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TROUT (Salmo trutta), a fish closely related to the salmon. Most modern ichthyologists agree in regarding the various North European forms of trout, whether migratory or not, as varieties or races of a highly variable and plastic species, to be distinguished from the sahnon by a few more or less constant characters, the most readily ascertainable of which resides in the smaller scales on the back of the caudal region of the body, these being 14 to 16 (rarely 13) in an oblique series between the posterior border of the adipose fin and the lateral line, and in the greater length of the folded anal fin as compared to the depth of the caudal peduncle. The gill-rakers are also usually fewer, 16 to 18 on the anterior bronchial arch. The young may be distinguished from salmon-parr by the greater length of the upper jaw, the maxillary bone extending beyond the vertical of the centre of the eye, and in specimens 6 in. long often to below the posterior of the eye. The young are brown or olive above, silvery or golden below, with more or less numerous black and red spots in addition to the parr marks, and, contrary to what is observed in the salmon, black spots are usually present below the lateral line. Except for the gradual disappearance of the parr marks, this coloration is retained in the form known as the brook trout or brown trout (S. fario), which is non-migratory, and varies much in size according to the waters it inhabits, in some brooks not growing to more than 8 in., whilst in larger rivers and lakes it may attain a weight of 20 ℔ or more. The coloration of the young is more strongly departed from in the races known as sea trout (S. trutta) and sewing (S. eriox or cambricus), anadromous forms resembling the salmon in habits, and assuming in the sea a silvery coat, with, however, as a rule, more black spots on the sides below the lateral line.

The principal British races of trout are the following: the northern sea trout (S. trutta, sensu stricto), silvery, losing the teeth on the shaft of the vomer in the adult, and migratory like the salmon; the southern sea trout (S. eriox or cambricus), similar to the preceding, but with the hind margin of the gill cover more or less produced, the lower bone (suboperculum) projecting beyond the end of the upper (operculum); the brown trout (S. fario), non-migratory, usually retaining the teeth on the shaft of the vomer, brown or olive with black and red spots, rarely more silvery, with numerous black spots; the Lochleven trout (S. levenensis), distinguished from the preceding by a more silvery coloration, frequent. absence of red spots and a pink or red Hesh; the estuary trout (S. gillivensis and S. orcadensis), large brown trout living in salt water without assuming the silvery coloration; the Gillaroo trout (S. stomachicus), in which the membranes of the stomach are conspicuously thicker than in the other trout, more so in adult examples than in young ones. But all these forms are ill-defined and subject to such variations when transported from one locality to another as to render their recognition a matter of insuperable difficulty. The instability of the characters on which S. levenensis is based has been conclusively shown by the experiments conducted by Sir James Maitland at Howietoun. Large specimens of migratory trout are often designated as bull-trout, but no definition has ever been given by which this form could be established, even as a race.

Other European varieties are the trout of the Lake of Geneva (S. lemanus), of the Lake of Garda (S. carpio), of Dalmatia (S. dentex), of Hungary (S. microlepis), of the Caspian Sea (S. caspius), &c. The size of trout varies much according to the waters in which they live, the anadromous forms nearly equalling the salmon in this respect, specimens of over 4 ft. and weighing up to 50 ℔ being on record.

The habitat of S. trutta extends over the whole of Europe, the Atlas of Morocco and Algeria, Transcaucasia, Asia Minor and northern Persia. By the agency of man the species has been thoroughly established in Tasmania and New Zealand, where it thrives in an extraordinary manner, and attains a very large size.

Closely allied species are found in North America, west of the Rocky Mountains, the best known being the rainbow trout (S. irideus or shasta), which has been introduced into many parts of Europe as well as the eastern states of North America, New Zealand and South Africa. It is more hardy than the English trout, and accommodates itself in almost stagnant waters, and has thus proved a success in many ponds which were regarded as fit for coarse fish only; but in many places it has caused disappointment by going down to the sea, whence it is not known ever to return. It is a handsome trout, bluish or purplish above, silvery or golden below, more or less profusely spotted with black on the body and uns, and with an orange or red lateral band. Its range extends from Alaska to North Mexico. The rainbow trout merges into a larger form, S. gairdneri, which resembles the British sea trout.

A remarkable European trout is the short-snouted trout, S. obtusirostris, a non-migratory species from Dalmatia, Herzegovina, Bosnia and Montenegro. It has a small mouth with a feeble dentition, resembling that of the grayling. A closely allied form, S. ohridanus, has recently been discovered in Macedonia.  (G. A. B.)