Post-vaccination intra-abdominal adhesions as a marker to identify Atlantic salmon, Salmo salar L., escaped from commercial fish farms

Intraperitoneal vaccination of Atlantic salmon, Salmo salar L., against furunculosis using oil adjuvanted vaccines causes visible intra-abdominal markers of apparently permanent character. The feasibility of these side-effects for discrimination between fish from vaccinated and unvaccinated populations was evaluated in an open study using samples of vaccinated and unvaccinated farmed salmon, and in a blind study using individually tagged fish from a sea ranching vaccination trial. Vaccines were trivalent, oil adjuvanted bacterins extensively used in Norwegian aquaculture. Adhesions between internal organs and the gut wall were more sensitive and more specific for identifying fish from vaccinated populations than pigmentations on the peritoneum or on the viscera of the fish. Samples of farmed fish drawn from 3 to 35 months after vaccination yielded sensitivities of 93–100% and a specificity of 100% for this marker trait. In the blind study, intraperitoneal inoculation of saline as a placebo was found to induce minor adhesions, thus reducing the specificity of the adhesion marker to 75%. Using the observed sensitivities and specificities in a data simulation, the intra-abdominal adhesion marker proved feasible for the identification of fish from unvaccinated populations even under unfavourable sampling situations. Provided the current practices of vaccinating farmed salmon prevail, exclusion of individual fish carrying the intra-abdominal adhesion marker is recommended when estimating the number of wild salmon stocks, and when selecting broodfish for propagation of specific river populations of Atlantic salmon.