We have investigated how the maintenance, net cost of swimming and total (maintenance + net cost of swimming) metabolic rates of red, hybrid tilapia (Oreochromis mossambicus ♀ × O. hornorum ♂) responded to different acclimation salinities, and if these responses correlated with changes in ion-osmoregulation (= osmoregulation) costs.

Three groups of fish were acclimated to either fresh water (FW, 0‰), isosmotic sea water (ISW, 12‰) or full strength sea water (SW, 35 ‰) and oxygen consumption was measured while they swam at 10, 20, 30 and 40cms-1. Maintenance oxygen consumption (estimated by extrapolation), for an average fish (63g), increased among groups in the following order: FW < ISW < SW. The net cost of swimming increased in the order ISW < SW < FW, and total oxygen consumption (maintenance + net cost of swimming) increased in the order ISW < FW < SW.

We assumed that the contribution of cardiac, branchial and swimming muscles to the net cost of swimming was proportional to swimming speed only, and therefore, at similar speeds, differences in the net cost of swimming among salinities were due to changes in the activity-related cost of osmoregulation. Consequently, the order in which the net cost of swimming increases from one group to another is the same as the order in which the cost of osmoregulation increases. Since the sequences for maintenance and total metabolic rates differed from that for the net cost of swimming, salinity-related increases in these rates cannot be attributed exclusively to changes in osmoregulation cost.

We conclude, based on the differences in the net cost of swimming, that osmoregulation in FW is more expensive than in SW, and that it is cheapest in ISW. Although we were not able to estimate the total cost of osmoregulation in FW and SW, we estimated the activity-related cost, relative to the cost in ISW, at different swimming speeds (net cost of swimming in FW or SW minus net cost of swimming in ISW at each speed). For a 63-g fish in FW, this cost increased from zero at rest, to 41mgO2kg−1h−1 (16% of the total metabolic rate, 24% of the net cost swimming) at 40 cms−1. In SW the same cost increased only to 32 mgO2 kg−1h−1 (12% of the total metabolic rate, 20% of the net cost of swimming) at 40cms−1. The net cost of swimming in FW or SW increased with swimming speed at a rate 3×4 times faster

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