PIT-Tag Detection Histories Suggest Differences in Ocean Survival Between Hatchery and Natural Populations of Umatilla River Origin Summer Steelhead

The Umatilla Hatchery uses 100% natural origin steelhead Oncorhynchus mykiss broodstock in a hatchery supplementation program designed to bolster the abundance of an ESA listed Umatilla River steelhead population. Despite the use of natural broodstock, adult returns of hatchery steelhead to the Umatilla River have fallen short of program goals. In 2006, we increased our use of PIT tags implanted in hatchery and natural smolts to investigate whether differences in juvenile and adult migration patterns exist. In this study, we compared PIT-tag detection histories of natural and hatchery origin steelhead from the 2006 smolt migration through the 2014-15 adult spawning run. Natural smolts emigrated predominately at age-2, and were on average 38-mm smaller than hatchery smolts that emigrated as yearlings, a difference that was statistically significant (P<0.01). Despite the differences in size and age at emigration, travel times and Cormack-Jolly-Seber survival estimates to John Day Dam were not significantly different between hatchery and natural smolts (P=0.18 and P=0.86, respectively). Adult detections at Bonneville Dam revealed that naturally-produced Umatilla steelhead had greater smolt-to-adult return (SAR) proportions than their hatchery-produced counterparts (P<0.01). The similar smolt migration survival estimates suggest that differences in cohort-specific SAR values may be driven by differential ocean survival.