Extensive fecal cast production and growth of Vibrionaceae in heat-stressed Atlantic salmon post-smolts.

Atlantic salmon (Salmo salar) are known to reduce or cease voluntary feeding and show altered digesta consistency under heat stress. This study was performed to determine what bacterial species occur and to characterize bacterial taxa and quantify changes in gut microbial abundance under heat stress. Atlantic salmon (all female, 1.5 kg average weight) in seawater tanks at 15 °C were fed for 2 to 4 weeks. Tank temperatures were increased to a 19 °C "warm phase" until voluntary feeding abated at which point tank temperatures were cooled to 15 °C for 4 weeks. At the end of each temperature phase the fish were stripped of feces and microbiome profiles were determined using 16 S rRNA V1-V3 metabarcoding. The tank experiment was repeated three times in successive years. Abundances of bacteria were determined using qPCR. 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. Quantitative PCR (qPCR) estimated fecal Vibrionaceae cell populations increased 1.9-3.4 log units/g after the warm phase. This population then decreased by 0.3-1.1 log units/g by the end of the 15 °C recovery phase. The gut Vibrionaceae and non-Vibrionaceae compositions in the separate trials were different each time. The results indicated heat stress induced inappetence corresponds to increased cast production accompanied by predominance of Vibrionaceae. The predominance of different bacteria in each trial could be partly due to the different initial abundances of taxa in the inputted smolt. Vibrionaceae colonizing the Atlantic salmon gut should be the focus of studies on the microbiology of thermally induced inappetence and dysbiosis in Atlantic salmon.