Brook Trout Distribution Associated with Temperature and Dissolved Oxygen in a Stratified North Central Washington Lake

Owhi Lake, a 202 ha dimictic lake located in north central Washington on the Confederated Tribes of the Colville Reservation, has an abundant population of large (>400 mm) brook trout providing a prized local fishery for tribal members. Every summer Owhi Lake stratifies, restricting the amount of habitat brook trout can occupy.  Temperatures below 21° C were not found above 7 m depth and dissolved oxygen fell below 4 mg/L at depths greater than 9 m in this 22 m deep lake during 2010 summer.  Brook trout distribution was associated with the temperature and dissolved oxygen based on day time hydroacoustic surveys to identify their habitat use during the period of lake stratification.  Throughout the period of stratification, 90-100% of brook trout were found occupying the metalimnion which on one sampling date was restricted to 2 m (8- 10 m depth) in the water column.  During the early period of stratification, 100% of brook trout occupied depths with temperatures from 14-18°C with dissolved oxygen concentrations >9 mg/L, 5-9 m depth, even though the entire lake was below 21°C.  Later in stratification, 90% of brook trout occupied depths with temperatures from 17.5-21°C with dissolved oxygen concentrations >8.5 mg/L, 8-10 m depth.  Results indicate Owhi Lake brook trout prefer occupying temperatures between 14-18°C with dissolved oxygen concentrations above 8.5 mg/L, suggesting there is suitable habitat for brook trout to occupy throughout the entire summer.  However, the population abundance may be limited by reduced habitat availability, 2 m in the water column, associated with temperature and dissolved oxygen during brief portions of the summer.