The response of native and introduced trout to coarse woody debris: an abiotic and biotic evaluation.

KEYWORDS: Colorado River cutthroat trout; Utah; Wyoming; coarse woody debris; restoration; competition; brook trout

ABSTRACT

Colorado River cutthroat trout Oncorhynchus clarki pleuriticus Cope populations have declined considerably in areal extent and overall density. The main hypothesis invoked to explain this decline is that cutthroat trout are susceptible to hybridization with other salmonids. Another potential cause for the observed decline is that introduced salmonids (especially brook trout) compete for food and space. Survey data collected along the north slope of the Uinta Mountains in Utah and Wyoming support the notion of a negative relationship between introduced brook trout and native cutthroat trout. Additionally, we hypothesize that coarse woody debris is a mediating factor in this interaction possibly providing a competitive advantage to brook trout. We conducted two large-scale debris manipulative experiments to determine the role of coarse woody debris on both of these trout species and their potential interactions. A large scale manipulative experiment was performed to evaluate the response of an allopatric population of Colorado River cutthroat trout to the addition of large organic matter. The study was performed over a two year period in a number of streams located along the north slope of the Uinta mountains in Utah and WYoming. Coarse woody debris was added to a series of reaches located in one of the study streams devoid of any natural wood, while the remaining streams served as controls. The addition of large organic matter resulted in a number of significant abiotic and biotic changes: 1) particle retention significantly increased in those reaches with wood, 2) stream velocities and substrate sizes were more variable in those reaches with wood, 3) the invertebrate composition was altered by the addition of wood, and 4) the density of 1+ cutthroat trout was significantly enhanced by the presence of wood. In addition, we manipulated wood densities and fish community structure (cutthroat alone or with brook trout) to determine if added debris did in fact alter competitive outcomes. All of these results will be discussed in the context of restoration strategies for enhancing existing populations of Colorado River cutthroat trout.

RESUMEN

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