| Summary: | An initial protocol for a bacterial challenge with the pathogen <i>Vibrio anguillarum</i>, was developed for haddock in Chapter 2 with a view to use this method to test the efficacy of the vaccination trials that were carried out. The results demonstrate that, as previously reported in cod, even though vaccination does not induce a production of specific antibodies, it does protect fish against infection. The effect of a diet enriched with nucleotides was studied in cod larvae in Chapter 5. In the trial performed in this thesis, live food (rotifers and Artemia) were enriched with yeast based nucleotides. The expression of five immune genes was investigated by RE-PCR to look at the effect of the nucleotides on the immune system of the developing larvae. After analysis of al samples, no significant difference on the expression of the immune genes was observed among treatments. As part of the study of the gadoid immune system, four novel genes were sequenced in haddock in Chapter 3 and 4. Two pro-inflammatory cytokines, interleukin-1β (IL-1β), interleukin-8 (IL-8), and two genes that play an important role in the immune system of teleosts, immunoglobulin-M (IgM) and recombination activating gene-1 (RAG-1). The expression of the RAG-1 gene appears earlier in development than IgM. Using whole-mount <i>in situ</i> hybridisation, the expression of the RAG-1 gene was first detected in larvae of 6-7mm in length (25-29dph) in a region identified as the thymus by histology. The research described in this thesis reveals the similarity of some of the immune mechanisms in cod and haddock.
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