Introduction
Within the Atlantic salmon farming sector there is a growing adoption of Recirculation Aquaculture Systems (RAS) for the manipulation and production of smolts. While offering many advantages (e.g. biosecurity, water and waste management, manipulation of physiological windows), RAS systems are operated under relatively intensive and constant conditions (e.g. 24hr light, high temperature) compared to conventional ambient open water/flow through production systems and fish also experience markedly different water chemistry. Under RAS, it has been suggested that de-synchrony between photoperiod and temperature may occur (e.g. “winter” short-day at high temperature) leading to inconsistent smoltification success and suboptimal performance at sea. Osmotic ionic balance & efficiency may also be affected by interactions with rearing water chemistry. Given that the single most critical element of smoltification is the development of hypo-osmoregulatory ability to maintain hydro-mineral balance via excretion of ions, then it is imperative to understand how water chemistry variation between systems (RAS vs FT) interacts with the process of smoltification, and subsequent seawater performance (Kolarevic et al., 2014; van Rijn et al., 2020). In addition, freshwater history is likely to impact immune function and disease resistance at later sea water (SW) stages given immune suppression has been reported in smolt transferred to SW (Johansson et al., 2016), but the potential effects of RAS environment remain to be documented. Therefore, a large collaborative project was launched in 2019, ROBUSTMOLT, to test the hypothesis that environmental conditions experienced in RAS during the freshwater phase (e.g. water chemistry and microbiology, nutrition, temperature and photoperiod) may influence early life history traits of salmon that will subsequently impact the microbiomes, immune barriers, ion regulatory capacity, and ultimately robustness at sea. This communication will present results from several studies which compared growth and osmoregulation in parr reared in either FW RAS and open loch cage and performances following SW transfer including feeding, growth and disease resistance tested through an immune challenge.
Material and Methods
Analyses were done from two main studies using fish from a commercial salmon production company in Scotland reared in either a RAS or loch site during the freshwater phase and transferred to seawater either in the company marine production sites (study 1 – smoltification) or the Machrihanish Marine Environmental Research Laboratory (MERL) at the Institute of Aquaculture (study 2 – SW performance). Sampling in study 1 was performed in RAS between -500 degree.days (DD, June 2019) and 400 DD (late July 2019) from the onset of the spring photoperiod and consisted in water and blood samples for mineral analyses, gill samples for Na+ K+ ATPase (NKA including enzyme activity and gene expression by qPCR), and histological analyses, and physiological assessment (weight/length, condition and smolt index) at regular intervals from parr to smolt. Two different smoltification regimes were compared in RAS using two different identical streams, either photoperiod (RAS-P) or diet manipulations (RAS-D). Loch reared smolt were sampled similarly however SW transfer occurred later in 2019. Additional samples were collected at 1 and 4 weeks post SW transfer. In study 2, smolts produced commercially in either freshwater RAS or open loch cages were transferred to MERL between mid-February and early March 2020 (2 weeks window) with an initial weight of 92.2 g and 101.2 g, respectively. Smolts (480 from each origin) were stocked into 12 x 1.5m3 (n = 6; 80/tank) and reared for 5 months. Fish were assessed for seawater adaptation (including ATPase, chloride, survival), growth (weight/length, SGR, FCR), feed intake (daily waste feed collection) and immune parameters following an artificial viral challenge using Polyinosinic:polycytidylic acid (poly I:C) I.P. injection (1.25 mg of poly(I:C) tested against PBS and control, n=3, 6 fish/treatment/tank) at two time points post SW transfer (2 and 6 weeks). Fish were marked with panjet (alcian blue, Sigma-Aldrich, UK) to differentiate between treatments. Sections of head kidney, liver and spleen were dissected and preserved in RNA later for gene expression analysis by qPCR of innate immunity gene markers.
Results and Discussion
Results of study 1 showed similar growth profiles between cohorts (RAS-P/D and loch). However, differences were found in the temporal changes in smolt index and NKA in FW and blood ion balance post SW transfer. Gill NKA remained relatively stable in RAS-P fish while diet clearly promoted smoltification with a sharp rise between -500 and 150 DD and a more classic profile was observed in the loch fish. Post SW transfer, RAS fish appeared to be under osmotic stress within the first 4 weeks. Water mineral levels increased in RAS throughout the FW phase and reached levels much higher than in the loch. Further data on gill (and gut) NKA gene expression will be presented.
Study 2 showed clear differences between the RAS and loch cohorts in SW. Thermal growth coefficient was significantly higher in loch fish compared to RAS fish for the first 8 weeks post transfer. Loch fish feed intake (expressed as g/metabolic body weight/day) was significantly higher from the time to transfer indicating a better capacity of loch fish to cope with SW. Mortality remained low however was higher in RAS fish during the first 4 weeks post SW transfer further supporting the challenge experienced by RAS fish. Complement increased sharply in the RAS immuno challenged fish 2 weeks post SW transfer but not in the loch fish. The PolyI:C challenge worked effectively and elicited a strong immune response in fish. In the 6-week post-SW, a greater complement activity response was observed in the loch fish than RAS-reared salmon following the PolyI:C challenge. Expression of anti-viral genes showed significant differences between RAS and loch fish especially for LGP2, a modulator of cellular anti-viral response, in the head kidney and Mx, LGP2 in the spleen.
Conclusions
This study provides new scientific data on the impact of RAS compared to loch on osmoregulation and smoltification in FW and growth, feeding and immune response following SW transfer. Data supported anectodal reports from the industry and recently published data (van Rijn et al., 2020) with regards to RAS fish performance in the weeks following SW transfer. However, given the multifactorial differences between RAS and loch rearing conditions, further studies are needed to identify factors explaining the apparent reduced coping ability of smolts reared in RAS and develop mitigation strategies. This work was funded by UK Research Council (UKRI), the Scottish Aquaculture Innovation Centre (SAIC) and industry partners.
Bibliography
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