The cost of living for freshwater fish in a warmer, more polluted world

Little of the vast literature on the temperature physiology of freshwater f ish is useful in predicting the effects of global warming. In the present r eview a series of laboratory experiments is reviewed in which rainbow trout (Oncouhynchus mykiss) were exposed to simulated global warming, a 2 degree sC increment superimposed upon the natural thermal regime, in the presence and absence of two common freshwater pollutants, ammonia and acidity (low p H). Simulated global warming had little effect on the growth and physiology of trout fed to satiation over much of the summer. However, in late summer , when ambient water temperature was at its highest, the addition of 2 degr eesC caused a marked inhibition of appetite and growth, although this impac t was not exacerbated by a reduction in food availability. In winter, +2 de greesC stimulated metabolism, appetite and growth by approximately 30-60%. Exposure of satiation-fed trout to low levels of pollutants produced unexpe cted results. Ammonia (NH3 + NH4+ = 70 mum) stimulated summer growth and en ergy conversion efficiency, whilst acidification (pH 5.2) increased appetit e and growth but caused no disturbance of electrolyte balance. These pollut ant effects were additive upon, but not synergistic with, the effects of +2 degreesC. The ability of the fish to acclimate to the experimental conditi ons was tested with acute lethal temperature and/or toxicant challenges. Fi sh exposed to +2 degreesC had a slightly (0.2-1.0 degreesC) but significant ly higher lethal temperature than those exposed to ambient temperature when fed to satiation. However, there was no evidence of acclimation to either ammonia or low pH. It is concluded that the impact of global warming on fre shwater fish will vary seasonally. The additional temperature may provide g rowth benefits in winter, but may threaten fish populations living towards the upper end of their thermal tolerance zone in (late) summer.