Hybridization Status, Distribution, and Summer Movement of Interior Columbia River Redband Trout, Oncorhynchus mykiss gairdneri, in an Intermittent Stream

Ichthyofaunal research in intermittent rivers and ephemeral streams (IRES) of the Pacific Northwest, USA, is urgent given the likelihood of decreased summer stream flows and elevated temperature as predicted by regional climate change models. In this investigation, we determined the hybridization status, distribution, and summer movement patterns of resident Columbia River redband trout (Oncorhynchus mykiss gairdneri) in Dry Creek, an intermittent Idaho stream. In 2012, we collected fin clips from 136 Dry Creek redband trout for genetic analyses across 187 SNPs. In the summer of 2013, we surveyed 50 reaches in Dry Creek for redband trout. Temperature loggers were deployed in 35 reaches to determine whether summer stream temperatures affected trout occurrence. We implanted 407 redband trout with PIT (passive integrated transponder) tags, and fish movements were monitored using two 3-m in-stream PIT Tag Antenna Systems. Genetic analyses confirmed a nonintrogressed trout population. Redband trout occupied 19.0 km of Dry Creek during early summer but were absent from reaches within 12 km of the mainstem Boise River. Upper Dry Creek had a mean daily maximum summer temperature of 18.0 °C (range 15.0 °C to 21.0 °C), while sites in lower Dry Creek had an average daily maximum of 21.5 °C (range 18.1 °C to 27.1 °C). Fish were more abundant in the upper reaches (62 redband trout per 100 m) than in the lower reaches (32 redband trout per 100 m) of Dry Creek. Most tagged trout (88%) had no record of movement. Of those trout detected by our PIT-tag antennas (n = 49), 94% moved upstream, and movements peaked in June, prior to flow cessation and the drying of lower Dry Creek. Our results indicate a genetically pure population of Dry Creek redband trout and that the summer distribution and movement of trout are likely affected by temporal and spatial stream intermittency patterns.