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JOURNALOFTHE WORLD AQUACULTURE SOCIETY Vol. 34, No. 1 March, 2003 Effects of Low Salinities on Oxygen Consumption of Selected Euryhaline and Stenohaline Freshwater Fish ILHAN ALTINOK AND JOHN M. GRIZZLE^ Southeastern Cooperative Fish Disease Project, Department of Fisheries and Allied Aquacultures, Auburn University, Auburn, Alabama 36849 USA The amount of energy required for os- moregulation depends on the difference be- tween internal and external concentrations of ions (Rao 1968; Farmer and Beamish 1969), changes in corticosteroid hormone levels (Morgan and Iwama 1996), glomer- ular filtration rates (Furspan et al. 1984), gill and kidney Na+, K+-ATPase activity (McCormick et al. 1989; Morgan and Iwa- ma 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. Most researchers agree that salinities dif- ferent from that of the internal fluids of the fish must impose energetic regulatory costs for active ion transport (Morgan and Iwama 1998). However, there is less agreement concerning the magnitude of these costs (Rao 1968; Farmer and Beamish 1969; Nordlie and Leffler 1975; Nordlie 1978; Furspan et al. 1984; Febry and Lutz 1987; Morgan and Iwama 1991; Claireaux and Lagard&re 1999) and for most species there is little information about the energetic con- sequences of life in different salinities. Our objective was to investigate the ef- fects of low salinities (5 9.Woo) on oxygen consumption in experiments allowing com- parison of phylogenetically diverse species. I Corresponding author. The salinity range was selected to include approximately isosmotic and hypo-osmotic conditions. With the conditions we used, the oxygen measurements provided an es- timate of the resting routine metabolic rate, which requires fasted, quiescent fish (Cech 1990). To eliminate metabolic requirements associated with acclimation, fish were kept in test salinities for 2.5 mo before oxygen consumption was determined. Six species of stenohaline and euryhaline fishes were tested. Channel catfish Ictalurus punctatus and goldfish Carassius auratus are stenohaline fish species that can only tolerate less than half-strength sea water (Black 1951; Allen and Avault 1970). However, rainbow trout Oncorhynchus my- kiss, brown trout Salmo trutta, striped bass Morone saxatilis, and Gulf sturgeon Aci- penser oxyrinchus desotoi (a subspecies of Atlantic sturgeon A. oxyrinchus) are eury- haline species that can live in a wide range of salinities from fresh water to full- strength sea water depending on their size and life stage (Behnke 1992; Elliott 1994;
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