Twilight migrations and foraging activities of the copper sweeper Pempheris schomburgki (Teleostei: Pempheridae)

During 1976 and 1977, movements and foraging activities of Pempheris schomburgki were studied on the shallow coral reefs of northeastern St. Croix, U.S. Virgin Islands. P. schomburgki emerge from daytime refuges on the backreef at about local sunset. Fifteen to 20 min after sunset, local milling groups move beyond the confines of the reef and assemble into larger groups. Twenty-five to 30 min after sunset they move in several waves along complex routes through the reef to forereef feeding grounds. Migrations of up to 1 km occur along the forereef. The reverse sequence occurs in the morning, but is earlier with respect to ambient light levels. The principal stages of these activities appear to be triggered by a combination of absolute light level, rate of change of light and state of adaptation of the eye. Migrating aggregations gradually split up into small, well-dispersed feeding groups, relatively evenly spaced along the forereef. Few individuals feed on the backreef. The principal available food consists of meroplanktonic crustaceans not available during the daytime. P. schomburgki mainly select the larger-sized individuals (mean length 5 to 6 mm), although some particles less than 1 mm are taken. These events probably represent adaptations to optimize diurnal sheltering sites, feeding grounds and the avoidance of predator activity.