Global climate change has remarkably affected on aquaculture industry, especially the consequences of salinity intrusion in freshwater regions. Striped catfish is the most important species in Vietnamese aquaculture because its turnover reaches billions US dollars every year. However, effects of salinity increase on physiology and immune capacity in the species remain largely unknown. In this study, striped catfish juvenile (20-25 g) was chronically exposed to different salinities, namely 0, 5, 10, 15 and 20 ppt with the increase 0, 0.25, 0.5, 0.75 and 1 ppt per day respectively, during 20 days (D20). Then, the fish was continuously kept in its corresponding salinity two weeks (D34) to see how it acclimate to the salinity. All treatments had very good survival rates (over 90%), except treatment 20 ppt after the first 20 days exposure. Weigh gain showed the highest level in the fish submitted to 5 ppt in D20 and the indicator exhibited elevating pattern with salinity in D34. In terms of osmoregulatory responses, osmolality, sodium and chloride concentrations significantly increased in fish exposed to hyperosmotic environment during the experiment. Red blood cell and hematocrit had the same pattern that decreased after 20 days of salinity increase. Two immune responses (plasma lysozyme and peroxidase activities) were investigated but the lysozyme activity was only significantly elevated in the hyperosmotic conditions in D20. The fish reared in higher levels of salinity had stronger activities of digestive enzymes , especially the activities of aminopeptidase, leucine alanine peptidase and pepsin in D20. Interestingly, the study found that the scoring of interlamellar cell mass in gill (ILCM) was significant lower in fish submitted to higher salinity in D20 and this effect was more remarkable in D34. These results support the hypothesis that hyperosmotic stress may affect the striped catfish juvenile through changes of physiological and immunological responses, however the fish looks like possible to acclimate salinity after two weeks, except the one in 20 ppt.
KEYWORDS
Hyperosmotic exposure, striped catfish, immunity, histopathology, salinity.