ENVIRONMENTAL CLASSIFICATION 



39 



migratory form. The land-locked salmon of Maine and New Hamp- 

 shire remains smaller and stouter than the form from which it is de- 

 rived. 22 The smelt Osmerus eperlanus of the North and Baltic seas 

 reaches a length of 30 cm., whereas in the Finnish lakes it grows only 

 to 15 cm. It has been established that the growth of the salmon, in 

 comparison with that of the brook trout, is notably accelerated by its 

 emigration to the sea 23 (Fig. la). In the bass genus Ambassis, all the 

 fresh-water species are smaller than the marine. 24 The causes of this 



cm. 

 100 



^-Slowly Growing 

 Brook Trout 



2K 3Vi 4% 



Years 



5Ya 6V2 



Fig. la. — Growth curves of Atlantic salmon (S. salar), salmon trout (S. trutta), 

 and European trout (<S. jario). After Dahl. 



phenomenon are as yet unknown; reduction in size is connected with 

 a departure from the optimum, but that is not an explanation. 



Another contrasting secondary character is the decrease in the 

 number of eggs in fresh-water animals. A notable enlargement of the 

 eggs by yolk-masses goes hand in hand with the decrease in number, 

 but whether this is a cause or an effect is unknown. There are indica- 

 tions that fresh water checks fertility. Thus the flounder, Pleuronectes 

 flesus, which frequently ascends the European rivers, does not become 

 sexually mature in them. A number of fishes entering the eastern 

 Baltic from the North Sea do not spawn there, Cottus bubalis and 

 Gobius niger, for example. 25 The oyster plantations of the river mouths 



