Impact of pancreas disease (PD) outbreaks in salmon farming on marine survival and growth of Atlantic salmon post-smolts

Pancreas disease (PD) caused by salmonid alphavirus (SAV) is a serious challenge in Atlantic salmon farming. PD outbreaks in salmon farms may spread the virus to the surrounding environment and potentially cause infections in wild salmon. Screening for SAV has revealed very low infection rates in wild salmonids irrespective of fish farming intensity or the frequency of PD outbreaks. However, monitoring of infection and disease in wild fish is difficult and has limitations. Therefore, to investigate any potential population-regulatory effects of the SAV transmission from aquaculture to wild populations, we conducted extensive Atlantic salmon Salmo salar smolt release experiments in 2 rivers in western Norway where PD is endemic in commercial farms. Released fish either received a commercially available PD vaccine or were unvaccinated controls. Returning adults were registered in both rivers in the subsequent years. Date of capture, body length, weight and sex of a subset of returning salmon were recorded. Our results show no significant differences in marine survival, fish growth or return age between the fish originating from the vaccinated and control groups in either river or release year even though numerous PD outbreaks were reported in farms along the migration routes during the study period. These data support results from SAV surveillance conducted earlier, suggesting that wild Atlantic salmon smolts are at low risk of SAV infection from fish farms. Therefore, we conclude that PD outbreaks in salmon farming are unlikely to lead to SAV infection-derived reduced marine growth or increased mortality in migrating wild salmon post-smolts with significance at the population level.