Effects of a Restricted Ration on the Growth and Energetics of Juvenile Rainbow Trout Exposed to a Summer of Simulated Warming and Sublethal Ammonia

Abstract

Laboratory tests were conducted to determine the potential effects of a warmer and more polluted environment on the growth and energetics of juvenile rainbow trout Oncorhynchus mykiss fed a fixed restricted ration (1% wet body weight/d) during summer. The fish were exposed either to the naturally fluctuating ambient thermal regime (base, representative of inshore Lake Ontario) or to the ambient regime + 2°C (base + 2°C), both in the presence or absence of 70 μmol total ammonia(Tamm)/L (0.013 mg NH3-N/L at 15°C, pH = 7.6). The 90-d exposures lasted from June to September 1994 and were designed to mimic an earlier study in which juvenile rainbow trout were fed to satiation. Relative to the earlier study, the restricted ration markedly increased (4–9-fold) the metabolic costs of nitrogen retention, that is, oxygen consumption per unit protein growth. Rainbow trout from the present study exhibited O2 consumption, and specific growth rates that were 50–75% and 13–20%, respectively, of the O2 and growth rates of fish fed to satiation. In addition to the pervasive ration effects, juvenile rainbow trout exposed to +2°C in the present experiment managed to retain more nitrogen for growth, at a slightly decreased energetic cost. Fish exposed to +70 μmol Tamm/L also exhibited higher energetic costs, but this was accompanied by a comparatively large increase in nitrogen retention efficiency. Thus, their “costs of growth” were substantially reduced. We conclude that a restricted ration of 1%/d will not further impair the ability of juvenile rainbow trout to cope with a chronic small temperature increase. Moreover, sublethal ammonia even may be beneficial under these circumstances.