The reaction of young coho Oncorhynchus kisutch to declining oxygen levelsduring long-term exposure

The respiration of coho salmon, Oncorhynchus kisutch, weighing between 20 a nd 45 g was measured at gradually declining oxygen levels and at temperatur es ranging between 14 and 17 degrees C. The maximum and minimum oxygen conc entrations tested were 235 and 41 mu mol/L respectively. Respiration rates were measured for 24 h at 235 mu mol/L before the oxygen concentration was towered stepwise to 157 and 81 mu mol/L. In one single trial, the oxygen le vel was lowered to 66, 53, and 41 mu mol/L. Respiration was highly variable in time. Peak activities always occurred during the night. The standard me tabolic rate at normoxic conditions was estimated to be around 4 mu mol oxy gen/g/h. The highest rates reached values close to 15 mu mol oxygen/g/h. At reduced oxygen levels the standard oxygen demand slightly increased to 4.5 mu mol oxygen/g/h, indicating a higher demand for vital metabolic function s. Due to the decrease of swimming activity, the maximum oxygen uptake rate s dropped to < 8 mu mol oxygen/g/h below 81 mu mol/L oxygen concentration. Under long-term conditions, physiological and behavioural adaptations play an important role for survival and need to be considered for the design and operation of fish farm facilities.