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9 <br />Discussion <br />Piscivory is not documented in literature reviews of fathead minnow food habits <br />(Carlander 1969; Scott and Grossman 1973; Held and Peterka 1974). While Franzin and <br />Harbicht (1992) found walleye larvae in stomachs of fathead minnows captured in trap <br />nets, they were uncertain whether the predation was a natural event or an artifact of <br />confinement of prey and predator in the trap net. I have shown fathead minnows to be <br />effective predators on sucker larvae in tanks. It is possible that the observed predation <br />resulted solely from artificial confinement in tanks (e.g. La Bolle 1981). However, <br />coincident use of shoreline habitats by larval suckers and very large numbers of fathead <br />minnows in Upper Klamath Lake (Buettner and Scoppettone 1990; Dunsmoor, <br />unpublished data) makes the possibility of such predation an important management <br />question in view of the endangered status of the suckers. <br />Fathead minnows displayed some specific predatory characteristics in this study. <br />First, fathead minnows appeared to be effective as predators only when in a group of at <br />least three fish; when alone, no fathead minnows consumed sucker larvae. Second, there <br />was no discernable relationship between body size of sucker larvae consumed and mouth <br />size of fathead minnows (i.e. fathead minnows do not appear to be gape-limited when <br />foraging on soft-bodied sucker larvae). The typical gape-limitation paradigm applies to <br />predators that consume prey whole, a practice for which the size of some cross-sectional <br />body dimension of the prey fish (commonly body depth) relative to some cross-sectional <br />dimension of the predator's mouth or pharyngeal apparatus limits the maximum size of <br />prey fish that can be ingested whole by a predator of a given size (e.g. Lawrence 1958). <br />Such relationships can have important management implications, as exemplified by the <br />utility of the Available Prey to Predator Ratio (Jenkins and Morais 1978) and the insights <br />generated into size-structured mechanisms related to year class formation in largemouth <br />bass Micropterus salmoides (Gutreuter and Anderson 1985; Keast and Eadie 1985). In <br />this study, fathead minnows ingested some sucker larvae headfirst and whole, while <br />others were torn apart and eaten piecemeal by a group of minnows. Therefore, the <br />maximum size of sucker larvae that could be consumed by fathead minnows will be <br />determined by something other than body dimension relationships. Sucker larvae used in <br />this study ranged in size from 13-21 mm. This study did not define the upper limit in <br />larval body size that fathead minnows could consume, and until field studies show <br />otherwise it must be considered possible that sucker larvae larger than 21 ram are <br />susceptible to fathead minnow predation in natural habitats.