EFFECT OF PREY SIZE ON THE ESTIMATION OF BEHAVIORAL VISUAL RESOLUTIONOF BLUEGILL (LEPOMIS-MACROCHIRUS)

Unlike larger conspecifics (>35 mm SL), behavioral visual resolution o f young-of-the-year bluegill (Lepomis macrochirus) (11-32 mm SL) measu red as visual angle is not independent of prey size. Visual angles bas ed on the maximum location distances (MLDs) for bluegill less than or equal to 32 mm SL increased directly with prey size and differed signi ficantly among three prey size categories (Daphnia pulicaria: carapace length 2.3, 1.1, and 0.8 mm). Visual angles of small bluegill viewing their preferred prey were approximately half those for fish viewing t he largest prey and indicate that the behavioral visual resolution of small bluegill was previously underestimated. Errors in prey size meas urements have a much larger effect on visual angle than do errors in r eaction distance. In most instances, MLDs of small bluegill to large p rey were significantly longer than were MLDs to small-and medium-sized prey. MLDs for the smallest prey were about 70% those for the largest Prey; the difference between the MLDs within a fish size category is much less than predicted if visual angle is independent of prey size. Poor visual and locomotory capabilities of small bluegill probably con tribute to the comparatively small differences in reaction distances t o small versus large prey.