exceedingly desirable and important to breed hybrids between the Trout and the Salmon. The fry of the Salmon, which, by-the-bye, is perhaps the most delicately flavoured fish that exists in this country, although it lives and thrives in fresh water for two or three years, if kept in a locality where it cannot escape to the sea, yet, if kept longer than that time, pines away and dies. If, therefore, we could obtain a hybrid fish, bred between the river Trout and the Salmon, we should probably produce a fish which, being a mule, would be always in good condition; being crossed with a river fish, it would probably never require a visit to salt water to keep it in good health. Being crossed with a Salmon, it ought to get to a good size in a comparatively short period; and, if it would rise at the artificial fly, or the minnow, ought to afford first-rate sport to СКАЧАТЬ