Bull Trout (Salvelinus confluentus) found in Odell Lake, Davis Lake, and their tributaries in the central Cascade Range, form a small population isolated from other populations by a lava flow about 5,500 years ago. While Bull Trout are listed as threatened across their range in the coterminous United States, the Odell population is considered at especially high risk because of the isolation and small size of the population, habitat degradation, limited spawning and rearing habitat, incidental handling associated with angling, and negative interactions with nonnative fishes. The Native Fish Investigations Section, in partnership with ODFW's district fish biologists, USFWS, and the Deschutes National Forest, has been involved in research and monitoring in the Odell Lake Core Area for over two decades. Recent and ongoing work focuses on habitat characterization, spawning habitat improvement, mapping juvenile survival and distribution, and ecological interactions between bull trout and other fishes.
Characterizing Habitat — Identifying where suitable habitat exists is critical to conservation planning in the Odell Lake Core Area. Native Fish Investigations deployed an array of temperature data loggers throughout the core area and conducted habitat surveys to map both thermal and physical habitat suitability across the Core Area. We found thermally suitable habitat in Trapper and Crystal creeks, direct tributaries of Odell Lake, and Charhaven and Maklaks creeks, which flow into Odell Creek. Odell Creek was more variable: a cold segment influenced by tributary inflows had highly suitable spawning habitat, but a downstream segment still recovering from a stand-replacing fire in 2003 had reduced thermal suitability for rearing despite having excellent physical habitat. Within thermally suitable reaches, physical habitat features such as deep scour pools, spawning gravel availability, and fine sediment levels varied. A lack of spawning gravel was identified as a potential limiting factor in some areas with otherwise excellent habitat, leading to a targeted effort to augment spawning gravel (see below). These reach-scale assessments continue to guide the prioritization of habitat restoration actions across the core area and to inform long-term monitoring of thermal conditions.
Spawning Gravel Augmentation — The federal Bull Trout recovery plan identified low quality and quantity of spawning habitat as a threat to population persistence in the Odell Lake Core Area, and our habitat assessment work found that spawning gravel was scarce or embedded in key cold tributary streams. From 2021 through 2023, we excavated embedded substrate from 34 pool tailouts in Trapper Creek and filled them with approximately 40 cubic meters of spawning-sized gravel. The modified sites had ideal gravel composition after treatment, with mean surface fines dropping from 15% to 2% and mean gravel depth reaching 32 cm. Gravel was retained with no significant change in depth from 2022 to 2023, though a few individual sites lost material to high flows or debris. Bull Trout quickly adopted the improved habitat: 83% of redds observed in 2022 and 86% in 2023 were constructed in modified sites, and redds built in augmented tailouts were significantly larger than those in unmodified areas. Kokanee and Redband Trout also spawned in the modified sites. We continue to monitor substrate condition, gravel retention, redd abundance, and juvenile density annually to evaluate the long-term effectiveness of this work.
Survival and Distribution of Juveniles — Understanding where juvenile Bull Trout rear and how well they survive is essential for identifying habitat bottlenecks and evaluating conservation actions. Because so few juvenile Bull Trout remain in the Odell Lake Core Area, Native Fish Investigations used translocated juveniles from the nearby Metolius River drainage as study fish. During 2018–2020, the section PIT-tagged and released approximately 2,950 juvenile Bull Trout across seven sites in the core area. An array of seven fixed PIT interrogation stations and annual mobile PIT surveys tracked the movement and survival of tagged fish starting in 2018. Data collected through 2024 showed that tagged Bull Trout used most of the study area across all life stages examined and during most of the year. Annual survival probabilities estimated from these data compare favorably with estimates from other bull trout populations, indicating that stream conditions in the core area are generally suitable for juvenile rearing. Data collection for this project is ongoing.
Competition between Bull Trout and Other Fishes — Interactions with nonnative Lake Trout are thought to be one of the most severe and unmanaged threats to Bull Trout in Odell Lake. To evaluate ecological overlap between native and nonnative species, Native Fish Investigations used stable isotope analysis to assess relative abundance and dietary niche overlap among fishes in Odell Lake. Nonnative species dominated the fish community: Lake Trout outnumbered Bull Trout 27 to 1, and introduced Tui Chub (Siphateles bicolor) was the most numerically abundant species sampled. Stable isotope analysis showed that Bull Trout and Lake Trout both occupy the top predator position in the Odell Lake food web, with a 23% probability that individual Bull Trout fell within the isotopic niche space of Lake Trout. Niche overlap was even greater among prey-sized species — the probability that nonnative Kokanee (Oncorhynchus nerka) and Tui Chub fell within the isotopic niche of native Mountain Whitefish (Prosopium williamsoni) exceeded 95%. These results indicate that competition with Lake Trout is a plausible mechanism contributing to Bull Trout decline and that ecological effects of nonnative species extend across multiple trophic levels in Odell Lake.