NOTE Giorgi et al.: Migratory characteristics of juvenile Oncorhynchus tshawytscha 



875 



£ 250- 



200 



150 - 



a 100 - 



Figure 2 



Weekly passage indices of zero-age chinook salmon pass- 

 ing John Day Dam, Columbia River. The dashed, solid, 

 and dotted lines represent 1981, 1982, and 1983, respectively. 



ing 22 August, the same general time frame as in 

 the following years. 



Migrant size increased steadily over the course of 

 the sampling period in all years. The mean size 

 ranged from approximately 90 mm in early June to 

 near 145 mm by the end of August. By mid-Decem- 

 ber, the length of fish passing John Day Dam aver- 



aged approximately 170 mm. The smallest mi- 

 grants observed passing either McNary or John 

 Day Dam were 55 to 60 mm during the month of 

 June each year. The largest migrants approached 

 225 mm in December. The fish sampled at all three 

 sites (McNary Dam, John Day Dam, and within 

 the reservoir) displayed the same size distributions 

 during the sampling periods. 



Environmental conditions 



Each year the trend in river flow was similar: dis- 

 charge consistently decreased during the summer. 

 However, the absolute flow volumes varied con- 

 siderably over the three years of study (Tables 1- 

 3). The greatest differences in flow volumes were 

 observed each year from mid-June to mid-July. 

 Beyond that period, flow volumes were nearly the 

 same from year to year. The highest flow year oc- 

 curred in 1982 when discharge levels averaged up 

 to 11.11 x 10 3 cubic meters per second during the 

 last week of June. Overall, the lowest flows oc- 

 curred in 1983. For example, during the last week of 

 June, river discharge volumes were nearly half the 

 maximum level observed in 1982, averaging between 

 5.91 x 10 3 and 6.87 x 10 3 cubic meters per second. 



Water temperatures were similar among years and 

 displayed the same tendency to increase throughout 

 the summer (Tables 1-3). Early in the summer, water 



Table 1 



Summary of 1981 brand release and recovery data from groups of zero-age chinook salmon marked and released at 

 McNary Dam and recaptured at John Day Dam. Travel time is the number of days required to traverse the reser- 

 voir from McNary Dam tailrace to John Day Dam. The percentiles were calculated from the passage indices. 



' The passage index is calculated daily as the ratio of the number recovered to the sampling effort and summed over days. Sampling effort was the 



average proportion of the total river flow discharged through Turbine Unit 3 during the 10-hour period 2000-0600 h. 

 2 The average river-flow volume and water temperature over the 10-day period following release of the marked group. 



