THE EFFECT OF SHORT-DURATION SEAWATER EXPOSURE ON THE SWIMMING PERFORMANCE OF WILD AND HATCHERY-REARED JUVENILE COHO SALMON (ONCORHYNCHUS-KISUTCH) DURING SMOLTIFICATION

The critical swimming velocity (U-crit) and haematology of wild and hatchery-reared coho salmon (Oncorhynchus kisutch) juveniles were examined in either fresh water or seawater following a 24-h seawater challenge, at the time of smoltification. In fresh water, wild smelts swam faster than hatchery-reared fish but this could largely be accounted for by scaling for body size. Transfer to seawater significantly elevated resting plasma [Na+] and reduced subsequent U-crit in hatchery fish (by 12%) relative to that determined in fresh water but had no significant effect on resting plasma [Na+] and U-crit in wild fish. Swimming the fish a second time in seawater after the initial 2-h exercise period resulted in a significant reduction in U-crit relative to that in fresh water in both wild fish (16%) and hatchery fish (a further 14%); this relatively greater impairment in U-crit in hatchery fish was due to a reduced hypo-osmoregulatory ability following seawater transfer that impairs conditions for muscle contractility and aerobic metabolism. Aerobic metabolism in seawater-exposed fish was affected in part through a reduction in haematocrit and an increase in plasma volume, reducing oxygen carrying capacity of the blood relative to conditions in fresh water.