1904-5.] Salmon in transition from Smolt to Grilse. 
395 
A Specimen of the Salmon in transition from the Smolt 
to the Grilse Stage. By W. L. Calderwood. (With 
Two Plates.) 
(Read December 19, 1904.) 
In October of this year (1904) there came into my hands a very 
interesting specimen of a young salmon. In round terms, the fish 
is 1 pound in weight and nearly 14 inches long. 
Up to the present time very little is known of the life history 
of the salmon during the transition from the stage of the smolt 
leaving the river, a fish of about 3 ounces, and that of the grilse 
returning to the river for the first time, a fish of 3, 6, or 9 
pounds in weight. 
A great deal of speculation has arisen as to the length of time 
occupied in this change, and most of the earlier writers have 
upheld the view that three or four months is sufficient, or, in other 
words, that the smolt of May or June is the grilse which appears 
in the summer of the same year. This view was mainly based, I 
believe, upon results which it was held had been obtained by mark- 
ing the fish by the mutilation or removal of the adipose fin. But 
since the adipose fin grows again to a greater or less extent, a con- 
siderable amount of uncertainty in recognising the recaptures was 
inevitable ; and I may add that recent observations made in 
Devonshire by the instructions of the Duke of Bedford, in which 
the marking was carried on in precisely the same manner, have 
been held to show that the grilse do not come back the same 
season, or within four months or so of the seaward smolt migration. 
All the recaptured grilse obtained in the Tavy were caught in 
the succeeding season. If any still remained in the sea and 
ascended during the second season succeeding, they would probably 
be unrecognisable. Further, the few smolts which have been 
recaptured after being marked by the attachment of a foreign 
body of some sort — I refer to those of the Early Tweed Experi- 
ments — have been got as grilse in the summer of the year after 
that in which they were marked. 
