Investigations Into Haddock (Melanogrammus aeglefinus) Immunoglobulin     A. Roberts1, 2, E. B. Bentley1, 3, D. Gillett1, 3, Andrew Dacanay1, L. L. Brown1 and S. C. Johnson1   1 National Research Council-Institute for Marine Biosciences, 1411 Oxford Street, Halifax NS; 2 Department of Immunology and Microbiology, Dalhousie University, Halifax NS; 3 Department of Biology, Dalhousie University, Halifax, NS   The Canadian aquaculture industry is diversifying into the production of non-salmonid finfish species such as halibut, Atlantic cod and haddock.  The Atlantic cod and the haddock, both members of the family Gadidae, pose an immunological paradox. Studies of Atlantic cod have shown functional immunoglobulin genes and a more diverse range of expressed MHC genes than seen in higher vertebrates but a specific antibody response has yet to be elicited in this species. In our laboratory we have found haddock to have naturally high levels of circulating immunologbulin.  However, we have been unable to elicit a specific antibody response in haddock regardless of the route of vaccine administration or the recipient size.  We were able to detect an immune response in the same animals against the live feed they received as larvae. Size exclusion chromatography and non-reducing electrophoresis yielded data on the physical structure of the haddock immunoglobulin molecule. It exists as a non-covalently bonded tetramer, with no intermediate covalently bonded forms (dimers, trimers or halfmers). This, together with the low specific antibody titer of these animals, raises questions about the immunocompetency of non-covalently bound intermediate forms of fish immunoglobulin and the contribution of specific humoral immunity to the overall immune response in the gadis.