First description of health status and associated bacterial diversity from non- native Chinook salmon (Oncorhynchus tshawytscha, Walbaum 1792) naturalised in a large river system from southern Chile

Chinook salmon (Oticorhynchus tshawytscha Walbaum, 1792) was naturalised in many river systems from South America following aggressive introduction efforts. This study investigated the health status and bacterial diversity of Chinook salmon from the Tolten River in southern Chile. Ribosomal and gene-coding markers were sequenced to establish the presence/absence of pathogens common to farmed salmonids. Pathological examination of 30 adult Chinook salmon showed no internal/external clinical signs of disease. Nine specimens were used for microbiological assessments, with all bacterial isolates identified to the genus or species level using 16S rDNA sequencing. Tissue samples from the remaining 21 specimens were subjected to PCR analyses for known pathogens. Sequencing of 16S rRNA resulted in 48.48% (32 of 66) of the obtained isolates matching to the Vibrio, Psychrobacter; and Pseudomonas genera. Exiguabacterium, Arthrobacter, Aliivibrio, Shewanella, and Sphingobacterium genera were represented to a lesser extent (3-5 isolates each). Of the 66 isolates, 27, 22, and 17 were respectively obtained from kidney, spleen, and liver samples. Positive molecular amplifications were obtained for 11 of the 63 tissue samples (17.4%), with 9 being Vibrio ordalii, 1 being Piscirickettsia salmonis, and 1 being Aeromonas salmonicida. However, when RT-PCR protocols for viral diagnostics were applied (i.e. IPNV and ISAV); no amplification products were detected. Despite a wide range of detected bacteria, continued research is needed to determine if Chinook salmon act as disease vectors or if bacterial colonisation is normal and asymptomatic. The presence of common aquaculture-related pathogens suggests the assessed Chinook salmon population may have recently originated from captive broodstock, which would be consistent with anecdotal information and genetic data.