Sabo, John. 1995. Competition between stream-dwelling cutthroat trout (Oncorhynchus clarki clarki) and coho salmon (O. kisutch): Implications for community structure and evolutionary ecology. Ph.D.
In coastal, Pacific Northwest streams, the influence of competition as a regulatory mechanism may vary temporally depending on the predictability of flow, thereby exhibiting "seasonally biotic" control during low-flow conditions. It also has been hypothesized that animal populations experiencing differing degrees of competitive intensity may diverge as a result of the differential selective pressures presented by these various regimes (Connell 1980). Juvenile cutthroat trout (Oncorhynchus clarki clarki) and coho salmon (O. kisutch) co-occur in many streams throughout their native ranges and, as such, are potential competitors. Life-history characteristics such as egg size and spawning timing may confer a size advantage to coho salmon fry, which in turn may augment coho competitive success during the early growing season. Further, resident allopatric populations of cutthroat exist in habitats inaccessible to anadromous salmonids (e.g. stream reaches upstream from impassable waterfalls). Thus, the opportunity exists to compare the competitive abilities of cutthroat populations originating from environments differing with respect to the presence or absence of interspecific competition with coho salmon.
The goal of this study was twofold: 1) to investigate the effect of relative size of cutthroat/coho pairs on the outcome of competitive success and 2) to determine if differences in competitive abilities exist between allopatric and sympatric populations of cutthroat when faced with a coho competitor. Behavioral feeding experiments were conducted in laboratory stream channels, in which foraging success and aggression were used as measures of competitive ability. Allopatric cutthroat out-performed sympatric cutthroat against coho competitors in terms of foraging success and aggression. Further, competitive success of both populations of cutthroat was largely dependent on the relative size of cutthroat/coho matched pairs. Lastly, sympatric cutthroat competitive success against coho was higher in experimental head-to-head dominance bouts than previously predicted by field surveys and laboratory experiments on habitat segregation. These findings are discussed in terms of community structure and evolutionary ecology.