THE METABOLIC COSTS AND PHYSIOLOGICAL CONSEQUENCES TO JUVENILE RAINBOW-TROUT OF A SIMULATED SUMMER WARMING SCENARIO IN THE PRESENCE AND ABSENCE OF SUBLETHAL AMMONIA

Quantitative bioenergetic and physiological measurements were made on juvenile rainbow trout Oncorhynchus mykiss exposed over summer (June-S eptember 1993) to a simulated summer warming scenario of +2 degrees C in the presence and absence of 70 mu mol total ammonia/L (nominal; equ ivalent to 0.013 mg NH3-N/L at 15 degrees C, pH = 7.6) to determine th e metabolic costs and physiological consequences associated with their growth in a warmer, more polluted environment. With unlimited food, f ish exposed to +2 degrees C show better energy conversion efficiency a nd increased nitrogen retention at a metabolic cost equivalent to the base temperature group. Metabolic fuel use appears to have been optimi zed to support the bioenergetic demands imposed during maximum summer water temperatures. Low-level ammonia enhances nitrogen and energy con version efficiency by stimulating protein retention, which ultimately results in the most cost-effective growth. However, in the +2 degrees C ammonia treatment, the stimulatory effect of low-level ammonia is lo st during mid to late summer due to the greater energy demands when fi sh are forced to cope with the additional stress of a small further in crease in temperature.