Changes on cortisol level and digestive enzyme activity in juveniles of Japanese flounder, Paralichthys olivaceus, exposed to different salinity regimes

The aim of this work was to evaluate the effect of salinity changes on digestive enzyme activity and whole-body cortisol concentration in juveniles (46 DAH) of Japanese flounder.Two different salinity patterns were designed: a control group exposed to a constant flux of seawater (S = 32) and a treatment group exposed to changes from full seawater (S = 32) to low salinity water (S = 10) each 6 h. The duration of the experiment was 2 days. Samples were taken every 6 h for enzyme activity and cortisol determination.Survival in both treatments was 92% at the end of the experiments. Cortisol levels were significantly higher (P < 0.05) in fish exposed for the first time to low salinity. Cortisol concentrations in the test group were also higher during the reminder of the experiment, especially during exposure to low salinity, but these differences were not significant (P > 0.05) between control and treatment groups. Trypsin activity decreased significantly (P < 0.05) during the first exposure to low salinity water, the comparison with the control group also showed significant differences (P < 0.05). In the case of lipase activity, no significant differences (P > 0.05) were found between treatment and control group.This work showed that juveniles of Japanese flounder could survive fluctuating exposure to low salinities (S = 10) at least in short periods. The results suggest that juveniles of P. olivaceus may retain abilities to survive exposure to low salinities through shared phylogenetic relationship.