Abstract: Juvenile of tamed mullets [(3.37±0.19)cm in length and (0.36±0.02)g in weight] were cultured in waters being 20 (CK), 15, 10, 5, and 0, separately, in salinity for 20d in laboratory. Activities of Na⁺-K⁺-ATPase (NKA) and contents of malonialdehyde (MDA) in gills, muscles and intestines of these juveniles were determined on the 0^th, 5^th, 10^th, 15^th and 20^th days. Results show NKA activities and MDA contents were significantly higher in branchia and intestine than in muscle of the fish living in the water of CK, the same as the normal environment in salinity. Responses to lower salinity varied with the organ. However, NKA activities did more intensely in gills and intestines than in muscles (P<0.05), while MDA contents did reversely. During the first 10 days,  the juveniles living in waters lower in salinity were found much higher in NKA activity in all the three organs than those living in CK (P<0.05), which reveals that appropriate low salinity stress (0-15) could stimulate NKA enzyme activity in the fish, thus enhancing the fish’s ability to adapt to low salinity environment. During the first 5 days, MDA contents in the three organs were much higher in juveniles living in waters lower in salinity than in those living in CK (P<0.05) and generally negatively related to salinity of the water they lived in. With the experiment going on, NKA activities and MDA contents in the lower salinity groups declined gradually and leveled off, leaving a narrow gap from the levels in CK, which demonstrates that the fish is quite high in adaptability to salinity. However, the restoration of NKA activity and MDA content to the normal level took time, of which the length depended on organs, showing specificity and continuation of the organs.

Key words: juvenile mullet, low salinity chronic stress, Na⁺-K⁺-ATPase , MDA