Graduation date: 2007Recent studies suggest that competition from brook trout (Salvelinus fontinalis; EBT) may have negative effects (e.g. displacement) on Lahontan cutthroat trout (Oncorhynchus clarki henshawi; LCT). Results from these studies have failed to elucidate the responsible mechanisms and have not examined if changing environmental conditions result in competitive/subordinate role reversals. The primary objectives of this study were to 1) determine if LCT and EBT compete for resources in a stream in which they co-occur, 2) determine the mechanisms responsible for the apparent competition and 3) determine if the results from competitive interactions change under variable environmental conditions. A field study examined how LCT and EBT interacted in a colonization situation typical of many LCT reintroductions, and provided insights into how LCT responded, under natural conditions, to EBT under various habitat conditions. A laboratory experiment, informed largely from the field study, was designed to force competitive interactions and measure the responses of LCT under varying habitat and environmental conditions. Results from the field manipulation revealed that EBT re-colonized a section of stream farther and faster than LCT and that about 25% of the individuals of both species stayed in the relocation pool. When in sympatry, LCT lost five times as much weight as EBT during the three week trial (2.5g to 0.5g, respectively). Lahontan cutthroat trout almost always had empty stomachs whereas EBT usually had at least some food present in their stomachs. Under laboratory conditions where density, species proportions, and temperature were manipulated, LCT typically lost weight and exhibited elevated blood plasma cortisol levels, an indicator of stress, during the 9 day trials (-8% and ~60 ng/mL, respectively) whereas EBT typically gained weight and exhibited lower cortisol levels (+1% and ~23 ng/mL). When the water temperature remained at 13°C, EBT gained an average of 5% of their body weight and had low plasma cortisol levels (~16 ng/mL) whereas LCT lost an average of 6% of their body weight and had elevated plasma cortisol levels (~40 ng/mL). As temperatures increased to 23°C, both species lost weight and exhibited elevated cortisol levels but LCT lost more weight than EBT (11% vs. 5%). As the proportion of EBT increased relative to LCT in experimental trials, LCT lost more weight and generally exhibited higher cortisol levels. Additionally, feeding observations in tanks that contained both species indicated that EBT were the first feed greater than 90% of the time, while LCT almost never fed and remained inactive when in the presence of EBT. In tanks that did not contain EBT, LCT were active and readily fed. Collectively, these results suggest that EBT are the dominant competitor and that the mechanisms for competitive dominance may, in part, be both physiological and behavioral in nature.