VII. IO 



LIFE CYCLE OF SALMO 



205 



given various common names (phinock, Severn, Loch Leven trout, 

 brook trout, &c). There can be no doubt that interesting genetic 

 differences between these forms exist, but they have not yet been 

 fully studied. The salmon are much less prone to form purely fresh- 

 water races, though such are known. 



During the breeding-season characteristic changes take place in the 

 fishes and differences between the sexes appear. In the salmon the 

 jaws become long, thin, and hooked, especially in the male. The 

 animals make pairs and the males fight with others that approach the 

 female. As the gonads ripen, the other 

 parts of the fish, which were well supplied 

 with fat at the beginning of the run, 

 become progressively more watery. 

 Finally, spawning takes place, the female 

 laying the eggs in a shallow trough (redd), 

 which she has 'cut' in the gravel by move- 

 ments of her tail, while the male sheds 

 sperms over them. She then covers the 

 eggs with gravel by further cutting move- 

 ments. The young male salmon (parr), 

 which have not yet been to the sea, may 

 become sexually mature. They accompany 

 the fully grown fish, hanging around the 

 cloacal region and shedding their sperms 

 at the same time as the large male. It is possible that this develop- 

 ment of a kind of third sex serves to increase the variability of the 

 population. The spent parr eat some of the eggs and they then proceed 

 to grow, migrate to the sea, and return later. 



Male trout will follow a spawning salmon and fertilize her eggs if 

 her own male is not looking. Hybrids formed in this way can develop, 

 but are said to be less fertile than the normal types; indeed, the males 

 are wholly sterile. 



After fertilization the salmon are very exhausted (known as kelts) ; 

 the males seldom return to the sea. The females, however, may recover 

 and after a period in the sea return to breed again, and this process 

 may be repeated several times. 



Very young trout or salmon are known as alevins or fry and remain 

 mostly among the stones (Fig. 124). When they emerge they are called 

 parr and have a number of characteristic parr-marks along their sides. 

 After two to four years spent as parr in fresh water salmon acquire a 

 silver colour and pass to the sea as smolts. Young salmon returning for 



Fig. 124. Three stages in the 



development of the salmon. 



I and II are alevins; III, parr. 



(From Norman.) 



