Introduction
Over the last 40 years, Norway has become a world leader in salmonid production, but despite the economic growth, farmers continue to experience high fish mortalities after post-seawater transfer (1). The transfer success of farmed post-smolt Atlantic salmon to sea is largely dependent on the smolt’s physiological ability to counteract environmental challenges. Early life experiences, in particular, as a result of different rearing conditions, are known to differentially influence physiological responses and plasticity that lasts later in life (2). Over the last decade RAS systems have been adopted as a key technology in the production of juvenile salmonids and it is estimated that RAS fish currently make up the majority of the smolts stocked in Norwegian sea cages. In comparison to the more conventional extensive flow-through (FTS) system, RAS provide a more stable and controlled rearing environment, although it is not fully understood how this affect the physiological plasticity and ability to withstand abrupt and seasonal changes in environmental factors once the fish is transferred to the sea. In this study we compared growth performance, physiological traits, and environmental adaptation in Atlantic salmon (Salmo salar) transferred into sea cages from fresh water intensive RAS (high and constant temperature) to a similar group produced in fresh water under extensive FTS conditions (natural temperate water).
Materials and methods
Circa 300k fish from RAS and FTS systems, respectively, were released in 6 commercial cages (triplicates cages; 25 x 25 m, 40 m depth.) in one of the commercial breeding facilities of Lingalaks AS. Biometry data and biological samples were collected monthly, from before transfer to seawater and until slaughter (12 months later). Water conditions at different depths (temperature, salinity, current, pO2, fluorescence) was collected by SAIV-CTD during the entire seawater production.
Results
There were no significant differences in growth and smolt development between FTS and RAS-produced smolts at the end of the freshwater phase. However, after transfer to SW the RAS fish showed several underlying physiological and molecular differences. Remarkably the final slaughter weight was 3646 ± 8.3 g in RAS compared to 4229 ± 70.5 kg in FTS fish; i.e 600 g less. A distinct lower HSI, and plasma levels of lactate and triglyceride were observed in the RAS smolts, before and after seawater transfer. RAS fish showed also lower NKA activity, and plasma levels of Na+, Ca2+, and P+ compared to FTS fish once transferred in seawater, suggesting hypo-osmoregulatory maladaptation in the first months in seawater. Both FTS and RAS fish showed physiological adjustments during the seawater production which were mainly linked to the seasonal variation in water temperature, but compared to the FTS, RAS growth showed a stronger negative interaction.
Discussion
The physiological difference observed between the RAS and FTS fish once in sea cages was likely linked to pre-history water temperature in freshwater. The two groups were exposed to different environmental conditions in fresh water, likely allowing the FTS fish to develop a more physiologically plastic response to environmental changes than the RAS fish once in seawater.
In conclusion, the current study emphasises the crucial role that pre-history freshwater rearing conditions play in laying the foundation for the fish successful physiological adaptation to seawater, growth performance, and ultimately the success of farming production.
Funding
Supported by Research Council of Norway (grant NoFood2Waste; No. 317770)
References
1 Fiskeridirektoratet. Akvakulturstatistikk (2020)
2 Tang, P.A. et al. Aquaculture 560, 738458 (2022)