Panmictic population structure in the migratory marine sparid Acanthopagrus australis despite its close association with estuaries

Recent studies have revealed surprising levels of genetic structuring within populations of marine species that were previously thought to be widely dispersed. Such subdivision may reflect unexpected physical or biological barriers to dispersal, including philopatric behaviour. Here we investigate the genetic structure of the eastern Australian yellowfin bream Acanthopagrus australis - a widely distributed species that is thought to be highly dispersive but is also known to spawn in close association with estuaries. Our data from surveys of allele frequencies at 6 microsatellite DNA loci for 350 fish revealed high levels of genetic diversity within all sites but no genetic differentiation of groups of recruits collected from sites separated by a distance of up to 50 km (allele frequency differentiation: F sub(ST) = 0.002, p > 0.05). Moreover, there was no differentiation of adults spread across the distributional range of the species (several 100s of kilometers, F sub(ST) = 0.002). We conclude that A. australis spawning is opportunistically associated with estuaries in general, and that the species essentially forms a panmictic population with a genetic homogeneity reflecting the predicted active northwards dispersal of adults and the southwards dispersal of larvae as affected by the Eastern Australian Current.