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–September 1993) to a simulated summer warming scenario of +2°C in the presence and absence of 70 μmol total ammonia/L (nominal; equivalent to 0.013 mg NH3-N/L at 15°C, pH = 7.6) to determine the metabolic costs and physiological consequences associated with their growth in a warmer, more polluted environment. With unlimited food, fish exposed to +2°C show better energy conversion efficiency and increased nitrogen retention at a metabolic cost equivalent to the base temperature group. Metabolic fuel use appears to have been optimized to support the bioenergetic demands imposed during maximum summer water temperatures. Low-level ammonia enhances nitrogen and energy conversion efficiency by stimulating protein retention, which ultimately results in the most cost-effective growth. However, in the +2°C ammonia treatment, the stimulatory effect of low-level ammonia is lost during mid to late summer due to the greater energy demands when fish are forced to cope with the additional stress of a small further increase in temperature.