The effect of salinity on swimming performance in early juvenile (2-month old) snook was determined. While at rest the snook were able to osmoregulate effectively over the range of freshwater to seawater. But in freshwater, swimming fish had the lowest physiological fitness, as evidenced
by a reduced aerobic scope, and at the highest speeds (5–6 bl/s), a greatly increased muscle lactate, increased hematocrit, and an increase in plasma osmotic pressure. Freshwater fish also appeared to have a reduced capacity for handling stress. By contrast, fish swimming in isotonic
seawater (ISW) had the highest aerobic scope, the lowest maintenance costs and no increase in lactate was detected at the highest swimming speeds. The performance in full strength seawater was similar to that in ISW except that at the highest speeds, some fish had increased lactate, indicating
that they had reached or exceeded their aerobic limit. On the other hand, the activity related cost of osmoregulation was the same in all salinities suggesting that the snook were able to increase the efficiency of osmoregulatory processes to match the (presumed) increase in ion flux that
accompanies increased speed. We suggest that resting or routinely active fish have sufficient energy reserves to cope with a wide range of salinities and that these reserves are diminished during maximal exercise such that stamina is compromised in suboptimal salinities.
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