charted in Figure 4. Small walleye (200-400 mm 

 FL) primarily consume salmonids, cottids, and 

 shad, while midrange walleye (400-600 mm FL) 

 rely more heavily on cyprinids, cottids, and catos- 

 tomids. For large walleye ( >600 mm FL), suckers 

 are the most important prey and the importance of 

 cyprinids and cottids is reduced. Figure 5 contains 

 the length frequencies of walleye prey collected in 

 1981 and shows peaks which correspond to the size 

 of walleye most likely to consume that prey, i.e., 

 cottids, juvenile shad, and juvenile salmonids are 

 small (25-125 mm FL); cyprinids, excluding 

 juvenile peamouth, are midrange in length (125- 

 300 mm FL); and catostomids are present in a 

 large range of sizes (150-450 mm FL) with peaks 

 300 mm FL. 



40 



30 



c 

 m 

 u 



Si 



Q. 



20 - 



10 - 



|\ 



. />\/\A/' 



A i • / \SJ3 \° /R 



o Peamouth (n 362) 



• Chiselmouth (n = 553) 



a Largescale Sucker (n =1211) 



□ Bridgelip Sucker (n = 775) 



,D 



££.A < 



**&*?. jmbk 



V 



iP-n 



LtA^ 



c 

 (J 

 <u 



Q. 



a Sculpin (n = 292) 

  YOYShad (n 660) 

 • -YOY Chinook (n = 295) 



fr*-*~ 



100 200 300 400 500 

 Fork Length (mm) 



FIGURE 5. — Length frequencies of potential walleye prey col- 

 lected in the John Day pool of the Columbia River, April through 

 September 1981. (Sample size in parentheses.) 



Discussion 



Walleye have been described as opportunistic 

 (Eschmeyer 1950; Ryder and Kerr 1978), crepuscu- 

 lar, or nocturnal feeders which primarily search 



416 



the bottom for prey (Ali et al. 1977; Ryder 1977) 

 and, in areas of abundant prey, select prey based 

 on size preference (Parsons 1971; Wagner 1972). 

 Walleye of the mid-Columbia River fit this general 

 description; however, the species composition of 

 their diet is different from that reported 

 elsewhere. This difference is undoubtedly due to 

 differing arrays and abundances of potential prey. 

 Our data suggest no significant change in annual 

 or seasonal variation in abundances of adult catos- 

 tomids and cyprinids (chiselmouth) (Table 5); 

 therefore, we believe that the variations in wall- 

 eye diets (Tables 2,3,4) are the result of changes in 

 availability of juvenile prey fish (Table 5). 



Our data do not clearly explain the dietary role 

 of juvenile anadromous fish that normally have 

 seasonal abundances in excess of 10 million fish 

 (Sims et al. 1981, 1982). We hypothesize that dif- 

 ferent behavioral responses of walleye, juvenile 

 salmonids and shad, and alternate prey result in 

 the walleye's apparent low dietary utilization of 

 juvenile anadromous fish. The walleye's subreti- 

 nal tapetum lucidum greatly enhances its visual 

 acuity at twilight, when many potential prey have 

 reduced visual acuity and are inactive (Ali et al. 

 1977; Ryder 1977). Yellow perch, Perca flavescens, 

 are walleye's primary prey over most of their co- 

 extensive habitats (Colby et al. 1979), and the yel- 

 low perch's behavior in dim light is described as 

 settling to the bottom and becoming inactive 

 (Ryder 1977). Ali et al. (1977) suggested that were 

 it not for their complimentary behavior at dawn 

 and dusk, walleye and yellow perch interaction 

 would not be as significant as it appears to be. In 

 dimming light, emigrating juvenile Pacific salmon 

 rise to the surface, increase swimming activity, 

 and move downstream ( Hoar 1958; Ali 1959). Simi- 

 lar behavior has been reported for juvenile shad 

 (Loesch et al. 1982). Emery (1973) studied the diel 

 movements of 21 species of Catostomidae, 

 Clupeidae, Cottidae, Cyprinidae, and Percidae, 

 and all but two species of Clupeidae were on or 

 near the bottom at twilight and during the night. 

 Emery (1973) further reported that these fish 

 could be more closely approached by a diver at 

 night than during the day. Therefore it appears 

 that juvenile salmonids and shad are buffered 

 from walleye predation by an abundance of alter- 

 nate prey (Tables 2, 3, 5) of a wide size range (Figs. 

 4, 5) and by a separation in space and time during 

 one of the walleye's peak feeding periods (Fig. 3). 



We caught no walleye <200 mm FL (Fig. 2), 

 even though our gear captured numerous speci- 

 mens of other species <100 mm FL (Fig. 5). We 



