Effects of elevated summer temperatures and reduced pH on metabolism and growth of juvenile rainbow trout (Oncorhynchus mykiss) on unlimited ration

Juvenile trout (Oncorhynchus mykiss) were exposed to a simulated global warming - acidic water scenario over a 90-day summer period (control temperature range 13-24 degrees C). The addition of 2 degrees C to the fluctuating summer cycle of inshore Lake Ontario and H2SO4 to synthetic soft water resulted in four treatments: control, acidification of control, simulated global warming alone, and global warming plus acidification. The twice-daily feeding regime raised metabolic rates to similar to 75% of Mo-2(max). Large increases (from 4.5 to 11.5%) in whole-body lipid, smaller increases (from 12.0 to 15.5%) in protein, and compensating decreases in water content (from 77 to 71%) occurred in all treatments over time. The addition of 2 degrees C resulted in depressed appetites and growth, particularly after the period of peak temperature (days 60-90; 26 degrees C). Metabolic rate and nitrogenous waste excretion were also depressed. Overall, exposure to low pH resulted in increased appetites and growth, the increase of 2 degrees C reduced gross energy intake and increased fecal energy losses, and exposure to low pH resulted in increased energy intake and gain and better conversion efficiency. The lack of ionoregulatory disturbance in trout chronically exposed to pH 5.2 suggested that dietary NaCl may have compensated for branchial ion losses.