welded to the downstream end of the trap is kept 

 partly filled with water, hence all fish are 

 washed down the screen into the collecting pot, 

 where they are held alive . 



Four of the original five fingerling 

 traps are in operation at the present time. Tan- 

 ner Creek trap was destroyed by a rock slide 

 in December 1948; the trap was never replaced, 

 but the bypass channel is still used by down- 

 stream migrants (see fig. 2). Partial blocking 

 of the entrance of Tanner Creek bypass by ex- 

 tension of the navigation -lock wingwall in 1947 

 was probably responsible for the drop in catch 

 by that trap from 49 percent of the total catch 

 in 1946 to an average of only 2 percent in 1947 

 and 1948 . This condition should not have any 

 effect on the data of age, length, and time of 

 migration. 



Operation of the traps was continuous, 

 except when fishway facility repairs or severe 

 cold weather necessitated shutting off the water 

 through the auxiliary-water screen pits. 



MIGRATION OF WILD AND HATCHERY - 

 REARED FISH 



peak in January 1946 consisted of marked hatch- 

 ery fish released from the Little White Salmon 

 station in October 1945. Some of the years 

 have a small peak in March . In figure 9 the 

 weekly catches are plotted as cumulative per- 

 centages, showing graphically that most of the 

 blueback were caught within a few weeks. The . 

 weekly catches in figures 5-9 and table 2 con- 

 tain a few more fish than figures and tables 

 listing the number of migrants grouped by 

 length and age, because in some weeks one or 

 two migrants were caught but were not sampled. 

 No age or length was assigned to these fish. 



Age of Migrants 



The ages of downstream migrants 

 ranged from 1 to 5 years, with 93 percent being 

 2's or yearlings . Scales of these fish have one 

 annulus. This aging method was developed by 

 Gilbert (1913), who counted the age from the 

 time the eggs were deposited. The I's form 

 from to 2 percent of the catch, 3's from 3 to 

 7 percent, and 4's from to 0.4 percent. Only 

 one 5 has been caught at Bonneville. Figure 10 

 and table 3 show the age composition for each 

 year. 



Time of Migration 



Some down stream -migrant bluebacks 

 pass Bonneville during all months of the year, 

 but the fingerling-trap catches indicate that 

 most of the migration occurs during April and 

 May. This observation agrees with the obser- 

 vation of Fish and Hanavan (1948) that the blue- 

 backs migrate from Lakes Osoyoos and Wenat- 

 chee from March through May. As explained 

 earlier, the catch was summed by weekly 

 periods. The "modal week (in which the maxi- 

 mum number passed the dam) ranged between 

 week 17 and week 20 (April 23 to May 20), with 

 week 18 (April 3u-May 6) and week 19 (May 7- 

 13) each occurring 3 times in the 8 years in- 

 cluded in this study. The modal week contained 

 from 22 to 49 percent of the yearly catch, and 

 averaged 36 percent of the catch for all years . 



The catch rises sharply to a peak and 

 drops off sharply, few fish being caught four 

 weeks after the maximu^n catch week. Figures 

 5-8 and table 2 show the weekly catches. The 



New growth appears on most scales by 

 May 1 at Bonneville, and a wide zone of new 

 growth is apparent in July. An annulus appears 

 to be forming on some scales as early as Sep- 

 tember, whereas other scales show no evidence 

 of an annulus forming in January and February. 

 Scale growth, therefore, is not similar for all 

 bluebacks that pass Bonneville, even within age 

 groups migrating during the same week . This 

 lack of homogeneity makes the identification of 

 false checks and the interpretation of the time 

 when an annulus formed a constant problem, 

 and may have caused some error in aging. A 

 group that was especially difficult to age was 

 composed of fish between 115 and 155 mm. that 

 migrated in the spring. In this study these 

 questionable blueback were grouped with the 2's. 

 Bonneville catches of known 2's composed of 

 marked blueback released by hatcheries in the 

 spring were most commonly in this length 

 group. 



Future studies of the separate races 

 comprising the Columbia River blueback 



