The water balance of 0-group flounders was investigated in a range of static [0–100% sea water (SW), 100%≡ 34%‰] and cycling salinities (2–98% SW, 12 h 25 min period). The permeability coefficients of these juvenile fish were found to be higher than those quoted for adults. The permeability of juveniles in fixed salinities decreased with increased salinity whereas animals in the tidally cycling regime showed permeability changes that were directly proportional to the ambient salinity. However, comparison of the two groups showed that animals in a cycling salinity regime were less permeable to water than animals acclimated to fixed salinities. Drinking and urine production rates fluctuated within a tidal salinity regime, and 0-group flounders were found to modify their water permeability, urine production and drinking rates simultaneously, so maintaining their blood osmotic concentration and total water content within narrow limits throughout the range of salinities.
The relevance of measurements made in fixed and tidally cycling salinities to water regulation under natural estuarine conditions is considered.