Endocrine Responses of Yellowfin Seabream, Acanthopagrus latus in Adaptation to Different Environmental Salinities

The aim of this study was to determine the changes of cortisol and T3 (triiodothyronine) levels in Acanthopagrus latus, a marine euryhaline teleost, in response to a wide range of salinities. The first experiment was carried out following exposure of juvenile yellowfin seabream to 5, 20, 42 and 60% environments. In the second experiment, the fish were assigned to a gradual adaptation to freshwater. Yellowfin seabream was capable of tolerating direct exposure of salinities from 5% to 60% without showing mortalities. This species was also able to tolerate gradual decrease in salinity in the surrounding medium from seawater (42%) to freshwater through a period of 10 days and successfully adapt to freshwater without showing mortality. This is the shortest period reported in a true marine fish for adaptation to freshwater through an acceptable experimental duration. The plasma cortisol level was increased in groups exposed to 5 and 60%. the highest cortisol level was observed in 60% treatment 12 hours after changing in salinity. The cortisol level of the groups adapted to 60% and 5% conditions were resumed to initial levels after 7 days and 24 hours, respectively. Following 24 hours of treatment, the T3 level resumed to the levels similar to the control, after a little increase. There was no difference in cortisol and T3 levels among control and experimental groups during gradual adaptation to freshwater.