Full size Salmon RAS in low Salinity level – AQM experience and conclusions

Dr. John F. Taylor | Salmon R&D Specialist

27.04.2023

To date, our experiences regarding performance in low-salinity RAS have consistently yield market size fish of 4.5kg round weight within 20months of start feeding. Low salinity (≤2ppt) RAS rearing of salmon offer many significant positive benefits for growth and welfare compared to brackish (5-15ppt) or high salinity (20-35ppt).

AquaMaof believes freshwater RAS is the most cost-effective choice for salmon grow-out, which also allows for greater site selection options. Brackish or seawater salmonid RAS can create greater technological, biological & operational challenges for salmonids. Adapting RAS to seawater can challenge treatment processes, and there is a need for larger treatment units to account for reduced CO₂ stripping efficiencies; nitrification; and DO-holding capacity in seawater. In addition, brackish water and seawater present bromide/bromine risk when ozonating; and potential for sulfate reduction to hydrogen sulfide. To date we have experienced no H₂S “events” in our facility and levels are negligible. These differences in conjunction with construction material alterations make brackish-seawater RAS more expensive than a freshwater RAS of similar production capacity.

Our experiences over the years have given a clear understanding and knowledge base of the significant importance of managing water chemistry, in particular cation-anion ratios and salinity. By having a sound understanding of local water parameters, contribution of diet to ion loading, and the ability to carefully manipulate ion ratios within the water either through buffers choices or via our denitrification system, in relation to optimal low salinity levels, we have provided a unique rearing environment to minimize osmotic stress and support optimal physiological performance. Furthermore, by provision of optimal water chemistry, we enhance the microbial environment in our biofilters to maximize nitrification efficiency, and further support efficient CO2 removal. Understanding the temporal dynamics of the system microbiome & effects of procedures is crucial to operational success and consistency.

Low salinity RAS has shown minimal problems associated with nephrocalcinosis. Although CO₂ has been a primary focus, our experience and growing evidence in literature & industry, shows that salt addition to hatchery phases increases the risk of developing kidney related problems. By operating at <2ppt, we observe minimal prevalence (<20%) and low severity (Av. Score <1), in contrast to historic rearing at intermediary salinity of 5-7ppt (100% prev.; score 2-3).

Further our 5 years of experience and SOP development have clearly demonstrated that we can successfully manage smoltification and post-smolt physiology in low salinity RAS, without the need for traditional brackish or seawater transfer. We also see further benefit of rearing and purging at low-salinity (2ppt) by minimizing osmotic stress by avoiding transfer into high salinity purging. The greater osmotic “shock” increases metabolic stress and can increase weight loss and greater fillet nutrient depletion during purge than at lower salinity.

Taste differences between freshwater and seawater RAS reared salmon have been widely debated. Published literature and our own experiences show that there is minimal, if any difference between FW and higher salinity reared salmon. There is a misnomer, that fish get “saltier” in the marine environment, and this is simply not true. Evolutionary physiological ion homeostasis is tightly regulated with both juvenile (FW) and adult (marine) showing comparable whole body ion (salt) level. Growing evidence indicate that diet, raw material, amino acid profile, fatty acid ratios and mineral premixes has the greatest influence on umami “taste” properties than the salinity of the rearing environment. Carefully tailoring ofdietary components allows the “taste of the sea” to be achieved. To date AQM R&D works heavily on adapting diet formulation to raise the optimum taste characteristics.

Finally, we cannot shy away from a major challenge of RAS be that fresh- or sea-water facilities, and that is pre-harvest maturation, even when using diploid all-female stocks. To date maturation is 15-20%, and we have identified major risk factors that must be considered in Operation. By lowering juvenile & pre-smolt water temperatures (14 down to 12C), and lower initial PSG water temperatures we can significantly reduce maturation (40%+ to 15%). By not “pushing” too hard at the start, there is a long-term benefit of lower maturation rate, with subsequently higher growth rate & higher % superior harvest product. We have evaluated differing photoperiod regimes to curtail maturation, but with limited success unlike the traditional photoperiod regime success in open-pen aquaculture. This may be attributed to the higher constant temperatures, and lack of seasonal photoperiod as rearing is typically conducted under 24 hour lighting. We are reviewing the application of short daylength rearing and the effects on maturation. This is supported in the scientific community evidence of late for RAS. Correct grading & top-harvest management strategies are also essential to remove “maturing” fish before late-stage maturation accelerates and product quality deteriorates in those individuals.

Based on current genetic availabilities, and lack of successful photoperiod regimes, AquaMaof is a strong advocate of the use of triploid all-female salmon, to completely avoid maturation. However, scepticism within the industry remains on use of triploids due to poor historic experiences. Since early 2020 we have used triploids and view them as a “different species” and adaptation of egg thermal regimes; some environmental parameters; and dietary adjustment are essential in order to maximise their potential and avoid negative attributes. We carefully tailored rearing SOPs to meet their requirements where they differ from traditional diploid protocols. To date we have achieved triploids individuals of 6.85kg within as little as 625days (20.8 months) of first feeding.

Through tight control of operational factors, in association with a sound understanding of water chemistry, adjustments in thermal regimes, in conjunction with tailored nutrition and feeding protocols in each production phase, we have already reduced current operational grow-out to market size by 6 weeks relative to historical growth models. At present we can achieve a consistent harvest weight of 4.5kg with low FCR (1.1-1.2) within 20 months of first feeding, whilst maintaining high survival (<0.5% mortality / month) and welfare standards.

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Disclaimer:

The values below are not an absolute value, and part of it are examples of population sections. Actual results will depending on other factors