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. Two Indo-West Pacific triggerfish species, Balistoides viridescens and Pseudobalistes flavimarginatus (Balistidae), however, form spawning aggregations just prior to both new and full moons, and are noteworthy in that they spawn demersally with males arriving at the spawning aggregation site first to defend nesting sites and then attracting females to spawn in them. Pseudobalistes flavimarginatus males build or maintain existing nests of rubble and sand at the spawning aggregation site, attract females to spawn at or past sunset, and abandon the site by the following morning. Typically, the nests are located in sand and rubble fields of channels (e.g., Ulong Channel, Palau, southwest Micronesia) or on similar substrates bordering reef slopes at depths of ca. 30 m (e.g., Agat, Guam). Males arrive ahead of females, usually beginning five or more days prior the new or full moon and secure, repair and defend (against rival males) nest sites. As females begin to arrive at the spawning aggregation site, males increase their aggressive behaviors against neighboring males or other intruding species (including scuba divers), and engage in preliminary courtship displays towards females. These behaviors are consistent with a lek-like mating system, with the exception of male defense of a tangible resource (the nest). At Orote Point, Guam, Balistoides viridescens males defend holes or small fissures in the pavement that are used as nests, attract females to spawn, and engage in post-spawning defense of the nest until the day of the new or full moon before they appear to abandon the nest by the following day. Both males and females remain at the spawning aggregation site afterwards, however, and engage in limited feeding and social interactions that include “parading” by groups of males and females in the water column, as well as slowly-executed non-aggressive intra-and intersexual displays between individuals. Post-spawning defense of the nest site as the next moon phase (third or first quarter) approaches has not been observed, however. As dusk approaches each night that this species is resident on the spawning aggregation site, individuals seek shelter in holes located mainly in the lower spur and groove or upper bench of the reef (ca. 2-6m depths). Holes typically accommodate a single individual but some holes have been found that accommodate two fish. This species also displays elements of lek-like behavior in its mating system but like P. flavimarginatus is not a true lekking species. Currently, individuals of both species are being collected on Guam and tagged with internal acoustic biotelemetry transmitters in an attempt to determine catchment areas and temporal-spatial movement patterns between home range and spawning aggregation sites.

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