SALINITY TOLERANCE AND ITS SEASONAL-VARIATION IN THE FLOUNDER, PLATICHTHYS-FLESUS

1. Following transfer from 50% SW the increases in plasma chloride concentration and osmolality in SW compared to FW adapted fish were less marked in winter than in summer. 2. Urine chloride concentration and osmolality were higher and urine volume lower in SW than in FW adapted animals throughout the year. Urinary bladder volume was significantly restricted during winter by the presence of greatly enlarged gonads, particularly in females. This was associated with a reduced urine osmolality in FW and SW adapted animals in winter. 3. Activity of the caudal neurosecretory system varied in accordance with seasonal migration and change in reproductive status of the flounder.