Immunocompetence as a measure of the biological effects of sewage sludge pollution in fish.

1. Dab, Limanda limanda, exposed to nominal concentrations of 0 (control), 0.0032% (low) and 0.032% (high) sewage sludge in seawater for 12 weeks, were assessed for their immunological competence. 2. No effect upon total blood leucocyte and erythrocyte numbers was found, although significantly fewer thrombocytes were seen in the high-exposure group. 3. A decreased serum protein level was found in the high exposure group, but lysozyme and immunoglobulin levels showed non-significant differences between the groups. 4. Melano-macrophage centres were also affected in the high-exposure dab, which had increased numbers in the spleen and kidney. No effect upon spleen weights or oxygen free radical production by splenocytes was noted. However, oxygen free radical production by kidney leucocytes was inhibited in the low-exposure dab.