The effect of dietary salts on acid stress in rainbow trout (Oncorhynchus mykiss)

Acid rain and acidic snowmelt stresses fish in the wild and in aquaculture, resulting in ionoregulatory disturbances or even death. The effect of acid stress on rainbow trout (Oncorhynchus mykiss) fed increased levels of salt was investigated using two forms of dietary salt: NaCl and CaCl₂. Chloride amounts were also varied in the diets so that the rainbow trout were split into five different feeding groups: 1.5% NaCl, 6% NaCl, 12% NaCl, 5.6% CaCl₂, and 11.4% CaCl₂. After 4 days of feeding, the 11.4% CaCl₂ diet was discontinued since it was lethal to the trout. After 70 days of feeding experimental diets, the trout were exposed to an acute pH of 3.5 for 8 hours. Plasma chloride, plasma glucose, and plasma cortisol were measured every 2 hours. C1 loss was significantly reduced in the 12% NaCl and the 5.6% CaCl₂ diet groups, but the plasma cortisol concentration was similar across all diets. This indicates that C1 loss is ameliorated with high dietary salts, but acid stress still occurs because of the toxic effects of H⁺ on sensitive trout tissues.