Mobile Hydroacoustic Assessment of Distribution of Juvenile Salmonids At Cougar Reservoir, McKenzie River, Oregon

The 2008 Willamette Project Biological Opinion requires improvements to operations and structures to reduce impacts on Upper Willamette River Chinook and steelhead, including evaluations of the feasibility of installing new juvenile collection and bypass facilities at three Project Dams.  An understanding of fish distributions will be critical to successful development of structures to pass juvenile salmonids safely and efficiently.  Day and night mobile hydroacoustic surveys are scheduled once each month from April through December 2011 to quantify the horizontal and vertical distributions of fish throughout the reservoir.  A 9-ft long inflatable pontoon raft, outfitted with a frame holding six 6⁰ split beam transducers, real-time kinematic GPS, and compass, pitch, and roll sensors will be pushed 6 m ahead of the survey vessel to minimize fish avoidance.  Two hydroacoustic systems will be used for echo trace counting to estimate densities at night and during the day when fish are not schooling.  One of these systems will control one split beam aimed 18⁰ forward of vertical to sample fish in 1-m strata from 6 to 61 m deep, and the other will control forward looking split beams aimed 4, 11, and 18 degrees below horizontal to sample fish in the upper 6 m of the water column.  A third system, with one forward- and one down-looking split beam will be used for echo integration when fish schools are present during the day to estimate fish densities in 1-m depth strata from 0-61 m.  Target strength distributions will be used to categorize detections into four length classes (43-102mm; >102-203mm; >203-305mm; >305mm), and densities by length class will be applied to a GIS framework to visualize distributions spatially and temporally.  Sampling will be scheduled to coincide with USGS Lampara seine sampling of pelagic areas and ODFW trap-net sampling of littoral areas to obtain relevant species composition data.  Population estimates for the reservoir will be made for common species by expanding species apportioned densities to volumes of water in littoral and pelagic areas each month.  Our presentation will present results for spring and summer 2011.