Seasonal peaks and annual fluctuations in recruitment of juvenile scleractinian corals

Abstract

Recruitment of juvenile scleractinian corals to natural CaCO3 settlement plates varies seasonally and annually on 4 vertically separated reef front sites during a 3 yr study period. Early summer was the major recruiting season; of 1470 recruits observed on 184 plates over 2 yr, 89% were in spring-summer (Oct.-Feb.), 8% in summer-winter (Feb.-Jun.) and 3% in winter-spring (Jun.-Oct.). The peak early summer recruitment was significantly greater in 1980-1981 (32 juveniles plate-1) than in 2 subsequent years (1981-1982, 9 plate-1; 1982-1983, 15 plate-1). Sites of greatest recruitment were reef shoulder in the 1st year, upper reef slope (6 m) in the 2nd yr, and reef flat in the 3rd year. Maximum annual variations in recruitment occurred in 2 shallow sites (reef flat and shoulder). A mid slope site (12 m) had the lowest overall recruitment and did not vary significantly in annual numbers of recruits. Greater recruitment occurred on undersurfaces of plates at shallow sites but on upper surfaces at deeper sites during all periods. Recruits were mainly from 2 families: Acroporidae, which recruited primarily in spring-summer, and Pocilloporidae, which recruited in all seasons. A shading experiment in 1 spring-summer period indicated that juveniles of some families other than Acroporidae and Pocilloporidae required more protected sites for settlement than those of these 2 families. Acroporidae and Pocilloporidae may be early colonizers of exposed reef substratum, while some other corals may represent later colonists. The seasonal patterns of recruitment described here are compatible with seasonal spawning patterns now known to occur in many Great Barrier Reef corals. The earlier concept that coral larvae were available at all times of the year must be rejected.