Body oblong or moderately elongate, covered with cycloid, in scales of varying size. Head naked. Mouth terminal or somewhat inferior, varying considerably among the different species, those having the mouth largest usually having also the strongest teeth. Maxillary provided with a supplemental bone, and forming the lateral margin of the upper jaw. Pseudobranchiæ present. Gill-rakers varying with the species. Opercula complete. No barbels. Dorsal fin of moderate length, placed near the middle of the length of the body. Adipose fin well developed. Caudal fin forked. Anal fin moderate or rather long. Ventral fins nearly median in position. Pectoral fins inserted low. Lateral line present. Outline of belly rounded. Vertebræ in large number, usually about sixty.
The stomach in all the Salmonidæ is siphonal, and at the pylorus are many (15 to 200) comparatively large pyloric cœca. The air-bladder is large. The eggs are usually much larger than in fishes generally, and the ovaries are without special duct, the ova falling into the cavity of the abdomen before exclusion. The large size of the eggs, their lack of adhesiveness, and the readiness with which they may be impregnated, render the Salmonidæ peculiarly adapted for artificial culture.
The Salmonidæ are peculiar to the north temperate and Arctic regions, and within this range they are almost equally abundant wherever suitable waters occur. Some of the species, especially the larger ones, are marine and anadromous, living and growing in the sea, and ascending fresh waters to spawn. Still others live in running brooks, entering lakes or the sea when occasion serves, but not habitually doing so. Still others are lake fishes, approaching the shore or entering brooks in the spawning season, at other times retiring to waters of considerable depth. Some of them are active, voracious, and gamy, while others are comparatively defenseless and will not take the hook. They are divisible into ten easily recognized genera: Coregonus, Argyrosomus, Brachymystax, Stenodus, Oncorhynchus, Salmo, Hucho, Cristivomer, Salvelinus, and Plecoglossus.
Fragments of fossil trout, very imperfectly known, are recorded chiefly from Pleistocene deposits of Idaho, under the name of Rhabdofario lacustris. We have also received from Dr. John C. Merriam, from ferruginous sands of the same region, several fragments of jaws of salmon, in the hook-nosed condition, with enlarged teeth, showing that the present salmon-runs have been in operation for many thousands of years. Most other fragments hitherto referred to Salmonidæ belong to some other kind of fish.
Coregonus, the Whitefish.—The genus Coregonus, which includes the various species known in America as lake whitefish, is distinguishable in general by the small size of its mouth, the weakness of its teeth, and the large size of its scales. The teeth, especially, are either reduced to slight asperities, or else are altogether wanting. The species reach a length of one to three feet. With scarcely an exception they inhabit clear lakes, and rarely enter streams except to spawn. In far northern regions they often descend to the sea; but in the latitude of the United States this is never possible for them, as they are unable to endure warm or impure water. They seldom take the hook, and rarely feed on other fishes. Numerous local varieties characterize the lakes of Scandinavia, Scotland, and Arctic Asia and America. Largest and most desirable of all these as a food-fish is the common whitefish of the Great Lakes (Coregonus clupeiformis), with its allies or variants in the Mackenzie and Yukon.
The species of Coregonus differ from each other in the form and size of the mouth, in the form of the body, and in the development of the gill-rakers.
Coregonus oxyrhynchus—the Schnäbel of Holland, Germany, and Scandinavia—has the mouth very small, the sharp snout projecting far beyond it. No species similar to this is found in America.
The Rocky Mountain whitefish (Coregonus williamsoni) has also a small mouth and projecting snout, but the latter is blunter and much shorter than in C. oxyrhynchus. This is a small species abounding everywhere in the clear lakes and streams of the Rocky Mountains and the Sierra Nevada, from Colorado to Vancouver Island. It is a handsome fish and excellent as food.
Fig. 49.—Rocky Mountain Whitefish, Coregonus williamsoni Girard.