The Metabolic Costs and Physiological Consequences to Juvenile Rainbow Trout of a Simulated Winter Warming Scenario in the Presence or Absence of Sublethal Ammonia

This experiment examined the metabolic costs and physiological consequences of growth and energetics of juvenile rainbow trout Oncorhynchus mykiss in a warmer, more polluted winter environment. Growth under the warm‐winter conditions was approximately three times less than equivalent growth of experimental and control groups previously observed under warm‐summer conditions. However, during winter exposure, wet weights and total lengths were roughly 30% higher in the “warmed” fish than in the base temperature group due to a combination of greater appetite and higher energy conversion efficiency. Oxygen consumption and nitrogenous (ammonia + urea) waste excretion rates were 30–40% higher for “warmed” fish but were less than one‐third of levels recorded in the summer. A corresponding increase in food intake was associated with elevations in whole‐body protein and lipid but not carbohydrate. Addition of 70 μmol ammonia/L elevated nitrogenous waste excretion much like in the previous summer exposure, but over this winter period it did not result in increased weight gain. Plasma total ammonia was not significantly higher in the ammonia‐exposed fish, unlike the summer experiment. Although nitrogen retention efficiency was much lower for overwintering juvenile trout fed to satiation, the metabolic cost of nitrogen retention (growth) was similar to that of juvenile trout exposed during summer. We conclude that overwintering juvenile trout fed unlimited ration and subjected to simulated warming, both alone and in combination with elevated environmental ammonia, will exhibit increased growth with only a slight elevation in energetic cost.