Effects of Low Salinities on Oxygen Consumption of Selected Euryhaline and Stenohaline Freshwater Fish

The amount of energy required for osmoregulation depends on the difference between internal and external concentrations of ions (Rao 1968; Farmer and Beamish 1969), changes in corticosteroid hormone levels (Morgan and Iwama 1996), glomerular filtration rates (Furspan et al. 1984), gill and kidney Na⁺, K⁺-ATPase activity (McCormick et al. 1989; Morgan and Iwama 1998), tissue permeability to water and ions, and gill ventilation, perfusion, and functional surface area (Rankin and Bolis 1984). Differences in the energetic cost of osmoregulation play a significant role in the difference in growth rate between seawater-and freshwater-adapted fish (Morgan and Iwama 1991; Ron et al. 1995; Wang et al. 1997). Oxygen consumption is an indirect indicator of metabolic rate in fish (Cech 1990) and can be used to determine effects of salinity changes on energy costs.