Tuesday, May 27, 2008
257

Effects of population density on feeding behavior by brook and cutthroat trout in an idaho stream

David Owens1, Joseph R. Benjamin2, Colden V. Baxter2, Fabio A. Lepori3, and Kurt D. Fausch3. (1) School of Natural Resources, University of Nebraska, 3310 Holdredge St., Lincoln, NE 68583, (2) Stream Ecology Center, Department of Biological Sciences, Idaho State University, Pocatello, ID 83209-8007, (3) Department of Fish, Wildlife, and Conservation Biology, Colorado State University, Fort Collins, CO 80523

This research investigated the effects of nonnative brook trout (Salvelinus fontinalis) on a stream-riparian ecosystem in Idaho. Previous studies demonstrated that brook trout reach higher densities and out-compete native cutthroat (Oncorhynchus clarkii) for aquatic insect resources.  Specifically, we examined whether brook trout alter their feeding with density, shifting from drift feeding to benthic foraging.  After isolating 30 meter reaches with fencing, we removed fish by electro-shocking and restocked either with cutthroat at their natural density, brook trout at their natural density, or brook trout at a reduced density equivalent to natural cutthroat density.  Fish were monitored visually, and each attempt to feed from the surface, drift, or substrate by different size classes was recorded.  The results indicate that large brook trout rarely feed from the benthos in natural density treatments, whereas smaller size classes feed on benthic resources.   In contrast, all sizes of native cutthroat fed on drifting invertebrates under natural density, as did all size classes in the low density brook trout treatment.  These results indicate that as brook trout compete with and displace native cutthroat, size class differences in feeding habits will result in greater feeding pressure on benthic invertebrates, further altering food webs in these streams.


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