FISHERY BULLETIN: VOL. 71, NO. 2 



after capture to stop digestive action. The 

 fish were later measured (fork length, FL) 

 and eviscerated. All measurements were taken 

 after 7 days of preservation to obtain con- 

 sistency, since Formalin causes some initial 

 shrinkage. Organisms in the stomachs were 

 identified individually to the lowest practical 

 category under a dissecting microscope with 

 the aid of appropriate taxonomic texts, classi- 

 fied according to their developmental stage, 

 and enumerated. Insects represented by chitin- 

 ous head capsules, particularly larval Chiro- 

 nomidae, were counted as complete organisms 

 whereas fragmented body parts were excluded. 



The 1968 study quantified only the food 

 organisms consumed. Methods in 1969 were 

 modified to provide data on fish length-weight 

 relationships and dry weight (biomass) of the 

 stomach contents. Fish were individually blotted 

 with absorbent paper to remove excess fluid 

 prior to weighing. After identification and 

 enumeration of food organisms, the entire 

 stomach content of each fish was placed in a 

 miniature watch glass, air dried at least 24 hr 

 in a controlled atmosphere, and weighed. 



Collecting stations were distributed along 

 a 38-km section of the Columbia River ex- 

 tending downstream from the Highway 240 

 bridge at Vernita, Wash. Six primary stations, 

 shown in Figure 1 (A, B, C, D, E, F), and 

 four supplementary stations were used. Samples 

 from all 10 stations were combined in summa- 

 tions of food organisms utilized (see Tables 

 1 and 2), but only data from the six primary 

 stations were used for subsequent statistical 

 treatment. 



The Chironomidae (midges) were the domi- 

 nant insect group utilized. Emerging subadults 

 and adults were captured in abundance, 64% 

 in 1968 and 58% in 1969. Midge larvae were 

 taken less extensively, 17% in 1968 and 18% 

 in 1969. Few midge pupae and no pupal 

 exuviae were noted. 



The order Diptera provided 83% and 78% 

 of all insects utilized in 1968 and 1969, re- 

 spectively. Other insect groups were of less 

 importance in terms of numbers, but not 

 necessarily in volume (or nutritional value) 

 since sizes of different species vary considerably. 

 The relatively large Trichoptera (caddisflies), 

 consisting primarily of Hydropsyche cockerelli 

 (Hydropsychidae), were numerically the second 

 most important order. Like the midges, most 

 caddisflies eaten were recently emerged adults 

 associated with the water-air interface. Other 

 groups of secondary importance were the 

 families Notonectidae (Hemiptera or true bugs), 

 primarily small Notonecta nymphs, and the 

 Hypogastruridae (Collembola or springtails). 



Few Ephemeroptera (mayflies), often im- 

 portant dietary items of salmonids in other 

 streams, and no Plecoptera (stoneflies) were 

 detected in the stomach contents. Unpublished 

 data from limited bottom samples, sporadic 

 drift samples, inspection of stones, and trap- 

 ping of adult insects by light attraction at 

 night indicate that populations of mayflies 

 and particularly stoneflies are low in the 

 central Columbia River. Zooplankton, originat- 

 ing primarily from the Priest Rapids reservoir 

 and present in the river drift, were utilized 

 in small quantities by only a few fish. 



RESULTS 



Food Organisms Utilized 



Throughout their sojourn at Hanford, over 

 95% of the diet of juvenile chinook salmon 

 consisted of insects. The prey included adult, 

 subadult, and larval stages of semiaquatics, 

 various developmental stages of aquatics, and 

 winged adults of terrestrials (Table 1). Com- 

 parison of the 1968 and 1969 data in Table 1 

 reveals that the organisms consumed were 

 essentially similar in two successive years. 



Seasonal Changes in Diet 



Some change occurs in the diet of juvenile 

 Chinook salmon from March to July (Table 2). 

 The Chironomidae accounted for the greatest 

 proportion of food organisms each month on 

 a numerical basis, with the most larval and 

 adult midges being consumed in March and 

 April. Hemiptera and Collembola, both con- 

 sisting of small forms, received maximum 

 utilization in April, May, and June when rising 

 river volumes inundated shoreline areas. Adult 

 Trichoptera were consumed primarily in June 



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