DAY AND NIGBT MOVIMENT 



Comparison between 19^2 and 1953 



Sampling from I9U6 to I95O indicated 

 that although most downstream-migrant 

 salmonids passed Bonneville Dam during 

 hours of darkness, some move seaward during 

 daylight hours. The hourly data collected 

 in 1952 and 1953 were separated into day 

 (5:01 a.m. to 7 p.m.) and night (7:01 p.m. 

 to 5 a.m.) periods. These hours were 

 chosen because they coincided best with the 

 hours preceding daybreak and following 

 nightfall; in this way no daylight was in- 

 cluded in the night period. 



The trap catches in 1952 had more 

 daytime than night migrants even though 

 the hourly data showed modes at dawn and 

 dusk in both 1952 and 1953. The numbers 

 of Chinook yearlings, bluebacks, and sil- 

 vers were not significantly different 

 (P "^ .05) between the day and night periods 

 in 1952; more chinook f ingerlings and 

 steelheads were caught during the daylight 

 than the night period (P<..01). Although 

 the time of year and duration of fishing 

 periods in 1953 were similar to those of 

 1952, all species were caught in signifi- 

 cantly greater numbers (P<: .01 for all 

 species) during the night periods in 1953- 

 The chi-square test was used to detemine 

 the significance of the differences be- 

 tween day and night periods. 



Table 3 susmarizes the data for each 

 year. Sampling in 19'*'7, 19^, and I95I was 

 not possible because other experlaents were 

 being conducted during those years. The 

 dally sampling periods were changed because 

 of the exploratory nature of the sampling 

 program. 



Factors Affecting Movement at 

 Bonneville 



factors that might affect movement aire (1) 

 operation of the dam, (2) races of fish, 

 (3) size of migrants, {k) number of migrants, 

 (5) weather conditions, and (6) physical 

 variables of the river (flow, temperature, 

 and turbidity). 



Operation of the dam introduces vari- 

 ables which are very difficult to evaluate. 

 Sane of these, such as changes in the spill- 

 way gate settings and powerhouse operations, 

 are changed over short intervals of time and 

 might affect catches. At night the demand 

 for power drops and the flow of water 

 decreases eis one or more generators are 

 closed down. At such times, spillway gates 

 may be opened to keep the water level below 

 the top of the spillway gates. Thus, flow 

 conditions in both channels are altered and 

 migration patterns KRy be Euffected. Varia- 

 tions between traps and also any daily 

 differences in catches of the same trap may 

 be caused by operational changes of the dam, 

 but the volume of auxiliary-water, which 

 draws the fish into the bypass areas, is 

 seldom changed over short periods of time, 

 and hence should not affect movement of the 

 fish within dally periods. 



Another possible variable is the dif- 

 ferent behavior of races. Rich (1920) 

 states there is evidence that Juvenile 

 Columbia River chinooks from pajrticuleur 

 tributaries tend to migrate at the same 

 time and school together during the seaward 

 migration. At present there is no practical 

 way to distinguish races at Bonneville; 

 hence, this variable was not tested. Salmon 

 are released by several hatcheries above 

 Bonneville. The distribution of hatchery 

 fish in the river may be different from 

 that of the wild migrants at the time they 

 pass the dam. Many releases migrate past 

 Bonneville within a short period of time; 

 thus, the proportion of migrants caught by 

 each trap would be affected by such schools 

 of fish. 



Several factoirs influence movement of 

 each species at Bonneville, resulting not 

 only in variations in the proportions 

 migrating within periods of time, but also 

 variations in the catches of each trap. 

 This variability in trap catches, although 

 expected, makes interpretation of the data 

 difficult. It is assumed here that the 

 combined catches of the trapc gives the 

 best estimate of the relative numbers of 

 migrants passing the dam. Seme of the 



The size of migrants may be closely 

 associated with races and time of year; 

 insufficient data are available for a study 

 of this variable. 



The effect of weather conditions on 

 migration at Bonneville would be difficult 

 to separate and analyze, becaxise the weather 

 is so closely «i8SOCiated with other vari- 

 ables such as condition of the river and 

 corresponding operation of the dam. 



