The change of scales (that is, the rapid reproduction of the worn part of the scales) coincides in the migratory species with their sojourn in the sea. The renovated scales give them a bright silvery appearance, most of the spots disappearing or being overlaid and hidden by the silvery scales. Now, some of the species, like S. fario, inhabit all the different waters indicated, even brackish water, and, in consequence, we find a great variation of colour in one and the same species; others are more restricted in their habitat, like S. salar, S. ferox, etc., and, therefore, their coloration may be more precisely defined.
With regard to size the various species do not present an equal amount of variation. Size appears to depend on the abundance of food and the extent of the water. Thus, the Salmon and the different kinds of great Lake-trout do not appear to vary considerably in size, because they find the same conditions in all the localities inhabited by them. A widely spread species, however, like S. fario, when it inhabits a small mountain pool with scanty food, may never exceed a weight of eight ounces, whilst in a large lake or river, where it finds an abundance and variety of food, it attains to a weight of fourteen or sixteen pounds. Such large River-trout are frequently named and described as Salmon-trout, Bull-trout, etc. Further, in Salmones, as in the majority of fishes and tailed Batrachians, there is an innate diversity of growth in individuals hatched from the same spawn. Some grow rapidly and normally, others more slowly, and some remain dwarfed and stationary at a certain stage of development.
The proportions of the various parts of the body to one another vary exceedingly in one and the same species. Beside the usual changes from the young to the sexually mature form observed in all fishes, the snout undergoes an extraordinary amount of alteration of shape. In the mature male the intermaxillaries and the mandible are produced in various degrees, and the latter is frequently more or less bent upwards. Hence the males have the snout much more pointed and produced, and the entire head longer, than the females; with the intermaxillary bone the teeth, with which it is armed, are also enlarged, sometimes to four times the size of those of the females. And if this development of the front part of the head happens to be going on while the individual is able to obtain only a scanty supply of food, the usual proportions of the head and trunk are so altered that the species is very difficult to recognise. Barren male fish approach the females in the proportions of the head and body, but hybrid fishes do not differ in this respect from their parents. The abundance or scarcity of food, and the disposition or indisposition of the Salmonoids to feed, are other causes affecting the growth or fulness of the various parts of the body. In well-fed fishes the head is proportionally not only smaller but also shorter, and vice versa.
The fins vary to a certain degree. The variation in the number of the rays is inconsiderable and of no value for specific distinction. The caudal fin undergoes considerable changes of form with age, and dependency upon the sexual development. Young specimens of all species have this fin more or less deeply excised, so that the young of a species which has the caudal emarginate throughout life, is distinguished by a deeper incision of the fin, from the young of a species which has it truncate in the adult state. As the individuals of a species do not all attain to maturity at the same age and at the same size, and as mature individuals generally have the caudal less deeply excised than immature ones of the same age and size, it is evident that the variations in the form of the caudal are considerable and numerous, and that it is a very misleading character if due regard be not paid to the age and sexual development of the fish. Further, species inhabiting rapid streams as well as still waters show considerable variations in the form and length of all the fins; those individuals which live in rapid streams, being in almost constant motion and wearing off the delicate extremities of the fins, have the fin-rays comparatively shorter and stouter, and the fins of a more rounded form, particularly at the corners, than individuals inhabiting ponds or lakes. Moreover, one and the same individual may pass a part of its life in a lake, and enter a river at certain periods, thus changing the form of its fins almost periodically.
Finally, to complete our enumeration of these variable characters, we must mention that in old males, during and after the spawning-season, the skin on the back becomes thickened and spongy, so that the scales are quite invisible, being imbedded in the skin.
After this cursory review of variable characters we pass on to those which are more constant, not subject to ready modification by external circumstances; and which, therefore, ought to be noticed in every description of a species of Salmo.
1. The form of the præoperculum of the adult fish. The præoperculum is composed of a vertical (posterior) and horizontal (lower) part (limb), both meeting at a more or less rounded angle. The development of the lower limb is a very constant character; in some species (as in the Salmon) it is long, in others (S. ferox, S. brachypoma) exceedingly short. The adjoining woodcuts will readily show this difference.
Fig. 287.—Præoperculum of A. Salmo salar; B. Salmo brachypoma.