The young fish living in fresh-water, like other young Salmon idae, have a series of dark bars along the side and are known as parr (q.v.) ; when about to migrate they become silvery and are termed smolts. The smolts generally enter the sea about May; in the sea they grow rapidly, and after their first winter, when they are termed grilse, may weigh several pounds, and towards the end of their second summer 10 lb. or even more. Grilse may return to the river from which they came in order to spawn, but many salmon do not spawn as grilse, and may spend several years in the sea before spawning. After spawning the salmon is termed a kelt; these may return to the sea and recover, but in many rivers a large number die after spawning. Probably few salmon live more than ten years or spawn more than three or four times; large fish—a weight of more than 8o lb. may be obtained—are not necessarily very old ; they are fish that have spent several years in the sea without spawning. The method of scale-reading has proved invaluable in elucidating the life-history of the salmon ; the concentric lines of growth are close together on the central part of the scale formed during the life of the parr in fresh-water ; the more rapid growth in the sea is marked by more rings farther apart, in which each winter is shown by a few rings, incomplete and closer together; if the fish enters fresh water to breed the scale wears away and gets an irregular edge, and if it recovers and gets back to the sea, the line of this former edge is seen inside the rings formed later, and is termed a spawn ing mark.
Fresh-water Colonies.--Salmon may form permanent fresh water colonies. The famous ouananiche of Quebec is a non migratory salmon, a small but very active fish, rarely reaching 8 lb. The Sebago salmon inhabits the lakes of Maine. In Europe there are fresh-water salmon in Lakes Ladoga and Wenern. Lake Wenern is now cut off from the sea by inaccessible falls, but there can be no doubt that formerly salmon entered it froiin the sea to spawn in its tributaries, and that some of the smolts' descending to the lake found it a sufficiently good substitute for the sea to stay there, and so founded a lacustrine race. In Wenern the salmon feed on fish and grow to a good size, but the fresh-water colony recently described by K. Dall from the Byglandsfiord in Norway (Salmon and Trout Mag. 1928) is of quite another kind, for the fish feed on minute crustacea and do not grow to more than a foot in length ; they are overgrown parr, probably originally tempted to stay by the abundance of parr-food. This discovery is interesting, for a similar explanation has been given of the presence in the rivers of Dalmatia of Salmo obtusirostris, a fish reaching a foot in length, very like a salmon parr, but with a smaller mouth and more numerous gill-rakers, and feeding mainly on ephemerid larvae. This may be derived from a colony of
salmon-parr formed in Glacial times, when salmon would have entered the Mediterranean.
The third Atlantic species of Salmo is the trout, S. trutta. (See TROUT.) In the north Pacific there are about eight species of this genus, which form a natural group, distinguished from the salmon and trout of the Atlantic by the structure of the skull, the ethmoid bone in the Atlantic species being large and blunt, in the Pacific ones smaller and pointed. The steelhead (S. gaird neri) of the Pacific coast of North America forms numerous permanent fresh-water colonies, and there is a related species in the rivers of Formosa. The other five species are found on both coasts of the north Pacific ; in them the breeding males have the jaws greatly prolonged and hooked, and it is believed that all the fish die after spawning. The largest and most valuable species is the quinnat (S. quinnat) which ascends rivers for long distances, and has parr that live in fresh-water. The blueback or redfish (S. nerka) also ascends streams, but the other species spawn at no great distance from the sea, to which the young fish soon make their way. The quinnat appears to have been successfully introduced into New Zealand, as has the trout (S. trutta), but attempts to introduce S. salar have been successful only in rivers in the extreme south, where the fish grow to be a fair size and breed, but feed in the lakes and do not go to sea.
The Salmonidae are to be regarded primarily as a group of northern marine fishes that breed in fresh-water, often forming colonies in lakes or rivers, and now including a number of species and even some genera that never go to the sea. An interesting feature of their distribution is the presence of species of certain genera, Salmo, Salvelinus, Coregonus, in lakes and rivers far to the south of their present range in the sea, which they must have reached from the sea in glacial times.
Mag. Nat. Hist. (1914). (C. T. R.)