Abstract

Transient spawning aggregations are formed by species that migrate periodically from relatively distant home ranges to specific sites where they persist for days or weeks during a spawning cycle before returning home. Many of these spawning aggregations form on a seasonal basis, with formation occurring just prior to the new moon, full moon, or both. Most species spawn in pairs or groups and have pelagic gametes. The Indo-West Pacific triggerfish species Balistoides viridescens (Balistidae), however, forms spawning aggregations just prior to both new and full moons, and spawns demersally with males arriving at the spawning aggregation site first to defend nesting sites and then attracting females to spawn in them. GPS-transect surveys of a principle site at Guam revealed patterns of spatial distribution of both male, female, and older juvenile triggerfishes on this site during daylight hours. Most individuals were concentrated on a shallow bench (4 - 8 m) adjacent to a cut in the fore reef with regular fidelity. Triggerfishes moved out from nocturnal shelter sites in the lower spur and groove zone to this area and were often observed patrolling this portion of the site while swimming in loose aggregations in the water column. During patrols, males and females displayed to one another, often rising to within 1m of the surface over the edge of the reef slope. These interactions appear to reinforce familiarization among individuals prior to courtship and spawning on the site.