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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, Schlosser and <br />Ebel 1989, Angermeier 1985, Gilliam et at. 1989). This is in contrast to studies of coarse or <br />rocky bottomed streams which showed no significant density effect of fish predation (Allan <br />1982, Reice 1983, Flecker and Allan 1984, Culp 1986, Reice and Edwards 1986). There are <br />many possible reasons for this, several of which are discussed by Allen (1983). One is that in <br />the soft sediments of slow moving streams or pools, where there is little current and the <br />organisms exhibit very little propensity for drifting, fish must actively forage for their prey. <br />In contrast, coarse bottomed streams usually have a swifter current due to higher gradient, and <br />are often dominated by fish species which wait in pools for their prey to drift by. In addition, <br />the rapid prey exchange caused by the steady immigration of persistently drifting insects in <br />rocky streams may overwhelm the local impacts of predators (Sih and Wooster 1994). <br />Another possibility for the difference in predation among substrate types may be the ability of <br />prey to find a suitable refuge. It is possible that rocky substrates offer more refuges than do <br />soft substrates (Allan 1983, Gilliam et at. 1989). <br />Despite the similarity of the results of our study with studies of soft bottomed streams, <br />the backwater habitats that we sampled are not usually associated with smaller streams. Except <br />for their lack of vegetation they were very similar to the littoral region of a lake or pond. <br />Backwaters have higher standing crops of invertebrates than the main river channel (Mabey <br />1993, Wolz and Shiozawa 1995). This higher food resource density is probably an important <br />factor in attracting fish to the backwaters.