Adaption of l and-based recirculating aquaculture system (RAS) has been pivotal in the significant development of Atlantic salmon production in the last decades. Since fish have little contact with the surrounding environment, t he systems offer stringent biosecurity measures against diseases and fluctuating environmental conditions . Despite this advantage, RAS is not risk-free for disease outbreaks. T here are still some potential transmission sources, including the intake of water and infected fish stocks, among others. High organic loads in these intensive systems likewise offer a favourable environment for opportunistic pathogens to thrive, establish and eventually infect the fish. Pathogen outbreaks in RAS have been reported, however, we have limited understanding of the extent of their risks since simulating these scenarios under controlled conditions is challenging.
A trial was conducted where a pathogen breach was simulated in brackish water recirculating aquaculture systems rearing Atlantic salmon smolts. Fish either received a traditional smolt feed or feed developed for RAS. Yersinia ruckeri , the causative agent of enteric red mouth disease (ERM) in salmonids was employed as the test pathogen. The pathogen was introduced into the system through the sump tank, simulating a pathogen breach via the intake water. The development of the disease was followed for 3 weeks. Thereafter, all remaining fish were taken out and the systems operated for another two weeks before the systems were disinfected through pH manipulation.
Preliminary results indicate that the pathogen did not manage to establish in the system as shown by its limited detection in the swab samples collected from the different locations, including, tank wall, drum filter, sump tank, biofilter wall and biomedia. Microbiota analysis provided further support for this. Key microbial phyla showed temporal variations during the trial, but the type of feed showed no clear effects. Key water qualities remained favourable and within the limits. Overall, the chemical composition of the sludge did not change, although, the phosphorus levels decreased during the disease development phase. There was no mortality following infection and fish did not show the classic pathologies for ERM. Despite the sporadic detection of Y. ruckeri in some fish samples, the load was relatively low. Infected fish mounted mucosal and systemic immune responses based on antibody and gene expression analyses, though the changes appeared not to be heavily affected by feed types . This was supported by minimal changes in the metabolomic and proteomic profiles of skin mucus. Biofilm carriers placed in the biofilters and sump tanks to monitor the establishment of pathogen and biofilm formation overtime, also confirmed the limited detection of pathogen, but showed stronger biofilm establishment in both biofilters and sump tanks for the traditional smolt feed RAS in comparison to the RAS feed ones . Infected biomedia were subjected to different disinfectants approved for use in Norway and those based on peracetic acid appeared to have the highest disinfection potential.
The work was supported by the Norwegian Seafood Research Fund ( FHF 901826 ).