130 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
COMPARISON OF THE SEXES. 
Relative changes in fresh ivater . — Before entering the fresh water the two 
sexes are identical in appearance. With the fall run the sex can often be distin- 
guished by the external appearance in specimens taken at the head of Suisun Bay, 
and it can always be distinguished in normal specimens by the time of their arrival at 
the spawning-grounds. Soon after entering fresh water several cartilaginous teeth 
appear in the front of the jaws of the males, and at the same time the jaws begin to 
grow longer. By the time the males reach the spawning-grounds the .jaws are much 
prolonged and hooked, and the teeth have grown to be large and solid canines; the 
body becomes deep and slab-sided, and the color usually more or less reddish. 
The principal change in the females lies in the diminution in the muscular tissue 
of the back and sides and in the distension of the abdominal walls on account of the 
development of the ova. Their color is usually more or less olive. After spawning 
the female is as thin as the male, but the jaws are not prolonged. 
The following illustrations indicate the changes better than descriptions : 
salmon, known as adult and grilse, that are found in the headwaters. Those here 
shown were nearly of the same length, though it is very rare to find as small a speci- 
men as the upper that has the adult form, and the lower was a rather large grilse: 
(See also plate 13, photographs of these same specimens.) 
The differences are obvious. The grilse simply fails to develop the character- 
istics of the breeding male, viz, the prolonged and hooked jaws, the large, hooked 
teeth, the deep, slab-sided body, and red color, and retains its salt-water appearance 
except in the loss of flesh. Grilse weigh from a half pound up, and intergrade with 
the adults both in weight and appearance; specimens with a length of 90 centimeters 
(35.5 inches) are occasionally found. I have seen two, which, from their olive color, 
could be distinguished as sea-run fishes, that were only 13 inches long. 
At Battle Creek fishery in 1900 the males were nearly all grilse, though there 
were almost as many of the adult males as there were females. The great prepon- 
Head of male salmon taken at Sacramento, September 5, 1900, 
showing the beginning of the jaw prolongation and canine 
teeth while yet cartilaginous. 
Head of female salmon taken at Sacramento 
September 5, 1900. This is also the head of 
the male in salt water. 
Two forms of males . — The illustrations on page 131 show the two forms of male 
