Masters Thesis

Modeling population interactions between native Yellowstone cutthroat trout and invasive rainbow trout in the south fork Snake River

The upper South Fork Snake River in Idaho supports one of the last remaining large- river populations of Yellowstone cutthroat trout (Oncorhynchus clarkii bouveri), which is threatened by competition and hybridization with introduced rainbow trout (O. mykiss). The Idaho Department of Fish and Game has implemented a three-pronged approach to preserving Yellowstone cutthroat trout that consists of reproductive isolation using barrier weirs, flow management, and angler harvest of rainbow/hybrid trout. This thesis presents an updated and expanded version of a model of the population dynamics of Yellowstone cutthroat trout and rainbow/hybrid trout that is used to conduct simulation experiments to predict the likely outcomes of multiple potential management scenarios and identify the management combinations most likely to result in long-term persistence of Yellowstone cutthroat trout in the study reach. A discrete-time, age-structured population model tracks same age cohorts of tributary-spawning Yellowstone cutthroat trout, river-spawning Yellowstone cutthroat trout, and rainbow/hybrid trout separately through life stages, population interactions, and mortality, including spawning and hybridization, potential peak spring flow-induced mortality of eggs and fry, age-0 competition for flow-dependent habitat during the first winter, and size-dependent angler harvest of rainbow/hybrid trout. Demographic parameters, flow parameters, and flow-dependent population attributes were estimated from site-specific data provided by Idaho Department of Fish and Game, drawn from literature wherever possible, or calibrated if necessary. We assumed that reproductive isolation remains in place and conducted 2000 simulations with environmental stochasticity for 25 years for each of 72 combinations of flow and harvest scenarios. The simulations showed that population trajectories and species composition are most sensitive to the level of harvest on rainbow/hybrid trout: the no-harvest scenarios showed a rapid decline in Yellowstone cutthroat trout, while scenarios with high harvest rates were fairly successful at controlling rainbow/hybrid trout numbers and preserving Yellowstone cutthroat trout. In scenarios that resulted in loss of the main river subpopulation, reproductive isolation in the tributaries would be the only mechanism for Yellowstone cutthroat trout persistence. Flow management within the relatively narrow range of feasible scenarios we tested had a relatively small effect compared to harvest, but may be of some value if high harvest rates cannot be maintained. Assuming the weir program continues, the model indicates that angler harvest is the most important tool for preserving Yellowstone cutthroat trout from displacement by rainbow/hybrid trout in the upper South Fork Snake River.