We conducted a field experiment to test the hypothesis that intragastric radio-tagging contributed to increased prespawn mortality (PSM) of adult Chinook Salmon Oncorhynchus tshawytscha after collection and transport to spawning sites above high-head hydroelectric dams. We assessed PSM rates of 970 wild and hatchery Chinook Salmon collected during trap-andhaul operations that were released untagged, tagged with passive integrated transponder (PIT) tags only, or double tagged with PIT tags and radio transmitters, and then recovered as carcasses in two Willamette River, Oregon, tributaries from 2009 to 2015. Results revealed no evidence that PSM rates were higher in PIT-tagged samples than in untagged (but not unhandled) control samples. The PSM rates in double-tagged samples were variable among years and between locations and indicated that radio-tagging effects were absent or small in effect size, on average, within each population. While we did not detect a consistent negative double-tagging effect across locations and years, results suggest that the potential for radio-tagging effects should be incorporated in study planning and design. We recommend that researchers use experimental designs that include control groups for directly evaluating tagging and handling effects on study outcomes.
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