Oxygen consumption of rainbow trout (Salmo gairdneri) in relation to activity and salinity

The oxygen consumption of the rainbow trout ranging in size from 23 to 196 g was measured at 5 °C and 15 °C in fresh water and salinities of 7.5‰, 15‰, and 30‰. The standard rate of metabolism and metabolic rates at various levels of steady swimming were determined.The cost of swimming and the cost of osmoregulation in terms of oxygen consumption for a 100-g fish were calculated on the assumption that there was no cost of osmoregulation in a salinity of 7.5‰. In fresh water and in 15‰ salinity the cost of osmoregulation was 20% of total metabolism while in a salinity of 30‰ it was 27%. This 27% of the total metabolism attributed to cost of osmoregulation does not appear to be proportional to the increase in osmotic gradient in 30‰ salinity.