PARENT SESSION
Oral Session #43: Fish: Ecology and Conservation.
Presiding: B. Harvey
Tuesday, August 6. 1:00 PM to 4:45 PM. Cochise Meeting Room, TCC.

Factors influencing the recovery of alpine lake zooplankton following removal of nonnative fish .

Knapp, Roland^*,1, Sarnelle, Orlando², ¹ University of California Sierra Nevada Aquatic Research Laboratory, Mammoth Lakes, California² Michigan State University, East Lansing, Michigan

ABSTRACT- Alpine lakes in the Sierra Nevada (California, USA) were naturally fishless and had a zooplankton community dominated by Hesperodiaptomus shoshone and Daphnia middendorffiana. Both species are now absent from hundreds of previously occupied lakes as a result of fish introductions. We had three expectations related to the recovery of these species following fish removal: 1) the probability of recovery for H. shoshone should be lower than for D. middendorffiana because of differences in reproductive mode (sexual versus asexual) and/or duration of resting egg viability; 2) the probability of recovery should be reduced at lower elevations due to the increased presence of potential predators; and 3) the probability of recovery should be inversely related to fish residence time because of egg bank depletion. To evaluate these expectations, we sampled zooplankton and sediments in 39 previously stocked but now fishless lakes. These samples allowed us to determine for each lake whether H. shoshone and D. middendorffiana were present historically and if so, whether these species had recovered following fish removal. As expected, the probability of recovery by H. shoshone was significantly lower than for D. middendorffiana (57% versus 79%). Based on the results of tree regression analyses, elevation and fish residence time were significant predictors of whether H. shoshone recovered while only elevation was a significant predictor of recovery by D. middendorffiana. These results suggest that returning alpine lakes to their natural fishless condition may often fail to allow full recovery of zooplankton community structure and ecosystem function. In addition, the insights provided by this study into at least some of the mechanisms responsible for recovery failure could be used to predict the success of future restoration efforts and guide the selection of restoration sites in which full recovery is most likely.

KEY WORDS: alpine lakes, nonnative fish, zooplankton, restoration