Diel Littoral–Pelagic Movements by Juvenile Bluegills in a Small Lake

Diel littoral–pelagic migrations by juvenile fish have been suggested as a mechanism that optimizes the trade‐off between predation risk and foraging return in open‐water habitats. However, previous study designs have not been able to conclusively demonstrate this behavior in the field. We applied a horizontally oriented hydroacoustic transducer, set at the open‐water edge of littoral vegetation, to observe diel fish behavior during two summers in Ridge Lake, Illinois, where the fish community is dominated by Largemouth Bass Micropterus salmoides and Bluegills Lepomis macrochirus . Based on acoustically tracked targets, most juvenile fish moved away from vegetated habitat at night and towards vegetation during the day. A greater number of predator‐sized fish were observed in the open water during the day than at night and weak acoustic targets, most likely zooplankton, were more abundant in the open‐water habitat at night. Therefore, it is likely that the reduced predator abundance and/or activity combined with potentially greater food availability in the open‐water habitat prompted an offshore migration at night that was reversed after sunrise. Diel horizontal migration patterns could affect the timing and location of interspecific interactions in systems where they occur and further research is needed to determine whether diel migrations alter ecosystem function.