The effects of sublethal ammonia on swimming performance in rainbow trout, Oncorhynchus mykiss

Adult trout Oncorhynchus mykiss fitted with a dorsal aortic catheter were exposed 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 fish 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 plasma total ammonia levels and pH were made at each speed. Control trout were treated identically but without exposure to ammonia. Ammonia exposure caused an increase in plasma total ammonia level to 436+/-34 mu mol l(-1), compared to 183+/-30 mu mol l(-1) in control animals (N=6). A significant reduction in total plasma ammonia level was found in both groups during exercise, despite a large negative concentration gradient in those exposed to an elevated concentration of ammonia in water, which may indicate an active excretory process. The overall increase in plasma ammonia levels in exposed trout 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 animals. Ammonia-exposed trout had a significantly higher maintenance metabolic rate (MMR) than control fish, when estimated as the gamma -intercept of the relationship between swimming speed and Mo-2. Active metabolic rate (AMR, maximum Mo-2 as measured at U-crit) was significantly lower in ammonia-exposed animals, leading to a profound reduction in factorial aerobic scope (AMR/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 recruitment and contributed to the reduced maximum tailbeat frequency and overall impairment of swimming performance in the ammonia-exposed fish.