Density effects on reproduction in natural populations of the staghorn damsel, Amblyglyphidodon curacao

Staghorn damselfish Amblyglyphidodon curacao were sampled underwater over 2 reproductive seasons from 4 reefs of differing coral cover on AustraMa's Great Barrier Reef. Plasma levels of estradiol (E2) [females], 11-ketostestosterone (11KT) [males], and testosterone (T) [both sexes] were correlated with reproductive development, and population density at each of the sites. In year 1, females from the site where population density was highest were smaller than fish from other sites, and fecundity was lowest. There were no site differences in plasma levels of T or 11KT in males, or T in females. Plasma E2 levels of females varied but not in relation to fish density. In year 2, fish densities were again highest at the same site as in year 1 and again females tended to be smaller than at other sites. Fecundity, plasma E2 and T of females, and plasma 11 KT levels in males were highest in fish from the site of lowest population density. The results suggest that high-density sites are characterised by small fish, lower fecundity, and on occasion, reduced plasma levels of T and E2 in females. High density was not accompanied by elevated androgen levels in males. The effects may result from increased competition for planktonic food at high density, and are consistent with similar effects in the brooding damsel Acanthochromis polyacanthus. These results are at variance with studies on temperate damselfishes where increased population density results in elevated plasma levels of gonadal steroids in males, suggesting that food availability may be the prime driver of reproductive function in tropical systems.