The respiration of young coho Oncorhynchus kisutch at gradually declining oxygen levels and during recovery from oxygen deprivation

The respiration of coho salmon, Oncorhynchus kisutch, weighing between 15 and 50 g was measured at gradually declining oxygen levels and at temperatures ranging between 14 and 17°C. The maximum and minimum oxygen concentrations tested were 250 and 40 μmol L−1, respectively. Respiration rates were measured for 1 h periods before oxygen concentration was lowered by 12.5 or 25.0 μmol oxygen L−1. At the end of these endurance tests the oxygen level was returned to normoxic conditions and respiration rates were determined for the recovery period. Under normoxic conditions (> 200 μmol L−1) the respiration of coho levelled around 5.1 μmol g−1 wet weight h−1. At intermediate levels between 150 and 200 μmol oxygen L−1, the average rate increased to 5.8 μmol g−1 h−1, which could be attributed to higher spontaneous activity of the test animals. At low oxygen levels (< 150 μmol−1) average respiration rates dropped to values between 5.5 and 5.7 μmol g−1 h−1, reaching a minimum of 3.8 μmol g−1 h−1 at oxygen levels below 50 μmol Lμ. First mortality was observed in this range. After exposure to reduced oxygen levels the fish maintained a higher respiration rate when again exposed to normoxic oxygen levels above 200 μmol L−1. Increased respiration rates were observed for a recovery period of 6 h.