THE ATLANTIC SALMON 



The following extract from the Report of the Research Committee of the 

 Edinburgh College of Physicians suggests a probable explanation of the 

 motives of the seaward movement: 



*• It has been generally assumed that the passage of the salmon from 

 the sea to the river is due to the nisus generativus. In considering the 

 question it must be remembered that the Salmonidae are originally 

 fresh -water fish, and that the majority of the family spend their whole 

 life in the fresh water. Salmo salar and other allied species have 

 apparently acquired the habit of quitting their fresh -water home for 

 the sea in search of food, just as the frog leaves the water for the same 

 purpose. When, on the rich marine feeding grounds, as great a store 

 of nourishment as the body can carry has been accumulated, the fish 

 returns to its native element, and there performs the reproductive 

 act. That the immigration of the fish is not governed by the growth 

 of genitalia and by the nisus generativus is shown by the fact that 

 salmon are ascending the rivers throughout the whole year with their 

 genitalia in all stages of development."* 

 If further investigation should establish the fact that a seaward migration 

 of early running fish before the spawning season is a normal phenomenon in 

 all rivers to which fish of that class resort, then it will appear no extrava- 

 gant hypothesis that these early salmon, having fared sumptuously for 

 months or years in the salt water, arrive at a period of satiety and repletion, 

 when the system becomes so stuffed with nutriment as to become tem- 

 porarily incapable of assimilating any more; that the animal's appetite 

 declines and ceases, and that it returns home for a period of repose and 

 abstinence. After fasting for some weeks or months, during which the 

 muscle slowly, but steadily, parts with its fat, it feels the need of fresh 

 nourishment, appetite revives, and the fish drops down again to the sea, 

 which habit and experience have taught it to regard as the only source 

 of sufficient provender, there to restore its vigour and vitality before under- 

 taking the exhausting strain of reproduction. 



Fifty years ago, or thereby, the late Mr Dunbar came to some such 

 conclusion regarding the movements of salmon in the Thurso. Into that 

 river, which is of very moderate volume, there is a considerable migration 

 of salmon during the winter months. These fish are quite distinct from 

 the ordinary run of small spring fish in February and March. They are 

 much heavier, weighing from 15 lb. to upwards of 20 lb., and Mr Dunbar 



*Rtp9rt, 1898, p. 169. 



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