Odell Lake Bull Trout

 Although the present size of the bull trout population within the Odell Lake Recovery Unit is unknown, it appears to be quite small. Spawning by bull trout of the Odell Lake Recovery Unit occurs only in the lower 0.8 km of Trapper Creek, below a falls presumed to be a barrier. Numbers of redds observed in Trapper Creek from 1996 through 2002 have ranged from one to 24. In addition to genetic effects caused by small population size, primary threats to the sustainability of these bull trout include limited spawning and rearing habitat in all streams, hybridization with brook trout in upper Trapper Creek, competition from other fish species, particularly lake trout in Odell Lake, and superimposition of kokanee salmon (O. nerka) redds on bull trout redds. The latter threat may be particularly detrimental because of the large number of kokanee that spawn in lower Trapper Creek.

To address concerns of bull trout redd superimposition by bull trout, the Native Fish Investigations Project has designed studies to:

1) Assess the impacts to bull trout reproduction caused by kokanee that spawn at the same location, and

2) Develop acceptable methods to separate bull trout and kokanee that spawn in Trapper Creek, Odell Lake.

The first objective will address the concerns that kokanee redds superimposed on bull trout redds will reduce the number of bull trout progeny. During the spawning season, we will cap several undisturbed bull trout redds and an equal number of redds superimposed with kokanee salmon redds. Emergent fry will be captured, identified, and enumerated; statistical comparisons of fry abundance will be made between both groups of redds. To adjust for spawner fecundity, estimates of the size of females that make each redd will be obtained visually. We intend to conduct these studies at Heising Springs on the Metolius River, which helps support one of the healthiest bull trout populations in Oregon.

The second objective addresses efforts to enhance the Odell Lake bull trout population. Recently, managers installed a weir to block passage of kokanee salmon to spawning areas in Trapper Creek; bull trout were captured and placed upstream. This activity led to opposition by anglers supportive of kokanee and loss of adult bull trout by an otter that invaded the trap. Our intent is to help managers design and implement methods that 1) prevent kokanee salmon from impacting bull trout redds in Trapper Creek, 2) enable bull trout to move into and out of Trapper Creek without capture and handling, and 3) enumerate bull trout spawning in Trapper Creek. Data collected in Trapper Creek should help with design and implementation of methodologies to separate the species during the spawning season.

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