Abstract
When different ages of juvenile allis shad Alosa alosa (18–74 days post‐fertilization) were exposed for 48 h to elevated salinity (25 or 30), all ages could tolerate moderates salinity (25) whereas only the older stages could tolerate the higher salinity. In 88 days post‐fertilization allis shad, acclimatation to full‐strength sea water for 2 weeks did not lead to significant changes in total water content and gill Na + /K + ‐ATPase activity whereas total Na + content increased within 5 days to values which remained stable thereafter. Together, these data suggest that juvenile allis shad develop a limited euryhalinity, which allows them to fully adapt to hyperosmotic environment immediately after transfer.