Introduction

Currently, adult Atlantic salmon (Salmo salar) returning to the Connecticut River are removed from the river and anesthetized in order to weigh, measure and collect scale samples. At this time, parasitic infestations are treated and the fish are given a combined antibiotic/bacterin injection. The injection consisted of 2.4 mg/kg fish oxolinic acid and 0.5 ml/kg fish Aeromonas salmonicida-Yersinia ruckeri bacterin. This treatment is given to reduce the numbers of fish that succumb to stress-induced epizootics of Furunculosis and Enteric Redmouth Disease.

Clearly, adult salmon reaching the hatchery facilities have been subjected to the natural stress of spawning migration and to other stressors, such as handling and transport. The ability of these fish to mount a protective immune response induced by bacterin injection is questionable because the physiological status of these fish is less than optimum at the time of capture and transport. The economy of vaccinating fish that may not be physiologically or immunologically responsive, therefore, is questionable.

Immunity of Atlantic Salmon

In attempts to increase survival of hatchery-release salmon and the number of adult returns, previous investigations have concentrated on the ability of vaccinated parr and smolts to survive challenge with specific pathogens. For example, Baum et al. (1982) vaccinated 2 year old Atlantic salmon smolts with a trivalent Vibrio bacterin. These fish were not challenged experimentally but were reared in commercial net pens. Neither survival among vaccinated and control fish nor the number of adult returns differed. Successful vaccinations of Atlantic salmon parr have been reported by Holm and Jorgensen (1987), Lillehaug et al. (1990) and Lillehaug (1990). These reports, however, evaluated the vaccination of Atlantic salmon against "Cold-water Vibriosis", a disease caused by Vibrio salmonicida.

Only a few studies concerning immunization of Atlantic salmon against Furunculosis and Enteric Redmouth Disease have been reported. Shieh (1985) reported that fingerlings immunized with extracellular protease from an avirulent A. salmonicida were protected against challenge with virulent A. salmonicida. Bruno and Munro (1989) indicated that fry vaccinated with a Y. ruckeri bacterin were protected against water borne challenge for up to 165 days. Data compiled since 1986 by the Northeast Atlantic

Salmon Restoration Project and the US Fish & Wildlife, Fish Health Unit (Lamar, PA) indicated that mortalities were reduced (as compared to non-vaccinated control groups) in groups of adult salmon given the combined bacterin and antibiotic treatment mentioned previously (Clifford, 1990). Reduction in mortality among the treated salmon can not be attributed to the effects of the vaccine alone, because antibiotic treatment may have contributed (solely or in conjunction with the vaccine).

Objectives

The ability of returning Atlantic salmon (Salmo salar) to mount an immune response induced by bacterin injection was questioned because of numerous stressors affecting returning fish. In addition, a health management plan which is both cost-effective and most beneficial to the adult Atlantic salmon maintained at brood stock facilities is needed. The first objective, therefore, was to assess the ability of adult Atlantic salmon to mount an immune response after returning to spawn. If these fish were not capable of mounting an immune response, the bacterin injection could be eliminated from the current protocol. Second, if an immune response was demonstrated, the protective nature of the response would be evaluated.

Present Study

For this study, the immune response of post-spawning males to A. salmonicida-Y. ruckeri bacterin was evaluated. Females that survive spawning are reconditioned as a routine procedure of the Atlantic salmon restoration efforts and are too valuable to be used for research purposes. Males, on the other hand, are normally sacrificed after spawning and can be used for research. Initial experiments indicate that post-spawning males produce a strong antibody response to A. salmonicida (mean - 11.1) and that the agglutinin response to Y. ruckeri serotypes 1 and 2 is weaker (mean 4.5 and 2.7, respectively). Antibody titers are expressed as the reciprocal of the last log 2 dilution showing positive agglutination. Brook trout passively immunized with sera from the Atlantic salmon were not protected from challenge with Y. ruckeri serotype 2. This serotype of Y. ruckeri, however, was not included in the bacterin used to immunize the salmon.

The post-spawning male Atlantic salmon maintained at the Richard Cronin National Salmon Station and sampled in this study mounted an immune response to the bacterin given at the time of capture. The protective nature of this response was not demonstrated by this study. Brook trout passively immunized with the Atlantic salmon sera and challenged with A. salmonicida and Y. ruckeri serotype 1, exhibited low levels of protection, however, the results are equivocal. Two problems occurred in these challenge experiments. First, low virulence was noted for both challenge isolates, 3.139 and 11.14. Mortality did not exceed 30% among fish immunized with phosphate buffered saline (pH 7.2) and challenged with either 3.139 or 11.14. More virulent isolates would have produced higher mortality in the non-immunized fish and differences between the immunized and control groups may have become more apparent. Second, mortality among non-challenged control fish further confounds the data. Pathogens were not detected in these fish and mortalities were attributed to non-specific causes. These mortalities, however, do not allow for differences to be detected among treatment groups. Further research is planned in cooperation with Atlantic salmon brood stock facilities and the US Fish & Wildlife, Fish Health Unit (Lamar, PA) to evaluate the protective nature of the immune response in adults upon return and after spawning.

Acknowledgements

This work was coordinated through the US Fish & Wildlife, Fish Health Unit (Lamar, PA) and the cooperation of the staff of the Fish Health Unit and the staff of the Richard Cronin National Salmon Station (Sunderland, MA) is greatly appreciated.

References

1.  Baum, E.T., E.S. Sawyer and R.G. Strout. 1982. Survival of hatchery-reared Atlantic salmon smolts vaccinated with a Vibrio anguillarum bacterin. N. Am. J. Fish. Mang. 4: 409-411.

2.  Bruno, D.W. and A.L.S. Munro. 1989. Immunity in Atlantic salmon, Salmo salar L., fry following vaccination against Yersinia ruckeri, and the influence of body weight and Infectious Pancreatic Necrosis Virus (IPNV) on the detection of carriers. Aquaculture 81: 205-211.

3.  Clifford, P. 1990. Control of Aeromonas salmonicida in Connecticut and Merrimack Rivers Atlantic salmon broodstock hatcheries. Proc. Atlantic Salmon Workshop, S. Baker (ed.), Northeast Fishery Center, Lamar, PA September 1990, p. 164-170.

4.  Holm, K.O. and T. Jorgensen, 1987. A successful vaccination of Atlantic salmon, Salmo salar L., against "Hitra disease" or coldwater vibriosis. J. Fish. Dis. 10: 85-90. Lillehaug, A. 1990. A field trial of vaccination against cold-water vibriosis in Atlantic salmon (Salmo salar L.). Aquaculture 84: 1-12.

5.  Lillehaug, A., R.H. Sorum and A. Ramsted. 1990. Cross-protection after immunization of Atlantic salmon, Salmo salar L., against different strains of Vibrio salmonicida. J. Fish Dis. 13: 519-523.

6.  Shieh, H.S. 1985. Vaccination of Atlantic salmon, Salmo salar L., against furunculosis with protease of a virulent strain of Aeromonas salmonicida. J. Fish Biol. 27: 97-101.