Localized Genetic Effects of a Long-Term Hatchery Stocking Program on Resident Rainbow Trout in the Metolius River, Oregon

Hatchery rainbow trout Oncorhynchus mykiss have been stocked in the Metolius River in central Oregon since 1938, and legal-sized (≥160 g) yearling trout were stocked annually from 1947 until 1995. In 1996, management objectives shifted to emphasize wild trout, and hatchery stocking ceased. We examined allozyme and mitochondrial DNA (mtDNA) variation among three naturally occurring populations of rainbow trout in the Metolius River to investigate possible hybridization with hatchery-produced rainbow trout. We also examined two commonly used hatchery strains of rainbow trout, one of which has supplied nearly all of the catchable hatchery trout in the Metolius. Both allozyme and mtDNA data showed the two hatchery samples to have genetic characteristics typical of hatchery populations derived from coastal rainbow trout O. mykiss irideus. Rainbow trout sampled from the lower Metolius River, approximately 30 km downstream of the headwaters, had allozyme and mtDNA characteristics typical of interior rainbow trout O. m. gairdneri. The two samples from the upper Metolius River, where stocking activities occurred, had allozyme profiles intermediate between interior and coastal types and mtDNA haplotypes characteristic of both interior and coastal populations. We attributed the upper-river results to hybridization between indigenous rainbow trout and the hatchery trout that had been stocked there for nearly 60 years. We attribute the lack of hybridization in the lower Metolius River to ecological isolation: the upper river meanders through park-like habitat, whereas the lower river has greatly increased water flows and velocities and a steep gradient, creating a habitat that may be inhospitable to hatchery-reared rainbow trout. Stocked hatchery trout that drift or migrate downstream into the lower river likely perish or are carried farther downstream into Lake Billy Chinook, where they are subject to lethal infection by the myxosporean parasite Ceratomyxa shasta and where a robust population of bull trout Salvelinus confluentus exists. If some fish from the genetically pure interior rainbow trout population in the lower Metolius River were to migrate to the upper river and spawn there, the hybridized upriver population would receive a steady infusion of genes from native fish. Future monitoring of life history and genetic attributes of the upper and lower rainbow trout populations could reveal whether such an infusion occurs.