Acute and sub-acute toxicity of bark tannins in carp (Cyprinus carpio L.)

The 24-h LC(I)50 and 96-h LC(I)50 of bark extract for carp (Cyprinus carpio L.) were estimated at approx. 65 and 50 mg/l total COD, respectively, in a semi-static system. Tannins, constituting 50-60% of total COD, are responsible for this toxicity. Considerable toxicity was also found in sub-acute studies in a flow-through system. In both systems oxidative polymerization of the tannins into high-molecular-weight polymers abolished the aquatic toxicity almost completely. Epithelial damage of the gill was the most important lesion of toxicity, supporting the hypothesis that the toxic action of oligomeric bark tannins is through binding to and cross-linking of cell membrane enzymes.