34 
BULLETIN OF THE BUREAU OF FISHERIES 
In the other distribution the eggs are distinctly larger, the log D ranging from 
about 0.34 at the beginning of the season to about 0.80 by the end of the season. 
This increase in size during the season is conspicuous and is in marked contrast to 
the condition observed in the group of immature fish. So far as the available data 
show, the segregation of the fish of this age group into mature and immature groups 
may be accomplished with very satisfactory accuracy by means of a study of the 
egg sizes. The percentage of mature and immature fish taken at different times 
during the season is treated on page 40 ff . Figure 28 shows a scale from one of the 
immature fish of this age group, which was taken on July 2, and Figure 29 one of 
the mature fish taken on the same date. 
It will be noticed in the collection made inside the river on July 7, 1919, that 
one immature fish of this age group was taken. It will presently be shown that there 
were other immature fish taken at the same time which belonged to other age 
groups — 2-year fish with ocean nuclei and 3-year fish with stream nuclei. The 
explanation of this occurrence and other associated facts is interesting in its possible 
bearing on the habits of the chinook sahnon. 
These immature fish observed inside the river were taken by seine on Sand 
Island, a low, level, sandy island several miles in length and about 1 mile in width, 
which is situated in the estuary of the Columbia River, only a mile or so inside the 
mouth of the river. The seining grounds are toward the upper end of the island, 
approximately 5 miles from the river's mouth. On the Washington side of Sand 
Island is Baker Bay, a broad, shallow stretch of water in which are situated most of 
the salmon traps in use on the Columbia River — about 200 in number. Under 
ordinary circumstances the water of this lower part of the Columbia estuary is fresh 
or only slightly brackish, depending upon the tides and the flow of the river. At the 
time of the "spring" tides, however, especially when the river is at a low stage, 
the lovv^er part of the estuary, up for a distance of several miles, becomes very brack- 
ish, approximating the salinity of pure sea water. On such occasions it frequently 
happens that small, immature fish are taken, both in the seines operating on Sand 
Island and in the traps in Baker Bay. It is generally beheved among the fishermen 
of the lower Columbia River, and some statistical evidence is available which woidd 
indicate that the belief is well founded, that the salmon enter the river in greater 
numbers on the spring than on the neap tides. Apparently the immature fish 
to some extent join those that are matming in a general movement into the river 
on these high tides, but do not pass beyond the upper limit of the approximately 
pure sea water. It is at these times that immature fish are taken on Sand Island 
and in Baker Bay. Presumably the immature fish return to the ocean again as the 
water freshens with the receding of the tide, while the mature individuals remain, 
in large measure, to continue their spawning migration. 
In this connection it is interesting to note that the stomachs of the immature 
fish thus taken inside the river are frequently filled with food, usually in an advanced 
stage of digestion. This is in contrast to the condition in the mature fish, the stom- 
achs of which have never been observed to contain any food.* 
' The writer has described (Rich, 1921) an instance, apparently unique, in which the stomachs of mature salmon taken in 
fresh water contained food. In the case described chinook salmon were found feeding on eulachon ( ThaUichthys pacificus) in the 
Cowlitz River, a tributary of the Columbia, some 70 miles above the ocean. 
