Competition as a Factor in Displacement of Native Cut Throat Trout by Nonnative Rainbow and Hybrid Trout

Abstract

Native salmonid fishes have been displaced worldwide by nonnatives through competition and hybridization, but dynamics of these factors are poorly understood. We apply a Lotka-Volterra population model to displacement of cutthroat trout (Oncorhynchus clarkii) by rainbow (Oncorhynchus mykiss)/hybrid trout in the Snake River, USA. Cutthroat trout are susceptible to hybridization in the river but are reproductively isolated in tributaries via removal of migratory rainbow/hybrid spawners at weirs. Hybridization is the primary mechanism for initial growth of the rainbow/hybrid trout population, but a model with hybridization alone does not explain observed trends. Two models, in which competition occurs 1) among river-spawned fish only and 2) among all fish, explain observed trends, but are indistinguishable from one another based on fit to data. When tributary-spawned cutthroat trout out-migrate as fry, competition with rainbow/hybrid trout results in extinction of cutthroat trout, even though reproductive segregation is maintained