Changes in urine flow rate and haematocrit value of rainbow trout Salmo gairdneri (Richardson) exposed to hypoxia

Measurements were made of urine flow rates and haematocrit of rainbow trout exposed to hypoxic conditions (3 mg O2/1) for 3, 4, 5 or 24 h. Elevated urine flow rates during short time exposure and concomitant increased haematocrit, followed by lower than normal urine flow rates on return to normal aerated water, suggest that haemoconcentration is the initial response to anoxia. Results from longer exposure to hypoxic conditions suggest that the initial period of haemoconcentration is followed by a period of water retention and a return of the blood volume to normal; the sustained elevated haematocrit is thought to be derived from release of erythrocytes from storage organs. During prolonged exposure rainbow trout also become more permeable to water.