Antibody response to diphtheria toxoid in rainbow trout (202 fish belonging to 28 full-sib pairs within 14 half-sib groups) and Atlantic salmon (791 fish belonging to 34 full-sib pairs within 12 half-sib groups) was determined by analyzing blood samples taken on three different occasions. The fish were immunized twice with the antigen during the experiment. Antibody levels were measured by an enzyme-linked immunoadsorbent assay (ELISA). A method of estimating titers from three serum dilutions was worked out to avoid end-point titration of the large number of samples. The rainbow trout responded well to immunization, and genetic variation in antibody titers was observed both before and after the first immunization (h2 = 0.2). The Atlantic salmon had high initial titers to diphtheria toxoid and the response to immunization was weak. This suggests an impact of natural or cross-reacting antibodies. The heritabilities for antibody levels were lower in this species (0.12 and less). There were high genetic and phenotypic correlations between the antibody measurements at the three samplings. Antibody response to diphtheria toxoid correlated negatively with survival after challenge with V. anguillarum. No correlation was observed between antibody level and production traits (mean slaughter weight and survival rate) for full-sib groups reared at different sea locations.

Lysozyme activity in blood was analysed in full- and half-sib families of rainbow trout, Oncorhynchus mykiss. 1075 fish from 80 full-sib groups within 30 paternal half-sib groups were sampled. The elapsed time between the initial crowding of fish for blood sampling and the collection of a blood sample from a particular fish, affected the lysozyme activity significantly. Statistically significant effects of sire on the lysozyme activity provide evidence of significant additive genetic variation in lysozyme activity in rainbow trout. The estimated heritabilities of the lysozyme activity were in the order of 0.2–0.3 with a standard error of approximately 0.05–0.15. Production parameters, such as mean weight at slaughter and mean survival during the sea-rearing period, were obtained for fish which were reared commercially and which belonged to the same full-sib groups as those analysed for lysozyme activity. The correlations between least squares means of lysozyme activity and production parameters, based on the full-sib records, were not significant.