SALMON OF THE YUKON RIVER. 

 Table 9. — Yukon Chum Salmon, 1920, Grouped by Age, Sex, and Weight. 



329 



The length-weight relationship, indicated in Table 10, is given without reference 

 to age. The average weight of all males and, separately, of all females that have the same 

 length is stated. According to this table, females average slightly lighter than males of 

 equal length, those from 23 to 28 inches in length averaging 97 per cent of the correspond- 

 ing males. The reverse of this might have seemed reasonable because of the slightly 

 lengthened jaws in the males. 



Table 10. — Average Weights, by Units of Length, Yukon Chums, 1920. 



GROWTH AND SCALE READINGS. 



In Figures 290 to 298 are presented photographs of a series of scales of Yukon chum 

 salmon that include representatives of all the year classes found in our collection. All 

 of these agree in belonging to the sea type — that is, the scales were wholly formed in 

 the sea, the fry having passed out of the river at a very early age, before even the nuclear 

 plate and the first scale ring had been formed. 



The Yukon chums agree in this respect with their southern relatives. All leave their 

 native streams as soon as the yolk is absorbed and they are free swimming. In more 

 southern districts this seaward migration is easily accomplished. The eggs are laid 

 in gravels not far removed from the tides, and the young, when free, easily drop down 

 with the current to the shore line. The case is less simple with the Yukon fish, many 

 of which have 2,000 miles or more to cover at a period when they average only 1% inches 

 long. No information is available concerning the dates on which this migration is 

 effected. Observations farther south indicate a very early descent to the sea in the 

 spring of the year. It is not known, however, to what extent hatching of the eggs and 

 development of the young on the Yukon are retarded by the very low temperatures to 



