240 BRITISH FRESH-WATER FISHES 



taken are eight or nine miles o{ winding, tidal, muddy estuary. 

 The migration, therefore, must have been deliberate, voluntary. 

 As for the cause of the movement, it does not seem very 

 obscure. The salmon, a native of fresh water, has acquired 

 the habit of resorting to the sea for food. It is imperative 

 that it should leave the sea for purposes of reproduction (salt 

 water having been proved! to be fatal to the vitality of salmon 

 ova), hence it has generally been assumed that the nisus 

 generativus is the only impulse to which the inland migration 

 is due. But the Research Committee have proved abundantly 

 that, while the act of spawning is restricted to a period varying 

 in different latitudes between the end of October and the end 

 of January, salmon leave the sea during every month in the 

 year, and with their ovaries and testes in every stage of 

 development. The cause which makes the fish leave the sea, 

 independently of the nisus generativus^ is that " 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." * When this return takes place in the 

 winter or spring months, it may well happen that the accu- 

 mulated nourishment, which is the source of energy in the fish, 

 becomes expended in the long interval which must elapse 

 before the spawning season, and resort is had to the feeding- 

 grounds for a fresh supply of energy to enable the salmon to 

 undertake the exhausting functions of reproduction. In short, 

 the salmon only goes to sea for one purpose, that of food ; t 

 but when that is accomplished, it hastens " home." 



Correct interpretation of the age of the fish in its successive 

 stages of parr, smolt, grilse, and salmon is of great importance 



* Life-History of the Salmon, p. 169. 



t This has received a striking illustration in late years from the 

 behaviour of English fresh-water trout introduced to the rivers of New 

 Zealand and Tasmania. They have acquired a sea-going habit precisely 

 analogous to our salmon, and are taken in nets at sea of great size and 

 with a silvery marine livery. 



