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 consequ ences 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 equiva lent growth of experimental and control groups previously observed und er warm-summer conditions. However, during winter exposure, wet weight s and total lengths were roughly 30% higher in the ''warmed'' fish tha n in the base temperature group due to a combination of greater appeti te and higher energy conversion efficiency. Oxygen consumption and nit rogenous (ammonia + urea) waste excretion rates were 30-40% higher for ''warmed'' fish but were less than one-third of levels recorded in th e summer. A corresponding increase in food intake was associated with elevations in whole-body protein and lipid but not carbohydrate. Addit ion of 70 mu mol ammonia/L elevated nitrogenous waste excretion much l ike in the previous summer exposure, but over this winter period it di d not result in increased weight gain. Plasma total ammonia was not si gnificantly higher in the ammonia-exposed fish, unlike the summer expe riment. Although nitrogen retention efficiency was much lower for over wintering juvenile trout fed to satiation, the metabolic cost of nitro gen retention (growth) was similar to that of juvenile trout exposed d uring summer. We conclude that overwintering juvenile trout fed unlimi ted ration and subjected to simulated warming, both alone and in combi nation with elevated environmental ammonia, will exhibit increased gro wth with only a slight elevation in energetic cost.