BECKER: PARAMETERS OF JUVENILE CHINOOK SALMON 



differ greatly from those in June. Yet a slight 

 increase throughout the season was evident. 



High variations within and between sam- 

 ples, related to feeding of individual fish, pre- 

 clude meaningful comparison between stations 

 on the basis of number of insects per stomach. 

 Number of insects contained under field situa- 

 tions is, at best, a rough index to nutrition 

 and subsequent growth. There are several 

 reasons. First, insects vary widely in size 

 from minute midges to large caddisflies; large 

 number of small insects in a stomach is not 

 necessarily equivalent to a few large insects 

 in terms of energy supplied. Second, the rela- 

 tive nutritional value may vary between like 

 amounts of different kinds of food organisms. 

 Third, stomach contents reveal only feeding 

 at the approximate time a sample was taken 

 and not the preceding meals responsible for 

 growth. Fourth, digestion rates, metabolism, 

 and energy consumption that result in growth 

 are highly temperature dependent, particularly 

 over the March to July range of 2° to 20 °C 

 that occurs in the central Columbia River. 

 Fifth, changes in water levels influence current 

 patterns, availability of food supply and, 

 more or less, expenditure of energy required 

 for a fish to obtain a "full meal." 



Total stomach biomass provides better infor- 

 mation on daily rations from natural river 

 ecosystems. Feeding intensity on the basis 

 of the relationship between fish size and stomach 

 biomass was calculated as: 



F/ = ,^ X 100; 



where Fl = feeding intensity, w = dry weight 

 of stomach contents in grams, and W — weight 

 of juvenile chinook salmon in grams (Olmsted 

 and Kilambi, 1971). 



Amounts of food in the stomachs of individual 

 fish varied widely. To minimize random sample 

 variations, feeding intensities were tabulated 

 on the basis of combined samples for each 

 collection date at all primary stations (Table 

 4). The few fish taken in March were available 

 only at Station D, where water temperatures 

 were somewhat higher than in the main channel 

 due to intragravel seepage of warm water 

 fi'om the shoreline. These fish revealed a rela- 



tively high feeding intensity compared to fish 

 at all primary stations in April. 



FI values from grouped samples generally 

 increased as the season progressed and the 

 river water warmed. The highest feeding in- 

 tensity in June and early July reflects pri- 

 marily an increase in the size of food or- 

 ganisms consumed, particularly by inclusion 

 of adult Trichoptera (Table 2). 



Table 4. — Feeding intensity (FI) of juvenile chinook 

 salmon in the central Columbia River, 1969. (Samples 

 combined by collection date.) 



Variability in Fish Lengths 



The expanding standard deviation in Figure 

 4 indicates an increasing size range from 

 month to month. Lengths of juvenile chinook 

 were relatively uniform at each station during 

 April and early May when recruitment to shore- 

 line zones was initiated and temperatures were 

 low. Variations in mean fish lengths within and 

 between stations appeared with further growth 

 in late May, as temperatures increased, and 

 these variations became extreme in June and 

 July. Statistical comparison of sample mean 

 lengths throughout the season, by Duncan's 

 Multiple Range Test, revealed significant dif- 

 ferences that supported this observation. 



Considerable turnover offish presumably took 

 place along the shore during the period of high 

 river discharge and thereafter. The inference is 



395 



