Comparison of Searching Behavior for Zooplankton in an Obligate Planktivore, Blueback Herring (Alosa aestivalis) and a Facultative Planktivore, Bluegill (Lepomis machrochirus)

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Abstract
Studies of zooplankton feeding by juvenile blueback herring (Alosa aestivalis), an obligate planktivore, and juvenile bluegills (Lepomis macrochirus), a facultative planktivore, indicate that they search by markedly different methods. The herring searches as it swims, sighting most prey above its horizontal course, and swims up to take the prey. The bluegill searches while stopped (hovering). If a prey is sighted, it swims to it, sucks it in, then coasts to a stop to resume searching. If no prey is sighted, it swims a short distance, coasts to a stop, and searches again. For uniform size prey (Artemia salina first instar nauplii) the time the bluegill hovers in a particular search position is positively correlated with the distance the prey is from the fish (r2 = 0.69–0.77, four fish). This, in conjunction with previous work on bluegills, suggests they take the first prey item seen, rather than choosing between prey. The bluegill is more reactive to moving prey of Daphnia size; however, blueback herring do not distinguish between motile and nonmotile prey, perhaps because any prey's image is moving across its retina as the herring swims. Stopping to search may be adaptive where there is a heterogenous (weedy) background, whereas moving prey are detected against the stationary background. Such a background is likely confusing to a moving fish.Key words: blueback herring, Alosa aestivalis; bluegiil, Lepomis macrochirus; predation, zooplankton