All dam-affected trout populations are not alike: fine scale geographic variability in resident rainbow trout in Icicle Creek, WA, USA

Abstract

Recognizing the genetic diversity within and among collections of allopatric rainbow trout is an important step in understanding and monitoring the dynamics of the metapopulation structure of a species like Oncorhynchus mykiss with resident and anadromous life history forms. Prior to the removal of a barrier and the recolonization of Icicle Creek with anadromous steelhead, we report the degree to which collections of above-barrier resident rainbow trout from 13 sites differ from downstream steelhead, and the pattern of genetic diversity and connectivity among resident collections using 14 microsatellite loci. Measures of genetic variability (He, AR, and A/L) are low in the upper-most collections of residents and estimates of Ne change approximately 4-fold from the upper tributaries (Ne~90) to the lowest main stem collections (Ne~360) over 35 river kilometers (rkm). The overall comparison of resident rainbow trout versus below-barrier steelhead is FST = 0.053. A STRUCTURE analysis of all 1,730 fish indicated three populations within the above-barrier collections of resident fish. Notably, two sets of upstream collections of rainbow trout, separated at a minimum of 16.4 rkm, had a mean FST = 0.128. Natural passage barriers account for some of the observed stock structure in Icicle Creek but the strongest differences are not associated with barriers by our analysis. No significant temporal variability was seen within four rainbow trout sites and one steelhead site; and no hatchery rainbow trout ancestry was detected in the watershed. In general these results highlight the need for conservation efforts to include fine-scale evaluations of population structure of riverine fishes above barriers to increase the accuracy of understanding and monitoring intra specific diversity and the biological effects of dams and dam removal.