Mortality at sea transfer is one of the major culprits for economic loss in the Norwegian salmon industry , reinforcing the tendency towards shortening the rearing time in open net pens . Recirculating Aquaculture Systems (RAS) have increased significantly in the last 10 years, RAS is expected to surpass flow-through systems in the next 5 years. C urrent production protocols favor high temperatures, the use of continuous light, and increased salinity to accelerate growth, where production of larger post-smolts (500-600 g) is becoming a common practice . Even though RAS is one of the most common production systems, there is a lack of standardized protocols, and the biological requirements of the fish are not always considered. Thus , optimizing the rearing strategies in RAS is crucial for produc ing robust (post-) smolts with better performance after seawater transfer. Th is study aims to understand the implications of different RAS protocols, in terms of photoperiod, salinity, and producing large post-smolts , on plasma metabolites related to energy status and expression of genes codifying the gut signals regulating the appetite and food intake and peptide transporters in Atlantic Salmon ( Salmo salar).
Materials and methods
Atlantic salmon (113 g) was reared in RAS at 12 °C and different photoperiod regimes simulating no winter (NW, constant light 24L/0D ), early winter (EW, standard winter signal 12L/12D and spring signal 24L/0D), late winter (LW, delayed winter signal of 12L/12D, normal spring signal of 24L/0D), and late long winter (LLW, prolonged winter signal of 12L/12D and standard spring signal 24L/0D ); two salinities (fresh- (FW) and brackish (BW) water); and two sizes at seawater transfer (300 and 800g). Fish were sampled before each transfer size. Pentra 400 was used to measure the metabolites related to energy status (glucose, triglycerides, cholesterol, and total protein) in the plasma. Expression of genes coding for gut appetite regulator hormones ( cholecystokinin; cck , and n europeptide Y; pyy) and peptide transporters (PepT1 ; slc15a1a , slc15a1b , and PepT2; slc15a2a , slc15a2b) were quantified in the anterior and posterior intestine, using the ΔΔCT method. Data were subjected to Two-way ANOVA, followed by Tukey (p < 0.05).
Photoperiod and salinity significantly affected the metabolites in 300 g fish , with NW show ing generally higher levels compared to EW and LW. On the other hand, neither photoperiod nor salinity had a significant effect on the metabolites in 800 g fish, except for triglycerides (Fig. 1). Expression of pyy was unaffected by the addressed parameters (data not shown) . We are currently analyzing the cck results that will be presented at the conference. At 300 g, the photoperiod regime was the only factor significantly affecting almost all peptide transporters . At 800 g , on the other hand , only slc15a2a was affected by the photoperiod . E xpression of slc15a1 showed a n approximately opposite trend to slc15a2; the former was upregulated in NW and EW , while the latter was in LW (Fig. 2).
Discussion and conclusion
Higher variations in the metabolite levels in 300 g fish might be because they have recently gone through the energy- demanding process of smoltification and are still adjusting their energy budget . While at 800 g, they have had more time to acclimatize to the environmental conditions. In agreement with this, the highest triglyceride levels were observed in NW and EW. This might suggest triglycerides as a more sensitive indicator of energy status in larger smolts. While plasma provides a good idea of the amount of ready-to-be-used energetic substances , it would be interesting to study the hepatic t riglycerides and glycogen reservoirs to obtain a completer picture.
The opposite trends in the expression of members of PepT1 and 2 may point to different physiological roles (nutrient absorption and/or molecule sensing) of these groups , differentially responding to environmental conditions ( Gomes et al., 2020). S hort-term starvation of Atlantic led to the downregulation of slc15a1 and b, while the upregulation of slc15a2a and b ( Del Vecchio et al., 2021). S imilarly, in our study, LW with the lowest energetic substances in the plasma, showed the lowest expression levels for slc15a1b , and the highest for slc15a2a and b.
Del Vecchio, G., Lai, F., Gomes, A.S., Verri, T., Kalananthan, T., Barca, A., Handeland, S., Rønnestad, I., 2021. Effects of Short-Term Fasting on mRNA Expression of Ghrelin and the Peptide Transporters PepT1 and 2 in Atlantic Salmon (Salmo salar ). Front. Physiol. 12:666670
Gomes, A.S., Vacca, F., Cinquetti , R., Murashita, K., Barca , A., Bossi , E., Rønnestad, I., Verri, T. , 2020. Identification and characterization of the Atlantic salmon peptide transporter 1a. Am. J. Physiol. Cell Physiol. 318, C191–C204.
Funding: SFI for ROBUST , CtrlAQUA - Ref. 237856. Analysis performed in the Fish Biology and Aquaculture high throughput state of the art laboratory (SacLab), NORCE.