Timing of Population Fragmentation in a Vulnerable Minnow, the Umpqua Chub, and the Role of Nonnative Predators

Abstract

AbstractWe examined the distribution of Umpqua Chub Oregonichthys kalawatseti, an endemic, vulnerable minnow in western Oregon, and whether six ecological populations (based on distribution patterns) had sufficient genetic cohesion to be considered evolutionary populations. We also evaluated the influence of Holocene geological events and recent nonnative predator introductions on the timing of population formation or fragmentation. Based on data from 10 microsatellite loci, we found evidence for four evolutionary populations of Umpqua Chub. One population, in the Smith River, is isolated by the Umpqua estuary and is more than 100 river kilometers from the other three populations: Elk Creek, Calapooya Creek–Olalla Creek, and Cow Creek–South Umpqua River. Quantile regression was used to examine the timing of genetic divergence among evolutionary populations assuming a genetic isolation‐by‐distance model. The quantile regression suggested that the genetic differentiation index (FST) should change by at least 0.0002/km; most fragmentation was recent and with similar timing, but the Smith River isolation event may have been about 2–4 times older. We could not distinguish whether the timing of the Smith River isolation corresponded to the last major tsunami event or the introduction of Striped Bass Morone saxatilis, a likely predator. All population fragmentation appears to be relatively recent, with the three upstream populations restricted to third‐ and fourth‐order streams, most likely fragmented by either nonnative Smallmouth Bass Micropterus dolomieu, which now dominate sixth‐order streams, or in the case of Elk Creek, a dam. The mid‐drainage Calapooya–Olalla population was the most genetically diverse and appeared to be a mix of the other populations, which showed a significant isolation‐by‐distance relationship to this population. We hypothesize that Umpqua Chub populations have formed and fragmented by peripheral isolation from a larger population, the remnant of which is the mid‐drainage Calapooya–Olalla population.