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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. [ABSTRACT FROM AUTHOR]