Graduate Thesis Or Dissertation
 

Prey availability overrides cover in determining growth and abundance of stream-dwelling cutthroat trout

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  • The abundance and biomass of cutthroat trout (Salmo clarki) have been found to be greater in logged than in forested streams of the Oregon Cascades. Although certain prey taxa increase in abundance following logging, habitat stability generally decreases and cover structures are removed. Research was conducted to examine the manner in which habitat features interact with the prey base to result in greater abundance of the trout in logged streams. Cutthroat trout were more effective in foraging on experimentally introduced prey (Culex spp. larvae) in a logged section of Grasshopper Creek (Lane Co., Oregon) than in a forested section. The differences in efficiency were related to prey size and to the amount of overhead shading and substrate crevices. Mean percentages of prey captured by trout were greater in logged control pools and crevicecovered pools of both sections than in forested control pools. Artificial shading of logged pools reduced capture success by the trout to levels found for trout in forested pools. Relative growth rates of cutthroat trout experimentally confined in pools were also greater in the logged than in the forested reach. Differences in growth rates were primarily due to differences between the reaches in invertebrate drift density. Higher drift density in the logged section probably reflected a greater habitat instability that increases the probability that benthic fauna will occur in the water column where they are more available as prey. Drift density significantly increased relative to controls in troutexcluded pools in the logged reach, but not in the forested section. This may have resulted from habitat features in the logged section that favor greater trout foraging success, and from the occurrence of a greater relative proportion of behaviorally drifting prey taxa, which represent a predictable food source. In laboratory experiments, food abundance played a relatively greater role than did cover in determining the abundance of cutthroat trout. In trials of one week duration, few introduced trout emigrated from channels having high food abundance, irrespective of the amount of cover. In channels with low food abundance, no, or very few, trout remained, irrespective of the amount of cover. These results support field data that suggest that, in spite of reduced cover, cutthroat trout are more numerous in logged than in forested sites in response to a greater foraging return.
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