Hybridization with rainbow trout alters life history traits of native westslope cutthroat trout

Hybridization with rainbow trout is a primary threat to the conservation of westslope cutthroat trout (Oncorhynchus clarkii lewisi; hereafter cutthroat) across its native range. Cutthroat conservation policy hinges on the degree to which populations are hybridized, yet little is known about differences in the life history of individuals with varying degrees of rainbow trout ancestry. We examined differences in growth, fecundity, and migration timing between cutthroat and hybrid trout in migratory components of populations from 2006 to 2009 in the Jocko River, Montana. We detected positive linear relationships between rainbow trout ancestry and somatic growth and egg size, but a negative relationship with fecundity. US federal policy suggests a threshold of 20% rainbow ancestry for cutthroat conservation. In our study, hybrids with ≥20% rainbow trout ancestry had significantly lower fecundity and larger egg sizes and migrated earlier at lower discharges and stream temperatures than individuals with <20% rainbow trout ancestry. Our results did not indicate threshold changes but rather continuous differences associated with rainbow trout ancestry, suggesting that establishing a biologically relevant cutoff for conservation purposes will be problematic.