Feasibility studies on the use of sewage sludge as supplementary feed for rearing tilapia i. Trace metal contents of the treated fish

The first part of the present project studied the trace metal content of different parts of tilapia, Oreochromis mossambicus, fed sludge cake as supplementary feeds. The feeding trial lasted for 60 days under stable aerated conditions. Various portions of sludge cake (5, 10, 20 and 30% w/w) and detoxified (removing trace metals) sludge cake (10 and 30%) were mixed with an artificial fish diet which contained 50% fish meal, 25% wheat flour, 20% peanut meal, 3% cod liver oil and 2% vitamin), while the control group were fed artificial diet only. The sludge supplementary diets (including the detoxified sludge) had significantly higher (p<0.05) amounts of all the trace metals tested (Cd, Cu, Cr, Ni, Pb and Zn) compared to the control diet, except Pb in the detoxified sludge. The concentrations of metals in different fish tissues (head, gill, viscera, flesh and bone) were related to the proportion of added sludge cake. The fish fed with the detoxified sludge cake supplement had significantly lower (p<0.05) metal contents than that fed with the sludge cake supplement. In general, the higher the portions of sludge incorporated into the diet, the higher the concentrations of trace metals in the tested fish tissues. Harmful effects were presumably reflected in lower growth rates of fish fed on sludge supplemented diets, with the sole exception of the 5% sludge cake supplement. Mucus secretion and coughing were induced in fish receiving higher portions of sludge diets.