The Genetic Relationship between Anadromous and Resident Oncorhynchus mykiss at a Putative Barrier with Implications for Habitat Improvement

Abstract

AbstractBig Bear Creek in the Potlatch River system, Idaho, contains an Endangered Species Act‐listed wild population of steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) that is the focus of extensive habitat restoration actions intended to increase population abundance. Both anadromous and resident O. mykiss populations occur in the Big Bear Creek drainage; however, anadromous individuals are known to spawn and rear only in the lower drainage, whereas putative resident fish occur in the headwaters. Big Bear Falls is a potential upstream migration barrier that is located between the two populations. Oncorhynchus mykiss of unknown origin are present within 1 km above the falls. We used analyses of genetic diversity, structure, and sibship reconstruction to determine whether Big Bear Falls limited the movement of steelhead to headwater areas. Known resident O. mykiss were sampled in headwater areas, known anadromous adults were sampled at a weir in the lower drainage, and juveniles of unknown life history were collected from areas below and above the falls. Allele frequency data indicated that anadromous and headwater resident populations were highly differentiated and that exchange of genetic material between populations was limited. However, juvenile O. mykiss in the above‐waterfall collection were found to be the offspring or recent descendants of steelhead that successfully navigated above Big Bear Falls, demonstrating that the falls is not a complete barrier. We also identified evidence of limited downstream gene flow, suggesting that resident fish contributed genetic material to the downstream anadromous population. This study documented the successful passage of adult steelhead above Big Bear Falls and the downstream movement of resident fish from the headwaters. However, uncertainties still exist regarding the annual variability in passage and the habitat limitations that impact O. mykiss utilization of areas immediately above the falls.Received March 14, 2015; accepted October 27, 2015