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DISCUSSION <br />The results of our experiment support the conclusions of several studies of fish <br />predation on invertebrates in pools and soft substrates (Wilzbach et at. 1986, <br />Schlosser and Ebel 1989, Angermeier 1985, Gilliam et at. 1989). This is in contrast to <br />studies of coarse or rocky bottomed streams which showed no significant density <br />effect of fish predation (Allan 1982, Reice 1983, Recker and Allan 1984, Culp 1986, <br />Reice and Edwards 1986). There are many possible reasons for this, several of which <br />are discussed by Allen (1983). One possibility is that in soft sediment slow moving <br />streams or pools, where there is little current and the organisms exhibit very little <br />propensity for drifting, the fish must actively forage for their prey. In contrast, coarse <br />bottomed streams usually have a swifter current due to higher gradient, and are often <br />dominated by salmonid species which wait in pools for their prey to drift by. In <br />addition, the rapid prey exchange caused by the steady immigration of persistently <br />drifting insects in rocky streams may overwhelm the local impacts of predators (Sih <br />and Wooster 1994). Another possibility for the difference in predation among <br />substrate types may be the ability of prey to find a suitable refuge. It is possible that <br />rocky substrates offer more refuge than soft substrates do (Allan 1983, Gilliam et at. <br />1989). <br />Despite the similarity of the results of our study and studies of other soft <br />bottomed streams, the backwater habitats that we sampled are part of a large river. <br />Because of the river size we were able to sample a habitat not usually associated with <br />smaller streams. The backwaters of our experiment were shallow and had no <br />10