Table 2. — Monthly changes in diet of juvenile chinook salmon 



sampling stations combined. 



FISHERY BULLETIN: VOL. 71, NO. 2 

 1969, all 



in 



' Major insect groups utilized. 



2 "J" = "jrace," less than 1% by number in stomach contents. 



and July, in association with summer emergence 

 of the univoltine caddisfly populations. A 

 minor group, the semiaquatic Pyralidae (Lepi- 

 doptera or moths and butterflies), were taken 

 most heavily in June. 



Some correlation of diet (Table 2) with fish 

 size (Figure 4) was evident. Chinook salmon 

 fry were relatively small (35-40 mm FL) and 

 had incompletely absorbed yolk sacs when 

 they emerged from the gravel and began feed- 

 ing in March and April. Food organisms 

 selected by fry were predominantly small forms, 

 primarily midges but some Notoiiecta nymphs 

 and Collembola were included. Adult Trichop- 

 tera in June and July were selected primarily 

 by large fish (>50 mm FL) capable of cap- 

 turing and swallowing these larger insects. 



Use of Drift Organisms 



The developmental stages of insects ingested 

 by juvenile chinook salmon reveal that most 

 were floating, drifting, or swimming in the 

 water when captured. This was apparently 

 the situation for the selection of most Chiro- 

 nomidae, Hydropsychidae, Notonecta nymphs, 

 and Hypogastruridae, the four main insect 

 forms utilized. Relatively few insect stages 

 normally adhering to epibenthic substrates 

 or living within gravel interstices were repre- 

 sented in the stomach contents. 



15 



5 - 



15 

 10 



5 



MEAN 



STANDARD DEVIATION 



MARCH 

 n = 48 



APRIL 

 n = 278 



STATION D 



10 



10 



5 - 



35 40 45 50 55 60 65 70 75 80 85 

 FORK LENGTH (MM) 



Figure 4. — Monthly length-frequency distributions of all 

 juvenile chinook salmon collected at Hanford in 1969. 

 (Data smoothed by a moving mean of three's.) 



Although autochthonous insects predomi- 

 nated, some allochthonous terrestrial forms 

 were ingested. The orders Homoptera, Hy- 



392 



