Overwintering Distribution and Postspawn Survival of Steelhead in the Upper Columbia River Basin

Abstract

Adult summer‐run steelhead Oncorhynchus mykiss overwinter in freshwater for several months prior to spawning. In systems supporting mixed populations of fisheries and conservation importance, understanding the distribution and survival of pre‐ and postspawn fish is necessary for informed management. The upper Columbia River supports hatchery‐origin components valued by anglers, natural‐origin components of conservation concern, and temporary strays from downstream populations. We used radiotelemetry and PIT telemetry to monitor the behavior, distribution, and survival of adult steelhead during fall, overwintering, spawning, and postspawn periods, with a focus on use of the main stem versus four major tributaries. Adult steelhead (N = 807) were tagged at Priest Rapids Dam in 2015 and 2016. One‐fifth of steelhead fell back below Priest Rapids Dam and did not reascend. A slight majority of tagged steelhead that overwintered upstream of Priest Rapids Dam did so in main‐stem reservoirs (54%; N = 548). Overwintering in the main‐stem Columbia River was more likely for later‐arriving steelhead and was concentrated in the upstream‐most reservoir. Winter tributary use was highest in the Wenatchee (26%; 2016) and Methow (18%; 2015) rivers, whereas no steelhead overwintered in the Entiat River. Harvest of hatchery‐origin steelhead was 18% in 2015 and was near zero in 2016, when the fishery was suspended due to low adult returns. After accounting for reported harvest, annual overwinter survival did not differ between main‐stem and tributary habitats, and relatively low adjusted survival of hatchery‐origin steelhead in 2015 suggested unreported harvest. In contrast to low iteroparity rates (<3%), the majority of postspawn steelhead (56.5%) exited tributaries as kelts; kelt survival to Bonneville Dam was 65% in 2016 and 23% in 2017. Collectively, the results highlight the importance of understanding patterns of habitat use and mortality in steelhead populations when managers are faced with balancing harvest and conservation goals.

Abundance and Migration Success of Overshoot Steelhead in the Upper Columbia River

“…The estimated annual mean number of wild steelhead overshoot fallbacks at PRD during the study period was 1,135 (SD = 729), or 25% (SD = 6.4%) of the adjusted wild steelhead count at PRD. A radiotelemetry study of adult steelhead, which was conducted at PRD between 2015 and 2017, reported similar levels of fallback (Fuchs et al 2021). In that study, radio tag-based estimates of wild steelhead overshoot fallback were slightly higher (mean = 22.1%; SD = 1.8%) than PIT tag-based model estimates but were based on a smaller sample size.…”

“…A radiotelemetry study of adult steelhead, which was conducted at PRD between 2015 and 2017, reported similar levels of fallback (Fuchs et al. 2021). In that study, radio tag‐based estimates of wild steelhead overshoot fallback were slightly higher (mean = 22.1%; SD = 1.8%) than PIT tag‐based model estimates but were based on a smaller sample size.…”

“…Although these percentages were not adjusted for detection probability, the mean detection probability for all IPDSs installed in every spawning stream upstream of Rock Island Dam was 0.894 (SD = 0.132) and ranged from 0.505 to 0.999 (Waterhouse et al 2020), suggesting that very few known unsuccessful overshoot steelhead went undetected. While several potential minor spawning areas were not monitored using IPDSs (Fuchs et al 2021), we assumed that most of the unsuccessful overshoot steelhead suffered the same rate of mortality in the main stem as non-overshoot fish or downstream passage-related mortality from turbine strikes, as opposed to successfully spawning in an unmonitored tributary above PRD.…”

“…Summer spill programs and juvenile bypasses are shut down for the season in late August or early September because the juvenile out-migration period has ended (UCSRB 2018). Unfortunately, this coincides with the period when overshoot steelhead initiate their downstream migration back to their natal streams (Fuchs et al 2021). However, surface spill passage routes are provided until November 15, but only at both Wanapum Dam (i.e., bypass) and PRD (i.e., sluiceway), specifically for adult steelhead (GCPUD 2006).…”

“…Hence, the response variable measured at the population scale (i.e., changes in abundance) should be an important consideration for adaptively managing protective measures that are intended to increase fallback migration success in the future. Fuchs et al (2021) reported that downstream passage at PRD began in early September through mid-December and resumed in early March. Extensive downstream migration studies are resource intensive.…”