28 THE LIFE OF THE SALMON 
examination of sea caught salmon that herring and 
sand-eels seem to be their favourite foods. Now 
Herr Dahl has caught the growing smolts out at sea 
as they follow the shoals of their marine “supporters.” 
Next we have to deal with the time spent by the 
smolt in becoming a grilse before fresh water is 
revisited. 
From the seasons at which young fish in the 
transition stage have been taken, as compared to 
their size and weight, as also from the early Tweed 
returns, which must be regarded as reliable although 
so few in number, it seemed certain that the view 
very commonly held in Scotland on this point was 
erroneous, and that a full year in the sea elapses 
between the time when the smolt leaves the river 
and the time when as a grilse it returns. 
To obtain direct and reliable evidence on this point 
it was desirable to mark a large number of smolts 
by the attachment of some foreign substance. The 
presence of small spring fish in many of our rivers at 
the same time raised a problem which seemed likely 
to be solved at the same time and by the same means. 
In 1903 I had experimented at Fochabers Rearing 
Ponds as to a suitable mark for attachment. A 
small silver disc was attached by a split silver pin to 
the gill cover so that it lay flat upon the outside 
and yet gave room for the necessary expansion. 
The delicate bones of the smolt’s operculum could 
not, however, stand the strain of this mark, and in 
a few weeks very many of the marked fish had 
dropped both disc and pin by the rupture of the 
gill cover. 
