TABLE 4. — Index of Relative Importance (IRI) (Pinkas et al. 1971) of foods found in 

 spring (April through June) and summer (July through September) 



1 1RI s are not additive across columns 



TABLE 5. — Catch-per-unit-effort (CPUE) for various juvenile 

 (Juv. i and adult fishes caught in the John Day pool of the Colum- 

 bia River, April-September 1980-81. 



Semes 



Dates 



Effort 



Sets 



Juv 

 Chinook 



CPUE 



Juv shad 



Juv 

 peamouth 



Apr-Jun 1980 

 Apr-Jun 1981 

 Jul -Sept 1980 

 Jul -Sept 1981 



Gill nets 



45 

 37 

 35 

 39 



16 65 



10 70 



2.65 



1.36 









 9276 

 42.87 









 688 

 5.77 



peamouth, and juvenile chinook salmon. However, 

 Sims et al. (1982) reported no significant sea- 

 sonal differences in the estimated numbers of 

 juvenile salmonids emigrating past the John Day 

 Dam (53,000 and 44,000 daily from 21 April to 30 

 June and from 1 July to 28 September 1981, respec- 

 tively), 90 km downstream of our study area. Simi- 

 lar estimates are not available for 1980, but the 

 smolt emigration past the John Day Dam was 

 estimated at 8.3 million (Sims et al. 1981) and 7.7 

 million (Sims et al. 1982) in 1980 and 1981, respec- 

 tively. Unfortunately cottids, the most important 

 food, were rare in our CPUE data for 1980, and in 

 1981, the electroshock CPUE was <0.1, a level too 

 low to detect changes. 



Seasonal shifts in walleye diets are often the 

 result of high spring-time availability of aquatic 

 insects and/or increased availability of prey fish in 

 the summer (Eschmeyer 1950; Parsons 1971). In 

 this study, invertebrates represented 4-13% of the 



numbers of prey items (Tables 2, 3), however they 

 contributed little to the total caloric intake of the 

 walleye because of their almost negligible volume 

 and poor assimilation by walleye (Kelso 1972). 

 Moreover, invertebrates did not exhibit significant 

 seasonal variation in walleye dietary importance 

 (Table 4). 



Diel Periodicity 



The mean index of fullness, measured as the 

 volume of stomach contents (ml) divided by wall- 

 eye body weight (kg), for all walleye sampled is 

 plotted against time of capture (2-h intervals) in 

 Figure 3. The shape of this curve suggests the 



'_ 24.0 



fZD-Salmonids 



QShad 



12.0 - 



1000 



1400 



Sunrise 



1800 2200 



Sunset 



Time (hours) 



FIGURE 3. — Index of fullness and numbers of juvenile salmonids 

 and shad consumed per walleye captured during 2-h intervals. 

 Data collected from the John Day pool of the Columbia River, 

 April to September 1980 and 1981. (Sample size in parentheses.) 



414 



