Salmonia; Or, Days of Fly Fishing / In a series of conversations. With some account of the habits of fishes belonging to the genus Salmo

by Sir Humphry Davy

Poiet.—I think Halieus is quite in the right to be a little angry at your observation, Physicus, in making him a disciple of a writer, who, as well as I can recollect, has deduced the genesis of the human being, by a succession of changes dependant upon irritabilities, sensibilities, and appetencies, from the fish; blending the wild fancies of Buffon with the profound ideas of Hartly, and thus endeavouring to give currency to an absurd romance, by mixing with it some philosophical truths. I hope your parallel will induce him to do us the favour to state his own notions more at large.

Hal.—Physicus has mistaken me; and I will explain. What I mentioned of the varieties of dogs as sprung from one type, he will, I am sure, allow me to apply, with some modifications, to all our cultivated breeds of animals, whether horses, oxen, sheep, hogs, geese, ducks, turkies, or pigeons; and he will allow, that certain characters gained by accidents, either from peculiar food, air, water, or domestic treatment, are transmitted to, and often strengthened in the next generation; the qualities being, as it were, doubled when belonging to both parents, and retained in spite of counteracting causes. It will be sufficient for me to mention only a few cases. The blood-horse of Arabia, is become the favourite of the north of Europe, and the colts possess all the superior qualities of their parents, even in the polar circle. The offspring of the Merino sheep retain the fineness of their wool in England and Saxony. Poultry, bantams, tumbling and carrier pigeons, geese, ducks, turkies, &c., all afford instances of the same kind; and in the goose and duck, not only is the colour of the feathers changed, but the form of the muscles of the legs and wings; those of the wings, being little employed, become weak and slender; those of the legs, on the contrary, being much used, are strong and fleshy; and it is well to know this, as, in the young birds, the muscles of the legs and thighs are the best parts for the epicure, a large quantity of flesh being developed there, but not yet hardened or rendered tough by exercise. These facts are of the same kind and depend on the same principles, as the peculiarity of the breeds or races in trouts. Fish in a clear cool river, that feed much on larvæ, and that swallow their hard cases, become yellower, and the red spots increase so as to outnumber the black ones; and these qualities become fixed in the young fishes, and establish a particular variety. If trout from a lake, or another river of a different variety, were introduced into this river, they would not at once change their characters; but the change would take place gradually. Thus I have known trout from a lake in Scotland, remarkable for their deep red flesh, introduced into another lake, where the trout had only white flesh, and they retained the peculiar redness of their flesh for many years. At first they all associated together in spawning in the brook which fed the lake, but those newly introduced were easily known from their darker backs and brighter sides. By degrees, however, from the influence of food and other causes, they became changed; the young trout of the introduced variety had flesh less red than their parents; and in about twenty years the variety was entirely lost, and all the fish were in their original white state. A very speculative reasoner might certainly defend the hypothesis, of the change of species in a long course of ages, from the establishment of particular characters as hereditary. It might be said, that trout, after having thickened their stomachs by feeding on larvæ with hard cases, gained the power of eating shell-fish, and were gradually changed to gillaroos and to char; their red spots and the yellow colour of their belly and fins increasing. In the same manner it might be said, that the large trout which feed almost entirely on small fishes, gained more spines in the pectoral fins, and became a new species; but I shall not go so far, and I know no facts of this kind. The gillaroo and the char appear always with the same characters: and I have never seen any fish that seemed in a state of transition from a trout to gillaroo or a char; which I think, must have been the case if such changes took place. I hope, after this explanation, Physicus will not find any analogy between my ideas and those of a school, to which I am not ambitious of being thought to belong; and that he will allow my views to be sound, or at least founded upon correct analogies.

Poiet.—Do you know any facts of a similar kind in confirmation of your idea that the par is a mule?

Hal.—I have heard of similar instances, but I cannot say I have myself witnessed them. The common carp and the cruscian are said to produce a mixed race, and likewise the rud and the roach; but I have never paid much attention to varieties of the carp kind. A friend of mine informed me, that in a branch of the Test, into which graylings had recently been introduced, his fisherman caught a fish, which appeared to be from a cross between the trout and grayling, having the high back fin of the grayling, and the head and spots of the trout: this is the more remarkable, if correct, as the grayling spawns in the late spring, and the trout in the late autumn or winter: yet I do recollect that I once took a grayling in the end of November, in which the ova were so large, as nearly to be ready for protrusion. The fisherman of the Gründtl See, in Styria, informed me, that he had seen a fish which he believed to be a mule between the trout and char, the fins of which resembled those of a trout, though the body was in other respects like that of a char. The seasons at which these two species spawn approach nearer to each other; but the char spawns in still and the trout in running water. In general the trout are mature before the char, yet I have seen in the Leopoldstein See, in Styria, a female char, of which the eggs were almost fully developed as early as June: the fisherman of the Gründtl See said, that these peculiar fish were very rare, and that he caught only one in about 500 char. It is not, I think, impossible, that it may be an umbla, a fish that might be expected to be found in that deep, cold, Alpine lake, a peculiar species and not a mixed variety. It is a fertile and very curious subject for new experiments, that of crossing the breeds of fishes, and offers a very interesting and untouched field of investigation, which I hope will soon be taken up by some enlightened country gentleman, who in this way might make not only curious but useful discoveries.

