ten^eratures were 43 to 45 degrees when 

 trapping was initiated and when fish were 

 found to be migrating. From the limited 

 data it is not possible to determine whether 

 a certain temperature triggers the fish 

 movement. For both seasons blueback were 

 migrating by the last week in April and tem- 

 peratures had reached 40 degrees by the time 

 of migration. 



Summer and fall fish migrations in 

 1955 occurred during the normal seasonal 

 rise and fall of river temperatures. Peaks 

 of abundance of both steelhead and chinook 

 occurred during the rise and fall of water 

 temperatures. The sudden drop in tempera- 

 ture November 11-15 forced cessation of 

 sampling at a time when some migration was 

 still in progress. The extent of migration 

 beyond this time is not known; however, it 

 is undoubtedly at a low level if it exists 

 at all. 



Time of Day of Migration 



Blueback salmon . — Blueback salmon were 

 found in the bypass trap throughout the day 

 and night, although by far the bulk of the 

 migration occurred at night between 6 p.m. 

 and midnight. Figure 19 gives the combined 

 counts for the hours 7 p.m. to midnight. 

 The peak hour varied during the season from 

 8-9, 9-10, or some times 10-11 p.m. At 



the height of migration the peak hour was 

 8-9 p.m. 



Chinook salmon . — Chinook salmon were 

 taken in the bypass trap throughout the 

 24-hour period of a day; however, as for 

 blueback, the main migration occurred during 

 hours of darkness. Figure 20, giving the 

 combined hourly counts in the spring fcr the 

 period from 7 p.m. to midnight, shows the 

 peak hour to be from 8-9 p.m. the same as 

 for the blueback. Actually, little differ- 

 ence was indicated between the hours of 8-9 

 and 9 - 10 p.m. 



During the summer and fall months when 

 the trap was emptied twice daily, at morning 

 and evening, the majority of the fish were 

 tallied in the morning. The counts, after 

 adjustment for equal fishing time, showed 

 that 72 percent of 13,052 chinook tallied 

 were counted in the early morning, indicat- 

 ing the nighttime preference for migration 

 of these fish in the Wenatchee River. 



Steelhead trout. — Steelhead also ex- 



700 



. 400 



z 



2 300 



200 



100 



7-8pn 



8-9 9-10 10-11 



ONE HOUR PERIODS 



M-12 



7-8, 



8-9 9-10 10-11 



ONE HOUR PERIODS 



11-12 



Figure 20. — Time of day of migration - 

 chinook salmon. Tumwater Dam, 1955. 



Figure 19. — Time of day of migration - 

 blueback salmon. Tumwater Dam, 1955- 



14 



