The effect of size on trace metal levels in raft cultivated mussels (Mytilus galloprovincialis)

Mussels are farmed in the coastal inlets of Galicia by means of floating raft culture. The growth of these bivalves is due to their great capacity to filter the water column, which unfortunately, also exposes them to dangerous contaminants, including heavy metals. Thus, it is imperative that mussels be monitored for metals. Size has sometimes been shown to be an important variable, but contradictory results have been found. In order to monitor metals in raft mussels, it is necessary to compile information regarding the number of size classes according to shell length, which must be taken into account to obtain a representative sampling, as well as the number of sample replicates that must be included. Also, to be considered is the cost incurred by carrying out the collection, preparation and analysis per sample. The purpose of this study is to provide information about the effect of size, indicated by shell length on the metal content of raft mussels. The ability of 10 mussel-pooled samples to discriminate real differences in metal concentration was also studied. In general, a positive relationship between metal content and shell length was observed; a similar relation was found between the weight of soft tissues and shell length. As expected from the similarity encountered between relationships of metal content and dry weight-length, the concentrations of the different metals in the soft tissues would not seem to depend on the shell length. Metal concentrations, in this study, were found to be approximately 45 ppm of Hg, 0.5 ppm of Cd, 0.7 ppm of Pb, 0.5 ppm of Cr, 0.6 ppm of Ni, 8 ppm of As, 5 ppm of Cu and 122 ppm of Zn; on a dry weight basis. No significant differences between metal concentrations at different shell lengths were detected. Another important observation was the high variability observed within one size, indicating poor homogeneity in subgroups of similar size, which must be minimized if the number of samples is not enhanced as indicated by power and size analysis.