But this period of melting snow and ice only lasted for a while and the cold boreal water grad- 

 ually receded to the north in the face of the encroachments of equatorial waters. The salmon 

 had to become adapted to these new conditions in the temperate seas. The char (Salvelinus 

 fonlinalis) no longer ventured into salt water with its abundance of food organisms but became 

 landlocked in the cold waters of Alpine and American lakes. But while they have become 

 purely freshwater fishes in our regions, they have retained their anadromous habits in the 

 rivers of the Arctic Ocean and on the eastern coasts of Canada and the United States. Polar 

 features are kept in being in these latter regions by the drift of the Labrador Current which 

 originates from the pack-ice. 



The Atlantic salmon (Salino salar) became more highly adapted, for it still leads an anadro- 

 mous existence down to the northern coast of Spain, although at Cape Finisterre it is no longer 

 to be found. So it would appear to tolerate waters with salinities from 34 to 35 "/pj but it cannot 

 endure those from 35..') to 36 °lg„. It has completely deserted the Mediterranean. However, 

 unwary fishes which had ventured into this sea at the end of the great glacial period have left 

 some descendants in the lakes of Italy, Albania and even of Algeria. Fiut like the Alpine char 

 they have given uj> their incursions into the sea — into waters that are too warm and salt y. Undoubt- 

 edly the Mediterranean is unsuitable for salmonid biology, for several attempts to acclimatise 

 the Pacific American salmon, the quinnat, in the Rhone have not met with success. Other 

 colonies of non-migrating salmon are still to be found in the Swedish lake Wener and in North 

 .Vmerica, where the " sebago " and l,he " ouananii'he ", belonging to the Allanlic species, no 

 longer go down to the sea. 



Trout have had imich the same kind of history. The salmon-trout or sea-trout (Salnto 

 bulla), which in our latitudes is si ill anadromous. has retained the iial)its of its ancestors. From 

 this species came river trout (Sdhno jario). This trout has many forms, which are named 

 after the localities or streams where they live, but all belong to the one species. Their colour- 

 ation tends to match that of their surroundings. Those living in clear waters are spotted, 

 while on gravel bottoms the black markings are X-shaped. Fishes living on mud have darker 

 colours, which become black in caves. In the peat-bogs of Ireland the body is dusky and takes 

 on a heavy appearance. The colours become whitish in clay-laden rivers. But a transplanta- 

 tion is quite sufHiient to change all these charai'lers. .\. young sea-trout cannot be distinguished 

 from a young river trout and in 1907 Cligny showed they belonged to the one species. 



The Pacific salmon are still anadromous fishes. Five species can be recognised and these 

 are grouped into the subgenus (turorliiinchits on account of the greater mnnber of rays in the 

 dorsal tin. They are of considerable importance in the canning industries of Canada and the 

 United States. The colour of the flesh varies from red to a very pale pink, in the following 

 order : The quinnat, chinook or king salmon (Sdlnio IscIkiii'iiIscIki) : the sockeye, blue-black or 

 red salmon ^^S. nerka) ; the coho or silver salmon (S. Icisnlrli) ; the humpback or pink salmon 

 (S. gorbiischa) and the chum or dog-salmon (S. held). There is another species of Salino 

 living in the same regions, the steelhead trout (Sdlino (jdirdncri). which is also anadromous. 

 In lakes and rivers are other trout which do not return to the sea and of these the best known 

 is the rainbow trout (S. iridcns). 



The coregonids or whiletlshes, with comjiressed bodies reminiscent of those of shads have, 

 like char, a dilTerent biology according to latitude. The ventiace ((^oirtjoniis albiila) and the 

 lavaret (C. fera) are anadromous in Siberia and Scandinavia, but do not leave the deep fresh 

 waters of the Alps. The "common whitefish " ((J. diipeijormis) is confined to the Great Lakes 

 of America, but keeping something of the migratory habits of its ancestors, it leaves the deeper 

 parts of the lakes to spawn in sizeable shoals in shallow waters. 



The umbers or graylings of northern lands are among the most beautiful of the salmonids, 

 with a blue-green back, silvery fianks and a very large dorsal fin looking like a butterfly's wing. 

 This fin is purple in colour and has four rows of dark spots. These splendid fishes live in fresh 

 waters and they leap in the waterfalls of Scandinavian rivers in the spring, when on their way 

 to spawn upon the break-up of the ice. 



In these ways, all of t he salmonids - and they are still e\<)l\ ing — have, as it were, a biolog- 

 ical memory of the great glacial period which presided over their dispersal. And according 



37 



