Examination of direct daytime predation by Coregonus artedi on Bythotrephes longimanus in Harp Lake, Ontario, Canada: no evidence for the refuge hypothesis

Since its introduction to Harp Lake, Ontario, Canada, summer abundance of the cladoceran zooplanktivore Bythotrephes longimanus has fluctuated substantially both among and within years. The principal planktivorous fish in Harp Lake is the cold-water Coregonus artedi (cisco). Previous studies hypothesized that Bythotrephes abundance was affected by the thickness of an ephemeral, dark, daytime refuge from cisco that potentially established at the bottom of the metalimnion. During summer of 2003, we estimated peak daytime refuge thickness by simulating light energy visible to cisco and found it was always negative and did not correlate with Bythotrephes death rates. Direct observations using gill-netting and acoustical methods suggested that cisco had frequent metalimnetic forays. Additionally, including years 2000–2004, the previous correlation between mean Bythotrephes abundance and refuge thickness no longer held. The refuge hypothesis appears to fail, as the amount of metalimnetic illumination was always above the level at which cisco reaction distance to prey is maximal. Selection of Bythotrephes by cisco instead appeared to increase once Bythotrephes became abundant, remaining consistent and nontrivial even after Bythotrephes population declined.