The influence of salinity on oxygen consumption and plasma electrolytes in grass carp, Ctenopharyngodon idella Val.

Fingerling grass carp, Ctenopharyngodon idella, 90–160 mm t.l., were acclimated to experimental salinities of < 1 (fresh water), 91, 195 and 317 mOsm/kg. Oxygen consumption rates of fish declined, as salinity increased, from 0.16 mg O2/g‐h in fresh water to 0.11 mg O2/g‐h at an ambient concentration of 317 mOsm/kg. Plasma electrolytes (Na+ and Cl−1) and total plasma ionic concentrations increased slightly following 10 days of exposure to ambient salinities greater than 195 mOsm/kg. At 317 mOsm/kg fish appeared to lose control of plasma electrolyte concentrations. As the osmotic gradient was reduced between the fish and the external medium it might have become advantageous to the maintenance of the osmotic equilibrium to reduce blood circulation to the gills, thus imposing a reduction in metabolic rate.