The Warner Sucker (Catostomus warnerensis), a freshwater fish endemic to the Warner Basin in southeastern Oregon, inhabits three principal tributaries — Twentymile, Honey, and Deep creeks — and the basin's shallow, intermittent lakes. The species was listed as threatened under the federal Endangered Species Act in 1985, due primarily to habitat fragmentation caused by irrigation diversion dams and threats from nonnative species. Recovery requires restoring passage within and among the three creek systems. The Warner Basin Aquatic Habitat Partnership, a collaboration of local landowners, irrigation districts, the Lake County Umbrella Watershed Council, USFWS, BLM, ODFW, and other partners, is working to modify or replace diversion structures across the basin. The Native Fish Investigations Section contributes to these recovery efforts through passage monitoring, distribution surveys, and the development of new monitoring tools.
Passage Assessment — Restoring fish passage at diversion dams is central to Warner Sucker recovery, but confirming that modified structures actually allow fish to move upstream requires direct evaluation. The Native Fish Investigations Section uses PIT-tagged fish and antenna arrays to assess whether Warner Suckers successfully pass through newly constructed fishways. Recent evaluations have focused on the JJ Diversion on lower Honey Creek, where irrigation ditch overflow was consolidated in 2023 to create a fish bypass route, and the O'Keeffe Diversion on Deep Creek, where a 208-foot fish ladder was constructed in 2023 to open 2.9 km of habitat below Deep Creek Falls. At the JJ Diversion, PIT tag monitoring confirmed that Warner Suckers successfully used the new bypass route to pass upstream. At the O'Keeffe ladder, high spring flows in both 2024 and 2025 prevented capture and tagging of local Warner Suckers for passage evaluation, though a Redband Trout released into the ladder was detected passing upstream, indicating that flow conditions within the structure are suitable for fish passage. Further evaluation with Warner Suckers under normal conditions is planned.
Distribution Surveys — Tracking where Warner Suckers occur and in what numbers is essential for assessing whether populations are stable across the basin — a key recovery criterion for delisting. Native Fish Investigations has monitored Warner Sucker distribution and abundance since 2007, using electrofishing and mark-recapture surveys across the species' range. A 2019 ODFW review compiled the available monitoring data and identified areas where additional survey work was needed. Since then, the section has collected updated distribution data for Twentymile Creek and conducted distribution and abundance surveys in Deep Creek. Work is ongoing to update surveys in other portions of the range.
eDNA Assay Development — Traditional mark-recapture surveys for Warner Sucker are labor intensive, require access to remote sites on private land, and involve extensive handling of a threatened species. To develop a less intrusive and more efficient monitoring tool, Native Fish Investigations has been field-testing an environmental DNA (eDNA) assay specific to Warner Sucker. A species-specific qPCR assay was validated in 2021, and field testing in 2022 revealed a strong positive relationship between eDNA concentration and Warner Sucker abundance in stream pools, suggesting that eDNA sampling could serve as a cost-effective index of population size. The study also found no significant effect of time of day on eDNA concentrations, despite the species' nocturnal habits. Further refinement is underway, but these results indicate that eDNA has strong potential as a practical monitoring tool for Warner Sucker across the basin.