Life History Characteristics of Juvenile Spring Chinook Salmon in Cougar Reservoir

We investigated spring Chinook salmon use of Cougar Reservoir (Willamette River Basin, Oregon) to provide critical information necessary to improve juvenile downstream passage conditions.  Our objectives were to describe the migration timing and size of juveniles entering the reservoi, distribution within the reservoir, and growth rate relative to stream-rearing juveniles.  We also assessed emigration timing and size of juveniles passing through Cougar Dam.  At least two year classes were present in the reservoir (subyearlings and yearlings) indicating that not all juvenile emigrate from the reservoir in their first year as would be expected in an unimpounded river system.  Fry comprised the majority (93%) of juveniles entering the reservoir each year with a peak migration in May.  Mean fork length of fry during this period was 34 mm.  Fry were distributed mainly in the upstream end of the reservoir near their natal stream in the spring but some were observed near the dam, a 7-km distance.  As surface water temperatures within the reservoir increased during summer, juveniles descended below the thermocline and were not observed near the surface until surface temperatures cooled in the fall.  Growth of reservoir-rearing juveniles was greater than their stream-rearing counterparts.  By the end of their first year rearing in the reservoir, juveniles were approximately 30 mm larger than juveniles rearing in the natal stream.  Most juvenile emigration from the reservoir occurred in late fall as the reservoir level is drawn down.  Survival through the dam was poor, with yearling juveniles suffering greater mortality.