Heat stress impacts the gut microbiome of Atlantic salmon by promoting growth of Vibrionaceae and is associated with extensive cast production

Atlantic salmon (Salmo salar) when exposed to heat stress have reduced voluntary feeding and exhibit changes in digesta consistency. This study was performed to determine what bacterial species occur and the overall bacterial abundance in the gut microbiome in heat stressed Atlantic salmon. For this Atlantic salmon in seawater tanks at 15{degrees}C were fed for 2 to 4 weeks. Tank temperatures were increased to 19{degrees}C until voluntary feeding abated at which point tank temperatures were cooled to 15{degrees}C for 4 weeks. At the end of each temperature phase the fish were stripped of feces and microbiome profiles were determined using 16S rRNA V1-V3 metabarcoding. The tank experiment was repeated three times in successive years. Vibrionaceae comprised most reads after the warm phase completed. The prominent levels of Vibrionaceae were accompanied by a large predominance of cast (sloughed intestinal mucosa) containing fecal samples. qPCR estimated Vibrionaceae cell populations increased 1.9-3.4 log units/g after the warm phase but and slightly decreased by 0.3-1.1 log units/g after the 15{degrees}C recovery phase. The results indicated heat stress induced inappetence corresponds to increased cast production accompanied by predominance of Vibrionaceae. Vibrionaceae colonizing the Atlantic salmon gut should be the focus of studies on the microbiology of thermally induced inappetence and dysbiosis in Atlantic salmon. Key ContributionThe experiments confirm that thermal stress increased cast production in Atlantic salmon. The increase in cast production coincides with increased predominance of Vibrionaceae. Though the growth of Vibrionaceae in the gut of salmon is not associated with acute disease it may represent a dysbiosis effect. The results suggest extended summers in regions where sea surface temperatures are at the tolerance limits require increased husbandry management of farmed fish. In addition, more studies are needed to determine if novel approaches can limit overgrowth of Vibrionaceae.