320 BULLETIN OP THE bureau of FISHERIES. 



The run culminated quickly within a week after that date, then maintained itself at a 

 fair level for about three weeks, and was practically over by the close of the first week 

 in July. Stragglers appeared during subsequent weeks in July and August but became 

 less and less numerous. , 



GROWTH AND AGE AT MATURITY. 



We have no knowledge concerning the feeding grounds of the Yukon salmon and 

 must leave the question open to what extent, if at all, the young traverse the passes in 

 the Aleutian Chain and attain their growth in the North Pacific. It is entirely possible 

 that throughout their life in the ocean, they remain within the confines of the Bering 

 Sea. None of them have been detected traversing the channels between the Aleutian 

 Islands, nor have they been recognized elsewhere along lines of their migration routes 

 in the sea. 



Conditions in Bering Sea, it would seem, must be less favorable for rapid growth than 

 in districts farther south. The northern part of the sea and a strip around the coasts, in- 

 cluding Bristol Bay, are covered with ice floe during the winter and early spring months. 

 The temperatures to which the salmon are then exposed must be near the freezing 

 point. At the time they seek the river mouth in May or June the surface temperature 

 in Bering Sea approaches 40 F. Under such adverse conditions growth during the 

 winter season must be at or near a standstill and in the spring might well not be 

 resumed before the beginning of the streamward migration. In that event the scales 

 would exhibit no growth accomplished during the year in which the fish was captured. 

 A salmon in its fifth year would indicate in its scale structure the completion of four 

 full years' growth, and the margin of the scale would be formed by the winter check of 

 the fourth year. In other districts to the southward the salmon of the spawning run 

 have already responded to spring conditions and have begun a period of rapid growth 

 before entering the streams. The scales have participated in this renewed growth, 

 and the margins exhibit a larger or smaller band of widely spaced rings, which lie out- 

 side the winter check of the previous year. But in the case of the Yukon king salmon 

 this is not present. The winter check of the previous year forms the margin, and 

 usually no trace exists of any growth belonging to the current year. A very few cases 

 form doubtful exceptions to this generalization, with the outer one to three rings more 

 widely separated at least in a portion of their course. This feature is shown dis- 

 tinctly in the accompanying series of photographs of the scales of Yukon king salmon, 

 ranging from those in their third to those in their seventh year (Figs. 276 to 285). 



Another striking peculiarity of the Yukon king salmon is found in their early his- 

 tory as fry and fingerlings. We did not secure any of the young, although attempts 

 were made to capture them with minnow seines on their downward migration, near the 

 mouth of the river. But the central areas of the adult scales contain records of the 

 early history and show conclusively in every instance that the young remained in 

 fresh water for a full year's growth before descending to the sea. In the photographs 

 that follow, the line " 1 " points to the outer margin of the stream growth, which 

 presents a nucleus of finely crowded lines, beyond which are the widely spaced lines 

 indicating rapid growth after reaching the sea. 



This habit of the Yukon kings is in striking contrast to what is observed in streams 

 farther south. In the Fraser River, the Columbia, the Klamath, the Sacramento, and 



