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In February 2022, unusual acute mortalities were observed in snubnose pompano (Trachinotus blochii) and permit (T. falcatus) broodstock held in sea cages in Nha Phu lagoon, Khanh Hoa, Vietnam. This phenomenon followed the appearance of an unknown, thick, brownish slimy material on the nets of flouting sea cages holding snubnose pompano (5.3– 10.9 kg) and permit broodstock (2.3– 4.1 kg), observed in December 2021. In February, local farmers decided to transfer these fish to other cages in order to clean the nets. Handling increased the fish's physical contact with the slimy material on the nets. The snubnose pompano and permit broodstock immediately exhibited strange behaviours once the fish were in close contact with the slimy material. Strange behaviours included vigorous swimming, loss of balance, and the appearance of white spots and patches on their skin. Approximately 80% of snubnose pompano and 100% of permit broodstock in the handled cages died within a few hours of exposure to the slimy material. Farmers left the remaining fish (approximately 80 individuals) undisturbed, and the mortality rate dropped to 1– 3 fish per week in the next 3 weeks. Similar slimy materials were observed on the nets of the cages holding barramundi (Lates calcarifer) and red drum (Sciaenops ocellatus) broodstock. Upon contact with the slimy material during handling, these fish immediately exhibited similar behaviours with white spots and patches on their skin, but no mortalities were observed. There was no abrupt change in temperature, salinity, or pH of the sea water during the mortality event.