EVALUATING THE EFFECTS OF BASS ON STEELHEAD
Prepared By:
Lindsy Ciepiela; ODFW Natural Resource Specialist 2
Ian Tattam; ODFW Supervisory Fish and Wildlife Biologist
The Lower Mainstem John Day River population is the only “Very Large” population in the John Day River MPG and is ecologically distinct from other populations. Steelhead in the Lower Mainstem John Day River basin occupy the lower elevation, drier Columbia Plateau ecoregion, and rely heavily on cool spring-fed tributaries for spawning and rearing. As such, steelhead rearing in the Lower Mainstem John Day River basin may express variations of life history traits (i.e., smolt age, adult return timing etc.) not expressed by the other four John Day River populations. Identifying and maintaining this life history variation within John Day River MPG will likely contribute to overall population stability and resiliency.
The major factors limiting steelhead productivity in the Lower Mainstem John Day River basin are similar to those in the other four John Day population groups and include degraded rearing and spawning habitat due to habitat degradation and altered flow, sediment and
temperature regimes (Carmichael and Taylor 2010). In addition to these limiting factors, the upriver range expansion of smallmouth bass (Micropterus dolomieu) into steelhead rearing tributaries presents a new and emerging threat to steelhead productivity throughout the John Day River basin (Rubenson and Olden 2020), especially in lower elevation and lower gradient tributaries to the Lower Mainstem and North Fork. Climate induced stream warming is creating thermal conditions optimal for the continued upstream expansion of smallmouth bass into natal steelhead tributaries. Alarmingly, Rubenson and Olden (2020) estimate smallmouth bass currently occupy 3-62% of current salmonid habitat in the Columbia River Basin but are predicted to expand their range by 75% in 2080. A large portion of their expected range expansion is into steelhead rearing habitat in tributaries. Currently, longitudinal thermal sorting (i.e., stream temperatures that gradually increase, generally, from headwater to mouth) largely limits sympatry between smallmouth bass and steelhead in all John Day Basin populations except the
Lower Mainstem John Day River population.
In the North Fork, South Fork, Middle Fork and Upper Mainstem John Day River populations steelhead rear higher in the watershed where temperatures are cooler, whereas bass, who prefer warmer waters, occupy lower
elevation stream sections. In these populations sympatry primarily occurs where the two species are at the edge of their thermal tolerance, and where both species are found in lower densities, limiting opportunities for conflict, as documented by Rubenson and Olden (2020) in the North Fork John Day River via stable isotope and fatty acid
sampling. In the Lower Mainstem John Day River thermal sorting does not limit species interaction, in some locations, because cool spring fed rearing tributaries flow directly into the seasonally warm waters of the John Day River, where smallmouth bass densities remain high year-round. This thermal juxtaposition ultimately gives smallmouth bass
immediate access to productive steelhead rearing tributaries. Observations in Bridge Creek, a steelhead rearing
tributary to the Lower Mainstem John Day River, suggest that smallmouth bass are likely to enter steelhead rearing tributaries between May and June and exit between July and August (I. Tattam, ODFW unpublished data). Smallmouth bass may be immigrating into steelhead rearing tributaries in the Lower Mainstem John Day to spawn as smallmouth bass nests have been observed in Rock Creek near Arlington. In many Lower Mainstem John Day steelhead rearing tributaries, smallmouth bass are present during steelhead fry emergence, and are hypothesized to opportunistically feed on fry, or compete for food and space. Quantifying the effect of smallmouth bass presence on steelhead productivity in Lower Mainstem John Day River tributaries is critical for developing strategies to maintain and increase steelhead production in the face of current and future expansion by smallmouth bass. This includes differentiating between whether any bass induced mortality is additive (i.e., an extra mortality source that would not occur in the absence of bass), or compensatory (i.e., bass prey on steelhead that would otherwise have died later due to other predators or a food/space shortage) (Haeseker et. al. 2020). The Mid-C steelhead recovery plan hypothesized that Lower Mainstem John Day river tributaries have high intrinsic productivity for steelhead, however, little empirical juvenile monitoring data exist for these tributaries. Funding and access to many steelhead-bearing tributaries in the Lower Mainstem John Day River historically limited habitat improvement project implementation, life-cycle monitoring, and status and trend monitoring, ultimately limiting our understanding of the distinct Lower Mainstem John Day River population.
In 2017 we established steelhead monitoring in Thirtymile Creek, a steelhead bearing tributary to the Lower
Mainstem John Day River, to address key knowledge gaps in our understanding of Lower Mainstem John Day River steelhead population dynamics, productivity, and life history diversity. Establishing monitoring in Thirtymile Creek
provided additional opportunities to do a post-hoc control-impact evaluation of the effectiveness of a riparian
corridor fence at improving habitat conditions and steelhead productivity; as well as establish baseline conditions for future restoration effectiveness monitoring. Finally, monitoring at Thirtymile Creek provided the opportunity to
evaluate the impact of smallmouth bass on rearing steelhead.
