240 BRITISH FRESH-WATER FISHES 
taken are eight or nine miles of 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 wisus 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. 
+ 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. 
