T 1 1 1 1 I I 1 1 I 



1234 123412345 

 MARCH APRIL MAY 



Figure 12. — Timing of migration of juvenile liatchery- 

 reared soelveye salmon from the Snake River to Brown- 

 lee Reservoir, 1965. 



ESTIMATES OF IMMIGRATION 



Recruitments by age-group and popiUation were 

 estimated each year from data on release and re- 

 capture of marked fish. Estimated recruitment 

 (N) was obtained by dividing the recaptures of 

 marked fish (R) into tlie number of marked fish 

 released (M) and multiplying by the catch (C). 



In 1962. personnel of the Idaho Department of 

 Fish and Game working upstream from the 

 Bureau's migrant-dipper trap marked fall ohinook 

 juveniles and released them in the Snake River. 

 These marked fish were used for the 1962 estimate. 

 The marked fish group far exceeded the total catch 

 of the migrant dipper. As a result, a single esti- 

 mate was made for the Snake River population 

 over the entire season. The 1962 estimate of re- 

 cruitment for Weiser River chinook salmon was 

 based upon the total catch by the scoop traps of 

 the Idaho Department of Fish and Game and the 

 estimated efficiency of these traps. 



In 196.3-65, the Snake and Weiser River fish that 

 passed our trap were estimated each week and the 

 values summed to yield the estimated total immi- 

 gration. Fish for marking were obtained from the 

 migrant dipper catches. 



It was sometimes necessary to supply informa- 

 tion by extrapolation when reliable data were lack- 

 ing. Wlien data on catch, recovery of marked fish, 

 or marked fish released were unreliable or lacking, 



reliable data for 1 or 2 weeks preceding, bracketing, 

 or following were used to supply an approxima- 

 tion of the needed data. 



Studies on fish distribution in the Snake River 

 in 1964 indicated that fingerling salmon were more 

 concentrated near the surface during the day than 

 at night (Monan, McConnell, Pugh, and Smith, 

 1969). Moreover, the increased length of louver 

 leads in 196.5 accentuated the difference between 

 the percentage of the migration captured by the 

 traps by day and by night. This difference necessi- 

 tated the use of two catch figures in estimating the 

 1965 i-ecniitments. Table 9 shows yearly differences 

 in the ability of the migrant dipper traps to cap- 

 ture fish during the peak of migration. 



SNAKE AND WEISER RIVERS 



Tlie number of juvenile native chinook salmon 

 that entered Brownlee Reservoir after 1962 de- 

 creased each year. One of the reasons was that the 

 adult spawners were intercepted and diverted to 

 hatcheries downstream from Brownlee Dam. Our 

 highest estimate of recruitment was in 1962 when 

 529,000 young chinook salmon enteretl from the 

 Snake River (table 10) ; these fish were offspring 

 of adults that had been transported past the dam 

 and spawned in the fall of 1961. The highest num- 

 ber of juvenile chinook salmon from Weiser River 

 (122,500) was also in 1962; these fish were off- 

 spring of spring migrants that had spawned in 

 1960. 



The number of kokanee increased each year; 

 nearly one-half million entered the reservoir in 

 1965. 



Of the hatchery-i-eared fish released in the Snake 

 River, sockeye salmon had the highest survival to 



Table 9. — Ability of migrant dipper traps to capture 

 juvenile salmon during peak migration from the Snake River 

 into Brownlee Reservoir, 1962-65 



I All age-group I. 



! 6:00 a.m.-6:00 p.m. 



3 6:00 p.m.-6;00 a.m. 



* Not present in migration. 



> Not tested. 



214 



U.S. FISH AND WILDLIFE SERVICE 



