SALMON OF THE YUKON RIVER. 



325 



The nuclear area of the scales of Yukon king salmon is of extremely small size and 

 contains correspondingly few rings. Undoubtedly this indicates comparatively very 

 small size for the fingerlings at the time of their downward migration. Our table 

 indicates an average size for migrating fingerlings of 2% inches. This is based on the 

 assumption that the fry are 1% inches long when the first scale ring is formed. If, as 

 seems more probable, they are slightly longer than this, our computed lengths of 

 migrating fingerlings would be correspondingly increased but could not much exceed 

 3 inches. The greatest length indicated on any scale examined by us is 4^ inches. 

 The number of nuclear rings for each year class is as follows: 



Tablk 6. — Number of Nuclear Rings, Yukon King Salmon, 1920. 



THE CHUM OR DOG SALMON (Oncorhynchus keta). 



The chum or dog salmon of the Yukon does not differ from other races of chums that 

 frequent streams in the more southern portion of its range either in external appearance 

 or in any of the structural peculiarities that distinguish this species. As is the case else- 

 where, individuals captured in the sea or those that enter streams well in advance of the 

 spawning period are symmetrical silvery fish, easily mistaken at a glance for the sockeye 

 salmon. The discoloration of the skin and the elongation of the jaws, which are later 

 provided with greatlv enlarged teeth, are of universal occurrence in this species (as, 

 indeed, in all of the species of Pacific salmon) when sexual maturity is approached. In 

 shorter streams that are colonized by chum salmon, the fish delay entrance until nearly 

 ripe and when first seen have already lost their silvery livery. But in the Yukon, this 

 species penetrates to spawning gravels in the far upper reaches of the river, and it pop- 

 ulates as well practically all the tributaries in the middle and the lower course of the 

 stream. We find, accordingly, among the chums entering the mouth of the river a 

 mixture of colonies, some of which are bound for the headwaters, in reaching which they 

 will spend six weeks or more, and others that have not far to go. It is undoubtedly for 

 this reason that the entering fish vary so widely in different portions of the run in the 

 extent of development of those striking characteristics that accompany maturity. 



During the season of 1920 all the early chums were of bright silvery color and had 

 abundant oil and a pinkish flesh that turned a deeper red on drying. But in a short 

 time changes appeared, even at the mouth of the river. At first occasional individuals, 

 usually males, appeared in a more advanced stage, with brightly colored bars on the sides 

 of the body and with long hooked jaws. When these were first observed they stood out 

 conspicuously from their fellows, which were still in the "silver" stage. By the last of 

 June these seasonal changes had become obvious in the great mass of chums then 

 running. It was the rule for the males to exhibit elongated jaws, provided with canine 

 teeth, and to show the beginnings of the bright cross-bars that characterize the spawning 

 males of this species. It might be thought that this development would continue un- 



