28 
BULLETIN OF THE BUREAU OF FISHERIES 
The operation of these weirs each season is providing records of the size of the pink- 
salmon runs in each of the several streams on successive years. Hence it is possible, 
by means of these records, to make a study of the returns in numbers of adults 
resulting from the spawning of pink-salmon populations of varying size in an individ- 
ual stream. However, in making such a study it is essential to know the extent to 
which the pink-salmon fry, after reaching maturity, return to spawn in the streams 
from whence they came and likewise the age at which they mature and return 
to spawn. 
In life history studies of other species of Pacific salmon 3 it has been shown that 
their homing instinct and age at maturity can be determined by marldng the fry 
(removing two of their fins) as they leave a particular stream and then examining 
the subsequent runs of mature salmon for individuals bearing the marks. In this 
way the extent to which the adult salmon return to spawn in their parent stream 
and likewise the age at which they return may be readily determined. Two exper- 
iments of this type have been carried on with pink-salmon fry, one at the Federal 
hatchery on the Duckabush River in the State of Washington and one in Snake 
Creek at Olive Cove, Alaska. 
MARKING PINK-SALMON FRY 
The marking experiment at the Duckabush River hatchery was carried on in 
the spring of 1930 and is the first known attempt to mark pink-salmon fry at the time 
they normally migrate from the streams. The fry in this experiment, as well as in 
the Snake Creek marking experiment, were marked by the removal of both their dor- 
sal and adipose fins. This double mark was used because mature salmon are quite 
frequently found with one fin missing due to natural causes. 4 Furthermore it is 
not only necessary to remove 2 fins but likewise 2 fins that are widely separated on 
the body. In this way, the chances are remote of finding a mature salmon with an 
identical mark which is due to natural causes. 
The 36,000 fry that were marked in this experiment were taken from the hatchery 
tanks where they had previously hatched under artificial conditions. The dorsal 
and adipose fins were removed by clipping them close to the backs of the fry with a 
pair of straight-bladed finger-nail clippers. The two fry pictured in figure 1 show 
the appearance of normal and marked individuals. Owing to the small size of the 
fry it was necessary to select apparatus that would facilitate their marking and 
minimize their mortality during the marking operation. The apparatus shown in 
figure 2 was found to give satisfactory results. 
The shock due to the operation in removing the fins from the fry did not appre- 
ciably affect their mortality. In fact, the mortality of the fry during confinement in 
the tanks was practically the same in both the unmarked and marked stocks. Under 
normal conditions when there was an abundant supply of fresh water in the tanks 
the mortality during a 24-liour period did not exceed 2 percent of the total numbers 
in either of the stocks. However, if the supply of fresh water was reduced, the mor- 
tality would, in extreme cases, mount to 25 percent of their total number within a 
few hours. 
The marking experiment in Snake Creek at Olive Cove, Alaska, was carried on 
in the spring of 1931. The 50,000 fry that were marked in the experiment were 
s See the following references: Gilbert (1913) for coho salmon, Rich and Holmes (1928) for Chinook salmon, Snyder (1921 to 
1924) for Chinook salmon, and Foerster (1929) for soekeye salmon. 
1 See page 37 for a discussion of the errors resulting from the use of only single fin marks in salmon marking experiments. 
