Effects of water temperature on the gut microbiome and physiology of Chinook salmon (Oncorhynchus tshawytscha) reared in a freshwater recirculating system

Climate change and global warming pose particular concerns to the aquaculture industry as water temperature is one of the most important influences of health and growth performance in poikilotherms. Despite increased recognition of the importance of the gut microbiome in fish health, knowledge of temperature effects on the holobiont and implications for aquaculture performance are still limited. We assessed the effect of water temperature in a freshwater recirculating aquaculture system on the gut microbiome and the physiology, health and growth of New Zealand's most important farmed fish, the Chinook Salmon ( Oncorhynchus tshawytscha ). Fish were reared at four temperatures (8, 12, 16, 20 °C) for 41 to 49 days. Health, growth and feed intake were assessed at the beginning and end of the experiment. Haematology, plasma biochemistry, histology and tissue composition (e.g. fatty acids, amino acids, chemical composition) and gut microbiota (faeces and hindgut wall mucosa) were characterized. We found that increased temperature resulted in an increase in feed intake and growth (plateauing as it approached 20 °C) but did not impact feed conversion efficiency. Gut microbiome community composition assessed from both sample types changed with increasing temperature, with the dominance of Vibrionaceae ( Photobacterium phosphoreum and/or pisciola ) being gradually replaced by Fusobacteriaceae ( Cetobacterium somerae) and Brevenemataceae ( Brevinema ). A significant effect of temperature was also observed on fish histology, fatty acids, chemical composition, haematology, plasma biochemistry and several internal biometrics, with significant correlation of most of these data layers with the gut microbiota. Notably, glucose, lactate and magnesium concentration in plasma, whole-body palmitic and the sum of the saturated fatty acids, and Cetobacterium were positively associated with increased temperature. This study provides further knowledge on how the Chinook salmon holobiont is influenced by water temperature and how the changes observed relate to fish growth performance and health. These results will help make more informed decisions for monitoring fish gut microbiota and tailoring dietary requirements to rearing water temperature. • Fish physiology, health, composition and gut microbiota were significantly correlated and altered by water temperature. • Dominant gut microbiome taxa gradually changed from Vibrionaceae to Fusobacteriaceae with increasing temperatures. • Feed rate and growth both increased with temperature, with no effect on feed conversion efficiency. • Faecal and hindgut wall mucosa microbiomes slightly differed in terms of alpha and beta diversity but showed highly correlated communities.