1905-6.] The Smolt to Grilse Stage of the Salmon. 
321 
Note on the Smolt to Grilse Stage of the Salmon, with 
Exhibition of a Marked Fish recaptured. By W. L. 
Calderwood. 
(MS. received July 13, 1906. Read July 13, 1906.) 
In tracing the various stages of the salmon’s growth, precise 
information has been difficult as to the length of time occupied 
between the first descent to the sea as a smolt and the first return 
to the river as a grilse. The difficulty of attaching to so small a 
fish as a salmon-smolt a suitable mark for subsequent identifica- 
tion has, in the main, been responsible for this lack. The average 
smolt is 5J to 6 inches long (14-15 cm.), and weighs from 1 to 
2 ounces. In the next stage with which we are familiar the fish 
may weigh 3 to even 10 lbs. It is clear, therefore, that any mark 
attached to the smolt must not only be sufficiently small and 
light for a fish only a few inches long to carry without incon- 
venience and injury, but must be adaptable to the rapid and 
great increase of growth which takes place in the sea. 
In the past a method of marking has been repeatedly resorted 
to which, though having the merit of simplicity, is not really 
reliable ; I refer to the method of fin-cutting, especially and 
commonly the cutting or the removal of the adipose fin. The famous 
Stormontfield experiments, reported upon by Buist in 1867, and 
repeated in other localities by other observers since that date, were 
conducted, so far as the study of migratory movements went, almost 
exclusively by fin-cutting. These experiments have been chiefly 
responsible for the belief that the smolt which enters the sea in 
greatest numbers in the spring, returns to fresh water as a grilse 
in two or three months, i.e. during the immediately succeeding 
summer. The Duke of Bedford’s experiments in Devonshire were 
conducted by fin-cutting, and are held to show a different con- 
clusion. Only three cases are, I think, on record in which the 
smolts have been marked by the attachment of a foreign 
substance, and recapture effected. These cases come from the 
PROC. ROY. SOC. EDIN., YOL. XXVI. 21 
