FISHERY BULLETIN: VOL. 7), NO. 2 



salmonids in river habitats. However, the 

 precise species of prey will differ between 

 and even within various lotic systems because 

 the existence and production of insect taxa 

 is influenced by diverse edaphic factors. 



Visual stimulation is important to the feeding 

 of young salmonids (Chapman, 1966). Juvenile 

 Chinook salmon at Hanford exhibit considerable 

 selection of living food organisms since non- 

 living material, such as insect exuviae and 

 plant seeds, rarely occurred in their stomachs. 

 Apparently this selection was due, in large 

 part, to prey movement that evoked the feeding 

 response. A preference for suspended organ- 

 isms was also indicated, since benthic stages 

 of aquatic insects were relatively unutilized 

 by Hanford fish. 



Determination of preference for a particular 

 food organism depends on the ratios of ingre- 

 dients making up the food complex and their 

 occurrence in the stomach of fish (Allen, 1942; 

 Ivlev, 1961). Although I obtained some inver- 

 tebrate drift samples in the central Columbia 

 River, which demonstrated an abundance of 

 chironomid larvae, the data were inadequate 

 for accurate determination of ratios over the 

 entire season. Feeding apparently corresponded 

 roughly to food organisms occurring free in the 

 water, but not necessarily in proportion to the 

 food actually available. 



Chinook salmon fry consumed small midges 

 most extensively whereas fingerlings tended to 

 include larger insects in their diet. The relation- 

 ship of increasing fish size to increasing food 

 size in young salmonids has been recognized 

 (Lindstrom, 1955; Hartman, 1958). Food 

 utilized by small salmonids are subject to 

 limitations imposed by the size of the fish 

 whereas food utilized by larger fish can be very 

 diverse (Mundie, 1969). However, diversity is 

 clearly limited to what is available in a given 

 ecosystem. 



Ecological Aspects 



The central Columbia River remains a large 

 flowing river with a relatively vast water mass, 

 rapid current velocities, and minimum shoreline 

 habitat in relation to discharge volume. Living 

 in stream environments requires considerable 



expenditure of energy that must be balanced 

 by food consumption. Growth occurs only when 

 energy provided by food exceeds energy ex- 

 pended in feeding and other activities. Energy 

 can be conserved by juvenile salmonids in three 

 ways: (1) leaving stream conditions to enter a 

 lake or sea; (2) living in the stream below the 

 main impact of the current; or (3) living pre- 

 dominantly in slack water, in pools, and in 

 marginal back eddys (Mundie, 1969). 



Examples of habitat selection associated with 

 energy conservation can be noted. Young Chi- 

 nook salmon and steelhead trout, Sal mo gaird- 

 neri, in Idaho streams inhabit velocities and 

 depths in relation to body size, shifting to faster 

 and deeper water as growth occurs (Chapman 

 and Bjornn, 1969; Everest and Chapman, 1972). 

 Similarly, chinook salmon fry in the Big Quali- 

 cum River, British Columbia, occupy marginal 

 areas while the larger fish move into habitats 

 of progressively higher velocity (Lister and 

 Genoe, 1970). Since my samples were obtained 

 entirely from shoreline areas that could be 

 effectively seined, they reflect feeding in those 

 habitats. A possible shift of larger fish to 

 deep water would remain undetected. 



Because metabolic rates of cold-blooded 

 animals such as fish increase as temperatures 

 rise, more food must be consumed for growth 

 of juvenile chinook salmon to be maintained 

 as the season advances and the water warms. 

 My data show that feeding intensity, on the 

 basis of both number of insects and total 

 stomach biomass, tended to increase from March 

 to July. 



Although chironomids are small and indi- 

 vidually low in nutritional value, they are 

 utilized throughout the season by juvenile 

 chinook at Hanford and their abundance com- 

 pensates for a lack of size. The adult caddisflies 

 appearing in June and July are large and pro- 

 vide greater nutritional value per individual 

 at a time when temperatures are high and more 

 energy is required for fish growth. By dry 

 weight, 1 adult Hydropsyche cockerelU is equal 

 to 35 adult midges. Although the calories 

 available per gram of dry weight for chironomids 

 (5,424) and hydropsychids (5,386) are nearly 

 equal (Cummins and Wuycheck, 1971), con- 

 siderably less energy is required to capture 1 

 prey organism than 35. 



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