Diets of sympatric Lahontan cutthroat trout and nonnative brook trout: Implications for species interactions

Nonnative brook trout (Salvelinus fontinalis) have been implicated in declines of stream-living Lahontan cutthroat trout (Oncorhynchus clarki henshawi), a threatened trout endemic to the Lahontan Basin of northeastern California. southeastern Oregon. and northern Nevada. Brook trout may displace Lahontan cutthroat trout through 2 mechanisms: interspecific predation and competition for food. To evaluate the evidence for these alternatives, we examined stomach content of 30 trout of each species captured in the North Fork Humboldt River, northeastern Nevada, to compare number, size. and taxonomic composition of prey. Taxonomic dietary overlap was high (81.4%) between brook and Lahontan cutthroat trout. Both species were nonselective in their feeding habits. Lahontan cutthroat trout consumed over 2.5 times as many prey on average, but brook trout consumed significantly larger prey. No trout of either species occurred in fish diets. Only a single fish, a Paiute sculpin (Cottus beldingi), was found in stomachs, and the majority (>90%) of prey consisted of insect taxa. Size and number of prey consumed were positively related to fish size for Lahontan cutthroat trout but not for brook trout. These results tio not provide compelling evidence to suggest feeding by Lahontan cutthroat trout is limited by presence of large numbers of brook trout in the North Fork Humboldt River. However, fundamental differences in each species' utilization of food in this system indicate that a better understanding of observed differences may help to explain the variable success of brook trout invasions across stream habitats in the Lahontan Basin and their potential effects on Lahontan cutthroat trout.