Effects of sublethal ammonia exposure on swimming performance in rainbow trout (Oncorhynchus mykiss)

Adult trout Oncorhynchus mykiss fitted with a dorsal aortic catheter were e xposed to 288+/-15 mu mol l(-1) (mean +/- S.E.M.) total ammonia for 24 h in water at a pH of 8.39+/-0.02, while swimming at a speed equivalent to 0.75 body lengths s(-1) (BL s(-1)) in a Brett-type tunnel respirometer. The fis h were then exposed to stepwise increments in swimming speed (0.25 BL s(-1) every 30 min) until exhaustion. Measurements of oxygen uptake (Mo-2) and p lasma total ammonia levels and pH were made at each speed. Control trout we re treated identically but without exposure to ammonia. Ammonia exposure ca used an increase in plasma total ammonia level to 436+/-34 mu mol l(-1), co mpared to 183+/-30 mu mol l(-1) in control animals (N=6). A significant red uction in total plasma ammonia level was found in both groups during exerci se, despite a large negative concentration gradient in those exposed to an elevated concentration of ammonia in water, which may indicate an active ex cretory process. The overall increase in plasma ammonia levels in exposed t rout was associated with a significant reduction in critical swimming speed (U-crit) to 1.61+/-0.17 BL s(-1) from 2.23+/-0.15 BL s(-1) in control anim als. Ammonia-exposed trout had a significantly higher maintenance metabolic rate (MMR) than control fish, when estimated as the gamma -intercept of th e relationship between swimming speed and Mo-2. Active metabolic rate (AMR, maximum Mo-2 as measured at U-crit) was significantly lower in ammonia-exp osed animals, leading to a profound reduction in factorial aerobic scope (A MR/MMR). Reduced U-crit was also linked to a reduction in maximum tailbeat frequency. Calculation of membrane potentials (E-M) in the white muscle of fish swum to U-crit revealed a significant partial depolarisation of white muscle in ammonia-exposed fish. This may have prevented white muscle recrui tment and contributed to the reduced maximum tailbeat frequency and overall impairment of swimming performance in the ammonia-exposed fish.