The measurement of disease susceptibility in dab, Limanda limanda L., following long-term exposure to contaminated sediments: Preliminary studies

Abstract

Exploratory studies into the immunocompetence of fish held on contaminated and uncontaminated bottom sediments have been conducted at the Fish Diseases Laboratory, Weymouth. The approach has been to expose dab, Limanda limanda L. (Pleuronectidae) long-term (> 3 months) on highly contaminated harbour sludge and less-contaminated sediment, and to compare the immune response and uptakes of contaminants between the two groups. Following exposure to the sediments, mortalities and clinical signs of disease were recorded. The dab were then challenged individually by intraperitoneal injection with 0–1 ml of a 1% suspension of inactivated Vibrio anguillarum. The fish were examined for clinical signs of disease. Blood and samples of organs, including brain, were taken for histological examination. The remainder of these fish (and all fish dying during the experiments) were frozen for contaminant uptake analysis. Clinical and histopathological changes were observed in both groups of fish with skin ulcerations and gill hyperplasia, showing a slightly higher prevalence in the fish exposed to contaminated harbour sludge. Quantification of the immune response was by agglutination assay and an enzyme-linked immunosorbent assay (ELISA). Fish maintained on the contaminated harbour sludge had a lower mean antibody titre than those on the reference sediment and this difference in mean titres was statistically significant. However, because the fish on the contaminated sediment were still able to produce a marked response to the antigen, in biological terms there may have been no difference in the susceptibility to disease between the two groups. Further work will be needed to clarify this point.