Seawater adaptability of anadromous and landlocked forms of the ayu Plecoglossus altivelis.

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Abstract
Osmoregulatory ability of the anadromous and the landlocked forms of the ayu Plecoglossus altivelis was examined by following changes in plasma ion concentrations after transfter to sewater. On direct transfer from fresh water to seawater, 90% of the juvenile ayu weighing 0.5-5g died within 24h. When they were transferred to seawater after acclimation in 50% seawater for 3 days, the plasma Na concentration increased significantly after 1 day and decreased to a seawater-adapted level after 3 days. When the seawater-adapted fish were transferred back to fresh water, the plasma Na concentration decreased to the freshwater-adapted level within 6h. There was no difference in seawater of freshwater adaptability between juvenile fish of the anadromous form and those of the landlocked form. In contrast, juvenile rainbow trout weighing 8-10g were unable to adapt to seawater, even after acclimation in 50% seawater for 3 days. The seawater adaptability of the landlocked ayu was also maintained after gonadal maturation. The intrinsic euryhalinity of the ayu might have played an important role in differentiation of this species and acquisition of migratory habits of descending to the sea.

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