Oregon’s only remaining non-reservoir population of adfluvial bull trout is found in Odell Lake, in the headwaters of the Deschutes River. Limited spawning and rearing habitats, combined with the effects of introduced species and other anthropogenic changes to the basin have contributed to a significant population decline. The lower section of Trapper Creek, a tributary of Odell Lake, is the only known spawning area used by this species throughout the system. Large numbers of Kokanee (Oncorhynchus nerka) also spawn in the same reach of Trapper Creek, superimposing their redds on those dug earlier by bull trout. After superimposition by Kokanee, bull trout redds are virtually undetectable. The objective of this study was to assess the effect of such overlay on bull trout’s egg-to-fry survival rates. Emergent fry traps were used, in combination with egg burial depth measurements, scour chains, and gravel characterization to establish actual physical overlap between both species and degree of bull trout egg pocket disturbance caused by Kokanee. Our results indicated that most bull trout egg pockets are dug deeper than depths reached by spawning Kokanee. Bull trout fry emergence data suggested that redd superimposition does not affect egg-to-fry survival rates.
Additional Research Information:
Effects of Redd Superimposition by Introduced Kokanee on the Spawning Success of Native Bull Trout
M. Weeber, G. Giannico and S. Jacobs