Abstract: In order to study the effects of acute low salinity stress on osmoregulation of juvenile Thunnus albacares, juvenile Thunnus albacares were transferred from salinity 34 to salinity 29（experimental group）and salinity 34（control group）, and sampled at 0 h, 6 h, 24 h and 48 h after transfer to analyze the effects of acute low salt stress on serum osmotic pressure, prolactin（PRL）lactic acid, blood sugar and ion concentration of juvenile Thunnus albacares. The results showed that the data of osmotic pressure, serum lactic acid, blood glucose, sodium ion（Na⁺）, potassium ion（K⁺）, chloride ion（Cl^-）, calcium ion（Ca²⁺）, and bicarbonate（HCO₃^-）were significantly different between the experimental groups（P<0.05）. In the experimental group, osmolality and Na⁺ decreased to the lowest level at 6 h and were significantly lower than those in the control group, and then fluctuated at this level. Prolactin decreased to the lowest level at 6 h and gradually increased steadily at 24 h, which was significantly higher than that in the control group（48 h）. K⁺ gradually decreased and reached the lowest value at 48 h, which was significantly lower than that of the control group. Cl^– reached the lowest value at 6 h and was significantly lower than that in the control group. The results showed that the osmotic pressure of juvenile Thunnus albacares under low salt stress was affected by acute low salt stress, and the osmotic pressure remained stable within 6 h, and then decreased rapidly and gradually stabilized. By its effects, the prolactin index decreased in a short time, but increased rapidly in a long time. There was no significant difference in the concentration of K⁺ among groups（P>0.05）, but there were significant differences in the concentration of Na⁺ and Cl^– among groups（P<0.05）.

Key words: Thunnus albacares, acute low salt stress, serum osmotic pressure, prolactin, ion concentration