^(HJiia^t^ 0^ (Mi^ oH^^d^aa^a^ 



This study investigated not only the food preferences of river fishes but 

 also the importance of all organisms in the food chain of the river ecosystem. 

 Fish species adaptations and preferences for particular habitats are often 

 related to their food habits. Research of food habits assists in an under- 

 standing of the interrelationships among organisms and their environment. 

 Impacts on one segment of an ecosystem can affect other segments. 



Impacts on fish populations from reduced flow vary for each species 

 depending on food habits. Riffle insects provide food for sturgeon, goldeye, 

 catfish, longnose dace, and for immature burbot and sauger. Mature sauger, 

 burbot, and catfish rely (in part) on the backwater areas for their supply 

 of forage fish. Reduced flow would affect both riffles and backwaters. 



RIFFLE HABITAT 



Riffle insects rely on current for food, oxygen, and protection from 

 predators. Reduced discharge is detrimental to benthic populations because: 

 (1) velocity requirements of riffle species are no longer met and (2) the 

 wetted bottom area of the channel is decreased. R. L. Newell (see Report No. 5 

 in this series) estimated that insect density at Intake could decrease by 

 10 percent for each 28.3 m^/sec (1000 cfs) reduction in discharge below 

 255 m3/sec (9000 cfs). 



Insects which prefer the greatest velocities, such as Traverella and 

 Hydropsyahe , would be among the first organisms to feel the impact of reduced 

 flows. According to Report No. 5 in this series, peak densities of Traverella 

 occurred at .69 m/sec (2.25 ft/sec) in August and .76 m/sec (2.5 ft/sec) in 

 October. Numbers of Hydropsyahe were highest at velocities of .46 m/sec (1.5 

 ft/sec) in October and .61 m/sec (2 ft/sec) in November. Faster currents are 

 preferred by caddis larvae in later months as they grow in size. 



Low flows decrease the wetted perimeter of the river channel. The loss 

 of habitable space varies with the depth and width of the channel, the greatest 

 reduction occurring where riffles are wide and shallow. Insects are concen- 

 trated in higher densities as flows diminish. Glass and Bovbjerg (1969) found 

 that when numbers of Chenmatopsyohe larvae are high, aggression among individuals 

 maintains tolerable spacing. Invertebrates remove from riffles by drifting 

 when flows are reduced (Minshall and Winger 1968, Pearson and Franklin 1968). 



This study has shown that Traverella was the most important food item 

 of goldeye, catfish, and sturgeon during August and September. Traverella 

 albertana comprised 60 percent of the number of benthic fauna at Intake in 

 August (Report No. 5). Due to the high density of this mayfly species and 

 its water velocity preference, a reduction in Traverella numbers could be a 

 critical loss to insect-feeding fish during abnormally low flows in late 

 summer. 



137 



