EVALUATING THE EFFECTS OF BASS ON STEELHEAD
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.