EFFECT OF SHORT-DURATION SEAWATER EXPOSURE ON PLASMA ION CONCENTRATIONS AND SWIMMING PERFORMANCE IN COHO SALMON (ONCORHYNCHUS-KISUTCH) PARR

The effect of seawater (sw) on plasma ion concentrations and critical swimming velocity (U(crit)) was investigated in hatchery-reared coho salmon (Oncorhynchus kisutch) parr exposed to one of four treatments: 24 h of seawater exposure (SW1), 5-7 d of seawater (SW5), 24 h in seawater followed by 24 h in fresh water (SW-FW), and a freshwater control (FWC). Only the SW1 fish demonstrated a reduced U(crit) and, at rest, elevated plasma [Na+], [Cl-], and [SO4(-2)]. With exercise, SW1 fish were characterized by an increase in plasma ion concentrations and a decrease in both hematocrit (Hct) and muscle moisture content. There is a strong relationship between plasma [Na+] at rest and U(crit), where an optimal swimming velocity is obtained in animals with resting levels of approximately 147 mEq.L-1. Traditionally, the 24-h seawater challenge is used to test the hypoosmoregulatory ability in smolting salmonids, however, our data suggest that it may also predict the aerobic swimming potential of salmonids following seawater transfer. We suggest that the reduction in Hct and increase in plasma [Na+] result in reduced oxygen delivery to the muscle and that decrease in muscle moisture content impairs the contractile process.