, Volume 15, Issue 2, pp 387-403
Date: 22 Oct 2013

Archival genetic analysis suggests recent immigration has altered a population of Chinook salmon in an unsupplemented wilderness area

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Abstract

Detecting genetic population shifts (i.e. allele frequency differences) through time is a primary function of effective conservation monitoring, but it is equally vital to understand the underlying causative factors of change which may be revealed through analyses of long-term, temporal trends. We compared archival and contemporary Chinook salmon (Oncorhynchus tshawytscha) collections from the John Day River in Oregon, USA, to evaluate the temporal relationships among four primary spawning areas over a span of 28 years (1978–2006). Although it lies amid many hatchery-supplemented salmon populations of the Columbia River, the John Day River has itself experienced no directed supplementation. Using a combined panel of 13 microsatellite and 92 single nucleotide polymorphism loci, we observed significant temporal heterogeneity across sample sites and tested for two likely evolutionary influences: stochastic processes (i.e. genetic drift) and gene flow via immigration. Based on abundance and effective population size estimates, we found no evidence indicating a recent bottleneck. We observed a sharp temporal decline in probability of self-assignment of John Day River fish, particularly for the North Fork tributary. There was a corresponding increase in assignment to distant Snake River populations, attributed to accumulating introgression from out-of-basin sources over time. Our study demonstrates that low level immigration sustained over multiple generations can alter the genetic composition of natural populations, and while immigration may help maintain genetic population diversity, it risks reducing adaptive advantages in local ecosystems.