Plasticity in foraging behaviour of a lotic minnow (Rhinichthys cataractae) in response to different light intensities

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Abstract
Unlike most cyprinids, the longnose dace (Rhinichthys cataractae) forages primarily at night in streams in southern Alberta [Canada]. This nocturnal strategy may result from intrinsic capabilities, like dark-adapted vision, or extrinsic constraints due to biotic interactions during the day. Our objectives were to determine, in the absence of biotic interactions such as predation and competition, (i) whether this foraging habit maximized the foraging ability of the dace in terms of search time, reaction distance, and attack efficiency; and (ii) if dace foraging behaviour was sufficiently plastic to adjust to changes in light intensity. All measures of foraging ability were superior under twilight conditions. Although search times in starlight were double those in twilight, this increase was much smaller than was predicted using a hemispherical search volume model. The daces'' ability to offset small nocturnal search volumes was accomplished by changing their pattern of search behaviour. In particular, use of a benthic rooting behaviour increased and we hypothesize that this behaviour is associated with location of prey by olfaction. Although the nocturnal foraging strategy of dace does not appear to maximize net energy gain, this strategy may be a response to other constraints, such as high predation risk during diel periods with increased illumination.