stomachs of walleye collected in the John Day pool of the Columbia River in the 

 1980 and 1981. Numbers in parentheses are percent IRI. 



1981 



Prey taxon 



Salmonidae 



Oncorhynchus tshawytscha 



Unidentifiable Salmonidae 

 Catostomidae 



Catostomus columbianus 



C macrocheilus 



Unidentifiable Catostomidae 

 Cypnnidae 



Acrocheilus alutaceus 



Mylocheilus caurmus 



Ptychocheilus oregonensis 



Other Cypnnidae 

 Miscellaneous fishes 



Cottus asper 



Alosa sapidissma 



Other (unidentifiable 



fish; Lampetra spp . 



Ictalundae) 

 Invertebrates 



same bimodal feeding periodicity as reported for 

 other walleye populations during times of high 

 prey densities (Swenson 1977). We found no an- 

 nual or seasonal variation in this periodicity. 

 Numbers of juvenile salmonids and shad con- 

 sumed per walleye at various times of the day peak 

 from late night to midmorning, drop to a low level 

 at midday, and remain low through the evening 

 peak in walleye feeding. 



Size of Prey Consumed 



Parsons (1971) showed a positive relationship 

 between walleye length and length of prey con- 

 sumed in Lake Erie. Walleye in the mid-Columbia 

 River exhibit the same relationship, and size of 

 prey is correlated to different prey taxa. The 

 change in the percent of the IRI of various prey 

 groups, as a function of walleye fork length, is 



100 



KS2S2828ogo8ogogogogogogogogogogo^^^ 



Walleye Fork Length 



FIGURE 4.— Change in percent of total Index of Relative Importance (IRI) (Pinkas et al. 1971) of prey components as a function of 

 walleye fork length (100 mm increments). Walleye collected in the John Day pool of the Columbia River, April through September 1980 

 and 1981. (Sample size in parentheses.) 



415 



