Popular Science Monthly/Volume 4/November 1873/The Growth of Salmon

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Fig. 1. Fig. 2.
PSM V04 D037 Salmon eggs.jpg
New-laid Salmon Egg. Egg after about 35 Days.

Fig. 1 represents the egg––natural size––of a salmon just laid. Each female salmon carries, on an average, 800 to 900 of such eggs to every pound of her weight. They are generally of a pinky opal color, elastic to the touch, covered with a soft, horny membrane, with a minute opening through which a particle of the spawn—the soft roe—of the male fish enters, and the egg is fertilized. From this moment the young fish gradually develops, under the influence of the cold running water. At the end of about thirty-five days—more or less, according to the temperature, which should be about 40°—two little black specks can be seen, as at Fig. 2, which are .the eyes of the embryo fish; the vertebræ may be discerned in the form of a faint red line, and a small red globule, which shortly afterward appears, represents the vital organs of the embryo fish.

At the end of about 80 to 100 days from the deposition of the egg the fish has so increased in size that it bursts the "shell" and makes its début in the form represented at Fig. 3. The sac or umbilical vesicle

Fig. 3.
PSM V04 D038 Fish coming out of egg.jpg
Fish coming out of Egg.

attached to the under part of the fish contains a secretion resembling albumen, which affords nourishment to the infant fish for the first six weeks or so of its existence. By that time it is quite absorbed, and for the first time we see a perfect fish, Fig. 4, with its fins, gills, and scales, which have hitherto been present, but imperceptible except under the microscope, fully formed: and now the young salmon begins to feed. His growth is not very rapid for some months, the lines a, b, c, representing the average length of a salmon at two, three, and four months old. At two years old the fish is about nine to twelve inches long.

As soon as they are large enough and strong enough, the "smolts," as they are now called, descend to the sea; here they are lost sight of until they return up the river as "grilse." The actual duration of their stay in the sea is not yet known, from one to three years being variously estimated as the probable length of time. The object of this migration to the sea is to find the food which is necessary for the secretion of the fat of the fish, who lives on the Infusoria, smaller fish and crustaceans, and the spawn of sea-fish, which abound in our seas. The length of their stay in salt-water is regulated, no doubt, by various circumstances, and is not the same in every case. When the salmon has laid up a sufficient supply of fat in its body and on its pyloric appendages, which are a wonderful provision of Nature for the secretion of an amount of fat sufficient to supply it during its sojourn in fresh waters, it ascends the river, its roe or spawn developing as it ascends; till, about Christmas-time, or sometimes earlier, it reaches the shallow head-streams of the river, in the gravelly beds of which it deposits its eggs, returning immediately afterward to the sea, no longer in the bright, plump, muscular condition in which it ascended, but a lean, lank, ugly, wounded beast, which one would hardly recognize as Salmo salar. Fig. 5 represents the head of a "kelt," as those salmon are called which have newly spawned. The curved projection, or hook, on the lower. jaw, is a cartilaginous membrane, the use of which nobody knows. The fish is in a very weakly condition, as his fat is gone, and he perhaps assumes this appearance to frighten other animals, which might otherwise be tempted to attack him. The drawing is taken from the photograph of a salmon, weighing twenty pounds, which was found dead on the banks of one of our Welsh rivers.

Fig. 4.
PSM V04 D039 Six week old salmon.jpg
Young Salmon Six Weeks old.
a, b, c, size of salmon at two, three, and four months respectively.

This fish, had it survived, would have returned to sea, recovered its fat, and presently come back worth £2 or £3, whereas, by dying in this condition, it was worth nothing. It had, however, done its duty by depositing perhaps 16,000 eggs. Only a very small percent-age, however, of the eggs laid ever become adult fish. Floods wash them out of their gravel nests; ducks, and other birds, eat them; beetles and various insects attack them; they are smothered with mud, or left high and dry on the shore; the young fish are poisoned by pollutions, or diverted into mill-leats and canals, and so lost; trout eat them wholesale; in fact, the whole of their earliest existence is a very living death, and it is a wonder, with all the ordeals they have to pass through, that we have any salmon left. To kill them legitimately for food for ourselves is bad enough, and we ought to do all we can to protect them when young.

In the artificial system of breeding salmon the adult fish are caught just as they are on the spawning-beds, and the eggs taken from them; the ova and milt are properly mixed together, and the eggs placed in troughs of water so arranged as to imitate as closely as possible the natural conditions necessary for the development and growth of the fish. Properly managed, ninety per cent, of the eggs will hatch out: the young fish are turned into the river when they are about a year old; if they can be kept two years in tanks large enough, with plenty of running water, so much the better for the prospect of their reaching the sea in safety.

Fig. 5.
PSM V04 D040 Head of a kelt.jpg
Head of a Kelt.

When we can make up our minds to keep all our pollutions out of our rivers, and build "salmon-ladders" over all the wears, so as to give the fish a fair field, and enable them to run up-stream unimpeded, then, and then only, shall we see salmon as plentiful throughout the country as it is said to have been in the North a century ago, when apprentices are reputed to have stipulated in their indentures that they should be fed on salmon not more than three days a week. Without this, all our efforts to stock our barren rivers with artificially-bred fry will prove comparatively unavailing.—Nature.
