Chronic stress negatively affects growth, health, and welfare of fish and is characterized by elevated plasma cortisol levels over time. While the short-term action of cortisol in fish is well described, less is known about the health effects of chronically elevated cortisol levels , in particular under intensive production. Furthermore, it is unclear how cortisol affects the intestinal microbiota. Here, we investigated how elevated cortisol levels affect the health of Atlantic salmon in recirculating aquaculture systems and whether these interact with feeding a functional diet containing the DHA-rich microalgae Schizochytrium . Atlantic salmon post-smolts (~557 g ) were reared in six single RAS units of 1.5 m³, and three experimental groups in duplicate were established: a control group not receiving cortisol (CD-), a group fed cortisol at 15 mg/kg fish (CD+), and a group receiving cortisol in addition to a DHA-rich algae feed (SL+ ). Following four weeks of restricted feeding at 1.2% of the biomass, the fish were sampled to investigate effects on growth performance, plasma and organ health indicators, as well as the bacterial composition of the digesta using 16S metabarcoding.
Fish receiving cortisol showed reduced growth and increased feed conversion, along with a significant increase in plasma cortisol and glucose levels. Fish fed the algae feed showed intermediate growth performance. C ortisol levels also increased strongly in the RAS rearing water. Fish receiving cortisol had reduced condition and hepatosomatic index, as a consequence of increased energy mobilization . Histological analysis of the liver, heart, and intestine will give further insights into the health effects of chronically elevated cortisol levels and its potential interaction with a DHA-rich diet. The ongoing analysis of the bacterial communities in the digesta as well as the rearing environment will reveal the effects of cortisol and the functional DHA-rich feed on the microbiota.