Effect of high dietary ARA and EPA on intestinal health of Atlantic salmon (Salmo salar) before and after chronic stress

Sammendrag

The reduced use of marine oils in fish feed has led to a higher n-6/n-3 fatty acid composition in tissues and organs of Atlantic salmon (Salmo salar). As long-chain polyunsaturated fatty acids (LC-PUFA) have central roles in many biological mechanisms, an altered fatty acid composition can change the eicosanoid cascades and pose a great threat to fish health and welfare. However, little is known about how altered LC-PUFA in feed affects the inflammatory process and chronic stress mechanisms in fish. This study aimed to examine the effect of high dietary ARA and EPA on the intestinal health of Atlantic salmon (Salmo salar) both before and after chronic stress. Smoltified Atlantic salmon received diets containing either high a) ARA or b) EPA, with three replicate tanks per diet for two months before sampling. Following this, the fish were subjected to three weeks of unpredictable chronic stress (UCS)-regime with stressors common in the aquaculture industry. The intestinal health was examined by using histological and immunohistochemical methods to investigate the mid- and hind-intestine for signs of inflammation.
High dietary ARA led to reduced weight increase and specific growth rate throughout the whole experimental period compared to feeding with high dietary EPA. Additionally, fish fed ARA diet showed changes in gross morphology in the hind-intestine after two months of feeding. This did, however, not correlate to upregulation of immune cells. Three weeks of chronic stress regime led to reduced growth in fish fed both diets. No upregulation of cortisol or haematocrit was detected after chronic stress. However, there was a significantly higher haematocrit level in EPA fed fish compared to those fed ARA diet, throughout the whole trial. Moreover, there were observed morphological changes in the hind-intestine of both ARA and EPA fed fish as a consequence of the chronic stress period, in addition to upregulation of granulocytes in mucosal folds in mid-intestine and complex folds in hind-intestine. Feeding with high ARA during the stressing conditions appeared to enhance the negative effects on the intestinal morphology compared to feeding with EPA. These data did not transform to elevated inflammatory responses as expected through regulations of immune cells. Chronic stress did in fact lead to upregulation of MHCII cells in the hind-intestine and CD8 cells in the mid-intestine of fish fed EPA, whereas the same were not observed in those fed high dietary ARA. The present results demonstrate that altered LC-PUFA affects the intestinal health of Atlantic salmon and highlight the need for extended research on this topic.