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In the last decade, invasive suckermouth armored catfish Pterygoplichthys spp. spread among many river systems of Vietnam. Extended distribution of armored catfish might be associated with using brackish water in estuaries for fish spread from one river system to another. The first goal of our study was to assess the occurrence of armored catfish in the estuary of the Da Rang River (Phu Yen Province, Vietnam) and their distribution depending on the horizontal salinity gradient (4--25 PSU). Fish were mainly caught by stationary bottom traps in water salinity from 4 PSU to 18 PSU. The second goal of our study was to experimentally evaluate the ability of armored catfish to move and breathe in seawater (33 PSU). Fish moved in horizontal and vertical planes after transfer into seawater during the first 15 minutes. Fish moved around less by the 13th--15th minutes in seawater. Armored catfish moved around more in seawater than in freshwater. The exposure to seawater for 6 minutes led to deterioration of fish breathing. The results of our field and experimental studies established that armored catfish are found and able to move in brackish waters but avoid high salinity water. These facts provide support for the hypothesis of armored catfish invasion through the estuaries and coastlines. [ABSTRACT FROM AUTHOR]