Diel Distribution and Activity of American Eels (Anguilla rostrata) in a Cave-Spring

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Abstract
American eels (Anguilla rostrata) in a Florida [USA] cave-spring (total population .simeq. 142 eels, density = .simeq. 0.03 eels/m2) showed distinctive day versus night differences in distribution and activity. Larger eels occurred deeper at all times. Eels were diurnally quiescent, nocturnally active, and changed over between behavior modes roughly at dusk and dawn, although considerable variability existed in timing of changeover. Changeover at dusk involved movement from deeper regions of low food to shallower regions of high food concentration, with smaller eels more likely to migrate. Changing light levels apparently determined onset and cessation of activity, with an inhibitory threshold of 10-100 lx influencing movement between deep and shallow regions. An endogenous rhythm and hunger may have also affected activity patterns.