Date of Award:

1996

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Advisor/Chair:

Dr. Todd A. Crowl

Abstract

Populations of the introduced book trout, Salvelinus fontinalis, have recently become more widespread and abundant in western North American streams, possible at the expense of native Colorado River cutthroat trout, Oncorhynchus clarki pleuriticus. We examined the intensity and potential mechanism of competition between these species.

Feeding experiments in laboratory stream channels showed that cutthroat trout feeding efficiency decreases in the presence of a brook trout. Decreased feeding efficiency appeared to be due to interference, as cutthroat trout were inactive in the presence of the brook trout. Evidence for interference competition in the feeding experiments was also given by the fact that brook trout feeding efficiency was lower than the feeding efficiency of cutthroat trout. The decreased feeding efficiency of cutthroat trout in the presence of brook trout was due to decreased attack rates by cutthroat trout, and was not due to attacks and consumption of the food items by the brook trout.

A field enclosure experiment, in which riffle-pool sections of a stream were isolated by fencing, was performed to determine if the presence of brook trout had a negative effect on the growth, fat content, and diet of cutthroat trout. Cutthroat trout fat levels were significantly lower in the presence of brook trout. The growth of cutthroat trout was not significantly different in the presence and absence of brook trout, but there was a trend fro lower growth of cutthroat trout in the presence of brook trout. Diet choices and total biomass of prey consumed by cutthroat trout in the field experiment and in a survey of three streams were not affected by the presence of brook trout.

The observed decreased feeding efficiency of cutthroat trout in the presence of brook trout may be the mechanism responsible for the significant;y decreased fat levels during the relatively short, summer growing season and may result in reduced population sizes due to high overwinter mortality and delayed sexual maturity.

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