Heavy metals have been shown to produce a wide range of effects on fishes and, as a consequence, several mechanisms are developed to compensate such affects. At the metabolic level heavy metal toxicity produces tissue hippoxia, enzymatic disturbance and energy defficiency. Fish responses involve not only the way in which ATP is produced, but also the way in wich ATP is used, as the motor activity or energy expenditure. Aerobic metabolism response involves mostly higher rates of metabolic turnover, enzymatic activity or mitochondrial mass. In anaerobic metabolism strong increases of lactate concentration as well as other anaerobic metabolites as alanine, succinate or ethanol have been described, and alternative anaerobic pathways have been postulated to explain the accumulation of such metabolites. However, as long-term treatments show normal metabolite levels it is suggested that anaerobic metabolism would play a role as a short-term energy source, immediately after metal action, while longterm adaptations in aerobic pathway or in physiological mechanisms take place
