I I 1980 (n 268) 

 E3 1981 (n 266) 



225 275 325 375 425 475 525 575 625 675 725 775 

 Fork Length (Midpoint of 50mm increments) 



FIGURE 2. — Length-frequency distribution of walleye collected 

 from the John Day pool of the Columbia River, April through 

 September 1980 and 1981. 



identifiable fish remains, the next most important 

 foods were catostomids (largescale suckers, Catos- 

 totnus macrocheilus , and bridgelip suckers) and 

 cyprinids (primarily chiselmouth, Acrocheilus 

 alutaceus, and peamouth, Mylocheilus caurinus). 

 These species are generally associated with the 

 benthos (Scott and Crossman 1973; Wydoski and 

 Whitney 1979). In 1980, juvenile salmonids, 

 primarily chinook salmon, Oncorhynchus tsha- 

 wytscha, and juvenile American shad were equal 

 to cyprinids in importance; however, in 1981, the 

 importance of salmonids and shad was greatly 

 reduced (Table 4). 



All statistical tests were conducted with foods at 

 the lowest taxon, and the data for individual 

 species are presented in Tables 2, 3, and 4; how- 

 ever, in the interest of concise reporting we discuss 

 results based on the following groups: catos- 

 tomids, cyprinids, salmonids, cottids, shad, and 

 invertebrates. Numbers and volumes of individual 

 prey were significantly different between 1980 and 

 1981 (P < 0.005); to further investigate these dif- 

 ferences, we tested for seasonal variation within 

 and between years. We found no significant differ- 

 ence in numbers or volumes of prey between 

 spring (April- June) 1980 and spring 1981 (P > 

 0.25); however, there were significant differences 

 in diets (numbers and volumes) between summer 

 (July-September) 1980 and summer 1981 (P < 

 0.05) and between the spring and summer of each 

 year (P < 0.01). 



We examined seasonal changes in IRI to detect 

 which prey could account for significant differ- 

 ences in walleye diet (Table 4). From 1980 to 1981 

 there is a reduction in the importance of sal- 

 monids, cottids, and shad, and an increase in 

 importance of catostomids, cyprinids, and inver- 

 tebrates. Seasonal changes are not consistent; 

 however, there are reductions in importance of 

 cottids, increases in importance of cyprinids, and 

 no changes in importance of invertebrates from 

 spring to summer each year. Our CPUE data 

 (Table 5) reflect annual and seasonal changes in 

 the abundance of juvenile shad, juvenile 



TABLE 3. — Percent by volume, percent by number, and percent frequency of occurrence 

 of foods found in the stomachs of walleye collected in the John Day pool of the Columbia 

 River, April-September 1981. Sample size equals 236 walleye, with 39. (Fr empty 

 stomachs. (Raw data are in parentheses.) 



1 Volumes and numbers of individual prey taxa were significantly different from those of 1 980 (P < 0.005). 



413 



