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Oregon's only remaining non-reservoir population of adfluvial bull trout (Salvelinus confluentus) is found in Odell Lake, in the headwaters of the Deschutes River. The population size is unknown, but appears to be quite small. Limited spawning and rearing habitats, combined with the effects of introduced species and other anthropogenic changes to the basin have raised concerns as to the population's viability. The lower section of Trapper Creek, a tributary of Odell Lake, is the only known spawning habitat used by this species throughout the system. Large numbers of kokanee salmon (Oncorhynchus nerka) also spawn in the same reach, superimposing their redds on those dug earlier by bull trout. After superimposition by kokanee, bull trout redds are virtually undetectable.
The objective of the study was to assess the effect of kokanee redd superimposition on bull trout egg-to-fry survival. Due to uncertainty in the size of the Odell Lake bull trout population much of the study was conducted on bull trout and kokanee of the Metolius River system, a neiboring Deschutes sub-basin. Emergent fry traps were used, in combination with egg burial depth measurements, scour monitors, and gravel characterization to establish actual physical overlap between both species and degree of bull trout egg pocket disturbance caused by kokanee. Results indicate that most bull trout egg pockets are dug deeper than depths reached by spawning kokanee. Kokanee were found to be very superficial spawners in the streams studied. Bull trout fry emergence data suggest that kokanee redd superimposition does not affect egg-to-fry survival rates. |
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