In-Reservoir Summer Zooplankton Availability after Deep Reservoir Drawdown

Many Chinook Salmon juveniles use upper Willamette Valley reservoirs as rearing habitat. Zooplankton are a major food item for Chinook Salmon and other fish in these reservoirs.  Here, we examined whether there were impacts to reservoir ecology and food webs upon refilling of Fall Creek Reservoir (FC) following deep drawdowns to run-of-river, using nearby Hills Creek Reservoir (HC) as a reference site. We found that zooplankton in FC had lower densities than HC in early summer, but by late summer the densities were more similar. Daphnia abundances averaged 700 individuals/m³ in August in both reservoirs.  Although zooplankton have similar vertical distributions in these reservoirs, they may be differentially available to Chinook Salmon because of thermal conditions at specific depths. During our two years of sampling following deep drawdown in FC, optimal temperatures for Chinook Salmon growth (12-18°C) corresponded with depths where the prey, Daphnia, were most abundant. In HC, however, these optimal temperature zones did not contain high densities of zooplankton. Isotopic data indicates that Chinook Salmon in both systems feed similarly, so greater access to prey in FC may result in higher growth rates than in HC even during months when zooplankton are less abundant.