Aquaculture of the mudskipper Pseudapocryptes elongatus has expanded rapidly over the past 10 years in
the provinces of the Mekong Delta of southern Vietnam. The ish’s capacity for aerial breathing enables its farming in earthen ponds with no aeration or little water exchange. We compared the composition and abundance of algal and bacterial cells, the rates of oxygen consumption by micro-organisms, and daily luctuations in dissolved oxygen (DO) concentration in ish aquaculture ponds, and compared these to data obtained from shrimp aquaculture ponds in the same locality. We found consistently higher numbers (up to 1.8 × 107 cells/ml) of algae, comprising mostly “Chlorella sp.”, and of bacteria (over 108 cells/ml) in fish ponds than in shrimp ponds. The rate of oxygen consump-tion in the water of the ish ponds was able to deplete DO within a few hours at night, which was corroborated by a series of 24-h DO measurement data recorded in the fish ponds. The present study is the irst to clearly demonstrate the extremely high abundance of microorganisms and the resulting prevalence of nighttime anoxia in the water of P. elongatus aquaculture ponds.