Poiet.—So much science would be required to make these experiments with success, and there would be so many difficulties in the way of preserving fishes at the time they are proper for reproduction, that I fear very few country gentlemen would be capable of prosecuting the inquiry.

Hal.—The science required for this object is easily attained, and the difficulties are quite imaginary. The impregnation of the ova of fishes is performed out of the body, and it is only necessary to pour the seminal liquor from the melt upon the ova in water. Mr. Jacobi, a German gentleman, who made many years ago experiments on the increase of trout and salmon, informs us, that the ova and melt of mature fish, recently dead, will produce living offspring. His plan of raising trout from the egg was a very simple one. He had a box made with a small wire grating at one end in the cover, for admitting water from a fresh source or stream, and at the other end of the side of the box there were a number of holes to permit the exit of the water: the bottom of the box was filled with pebbles and gravel of different sizes, which were kept covered with water that was always in motion. In November or the beginning of December, when the trout were in full maturity for spawning, and collected in the rivers for this purpose upon beds of gravel, he caught males and females in a net, and by the pressure of his hands, received the ova in a basin of water, and suffered the melt or seminal fluid to pass into the basin; and after they had remained a few minutes together, he introduced them upon the gravel in the box, which was placed under a source of fresh, cool, and pure water. In a few weeks the eggs burst, and the box was filled with an immense number of young trout, which had a small bag attached to the lower part of their body containing a part of the yolk of the egg, which was still their nourishment. In this state they were easily carried from place to place in confined portions of fresh water for some days, requiring apparently no food; but, after about a week, the nourishment in their bag being exhausted, they began to seek their food in the water, and rapidly increased in size. As I have said before, Mr. Jacobi assures us, that the experiment succeeded as well with mature fish, that had been killed for the purpose of procuring the roe and melt, these having been mixed together in cold water immediately after they were taken out of the body. I have had this experiment tried twice, and with perfect success, and it offers a very good mode of increasing to any extent the quantity of trout in rivers or lakes, for the young ones are preserved from the attacks of fishes, and other voracious animals or insects, at the time when they are most easily destroyed, and perfectly helpless. The same plan, I have no doubt, would answer equally well with grayling or other varieties of the salmo genus. But in all experiments of this kind, the great principle is, to have a constant current of fresh and aerated water running over the eggs. The uniform supply of air to the fœtus in the egg is essential for its life and growth, and such eggs as are not supplied with water saturated with air are unproductive. The experimenter must be guided exactly by the instinct of the parent fishes, who take care to deposit the impregnated eggs, that are to produce their offspring, only in sources continually abounding in fresh and aerated water.

Phys.—But as every species of fish has a particular and usually different time for spawning, I do not see how it could be contrived to cross their breeds, or how the ova of a trout, which spawns in December, could be impregnated by the seminal fluid of the grayling, which spawns in May; for I conclude it would be impossible to preserve the eggs of a fish out of the body in a state in which they could retain or recover their vitality.

Hal.—I believe I mentioned before, that I had found instances, in which the ova of fish were developed at a different period from their natural one; and I have no doubt, that a little inquiry respecting the habits of fishes would enable us to acquire a knowledge of the circumstances, which either hasten or retard their maturity. Plenty of food and a genial season hasten the period of their reproduction, which is delayed by want of proper nourishment, and by unfavourable weather. Males and females likewise, confined from each other, have their generative powers impeded; and trout, grayling, and salmon, will not deposit their ova except in running water; so that by keeping them in tanks, the period of their maturity might be considerably altered. I have seen char even, which had been kept in confined water from September till July; and so slow had been the progress of the ova, that they appeared to be about this time fit for exclusion, though, in the natural course of things, they would have been ripe in the end of October of the year before. By attending to and controlling all these circumstances, I have no doubt many interesting experiments might be made, as to the possibility of modifying the varieties of the salmo, by impregnating the ova of one species with the seminal fluid of another. With fishes of other genera the task would be still more easy. Carp, perch, and pike, deposit their ova in still water in spring and summer, when it is supplied with air by the growth of vegetables: and it is to the leaves of plants, which afford a continual supply of oxygen to the water, that the impregnated eggs usually adhere; so that researches of this kind might be conducted within doors in close vessels, filled with plants, exposed to the sun. I have myself kept minnows and sticklebacks alive for many months in the same confined quantity of water, containing a few confervæ; and their ova and melt increased in the same manner, as if they had been in their natural situation.

Orn.—I conclude from your statements, Halieus, that nothing more is required for the production of fishes from impregnated eggs, than a constant supply of water of a certain temperature furnished with air; and of course the same principles will apply to fishes of the sea.

Hal.—There can be no doubt of it: and fishes in spawning time always approach great shallows, or shores covered with weeds, that, in the process of their growth, under the influence of the sunshine, constantly supply pure air to the water in contact with them.