286 A HISTORY OF FISHES 



the sea to recover their condition, others miss a year or even 

 allow two or more years to elapse before the call of reproduction 

 once more urges them to enter fresh water. Few Salmon live 

 beyond eight or nine years, and it is exceedingly rare for any 

 individual to spawn more than three or four times in its life. 

 It is of interest to note that with few exceptions Salmon always 

 return to the same rivers from which they originally came, 

 but this powerful homing instinct often receives a check 

 nowadays owing to the poisonous chemical effluents poured 

 into some of our Salmon rivers by factories. In former times 

 the Thames was a famous Salmon river, but pollution of its 

 lower reaches made the ascent impossible, and the last fish 

 was captured here in about 1833. Every year, however, a 

 few Salmon make their appearance at the mouth of the Thames, 

 and there can be little doubt that were the water to become 

 miraculously purified these fish would run up once more and 

 spawn in the upper reaches. 



The Salmon of the Pacific coast of North America, a natural 

 group of five or six species (sub-genus Oncorhynchus) which 

 includes the famous Quinnat or King Salmon [S. (0.) ischaw- 

 ytscha], have somewhat similar breeding habits, although many 

 of the features of their spawning cycle are more accentuated. 

 The Quinnat spawns in November at the age of about four 

 years and at an average weight of 22 lbs., but the ascent of the 

 rivers by these fish commences in the previous spring. Those 

 individuals which run first have the greatest distance to travel, 

 and in the Yukon the spawning grounds are situated near 

 Caribou Crossing and Lake Bennett, a distance of no less 

 than 2250 miles from the sea. The Blue-back Salmon or 

 Red-fish [S. (0.) nerka] also runs in the spring, ascending the 

 rivers for 1500 miles or more, but the remaining species — the 

 Silver Salmon [S. (0.) milktschitsch], Dog Salmon [S. (0.) keta]. 

 Humpback Salmon [S. (0.) gorbuscha], and Masu [S. (0.) 

 masou] — all ascend the rivers in the autumn. The differences 

 between the sexes in the breeding season are very much more 

 marked than in our own species, the males of the Quinnat or 

 Blue-back being hurnp-backed, with sunken scales, much 

 enlarged, hooked, bent or twisted jaws, and huge dog-like teeth 

 (Fig. 104B). The reproductive act seems to be more exhausting 

 to these fishes, for after spawning the male and female drift 

 helplessly downstream tail foremost, and no fish, either male 

 or female, succeeds in regaining the sea. If the spawning 

 grounds lie far inland the bodies of the fish may be covered 



