Abstract

New and previously published genetic data from 6000 barramundi comprising 50 collections across tropical Australia were analysed for evidence of population subdivision. Sixteen discrete populations were identified, including four populations that were identified from new collections. Duplicate collections from two localities were statistically homogeneous after seven years between collections. Environmental and genetic factors that yielded the observed genetic pattern were investigated. Geological evidence of sea-level changes, when compared with bathymetry data for the region, reveals that barramundi must have recently recolonized many of the coastal estuaries of tropical Australia. This recolonization resulted from the inundation of the Gulf of Carpentaria and Torres Strait by a rapid rise in sea level of at least 130 m between 18 000 and 6000 years ago. The genetic data clearly indicate that, as the population spread into new habitats, there was a corresponding decrease in genetic diversity. This observed decrease has been maintained despite continued migration between populations. The 'onedimensional stepping stone' migration model, which most closely fits the observed population structure, predicts that the observed level of population subdivision (F_ST = 0.064) is maintained against substantial gene flow between adjacent populations. This contrasts with the predictions of the often-used 'island model' which gives estimates of N_em at least two orders of magnitude lower than those from the onedimensional stepping-stone model.