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The Challenges of Producing Large S1 Smolt in Recirculating Aquaculture Systems (RAS)

Salmonids Husbandry Technology & equipment +5 more

Salmon farming companies have been switching their focus to the production of much larger smolt as a strategy for reducing netpen grow-out time and for reducing problems with sealice. Since recirculating aquaculture systems, with their warmer year round operating temperature, are great at promoting growth and producing larger smolt, they are more regularly being built for this purpose. But as with any shift in technology, there are always lessons to be learned and new challenges to conquer, writes David R. Russell.

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Pre-mature smoltification

Smoltification has three primary triggers: a large increase in day length, an increase in temperature, and an increase in nutrient availability from increased feeding. Of these, an increase in day length is the most important, yet the other two inducers of smoltification can result in smoltification or partial smoltification when fish are large.

Partially smoltified and smoltified salmon that remain in freshwater are more susceptible to disease and are difficult to re-smoltify at a later date after reversion to a freshwater state has occurred.
In RAS, the phenomenon of early smoltification and reversion prior to the intended date of seawater transfer is frequently encountered in S1 production.

In flow-through systems, cold winter time temperature has a “braking” action on hormonal changes associated with smoltification. This braking action delays smoltification until temperature, and with it, enzyme activity, feeding, and hormone levels have increased.

Since biofiltration of RAS has minimal temperature requirements for nitrification, operating temperature in RAS is almost always above the temperature threshold at which braking occurs. Thus in RAS smolt production facilities, with temperature not being cold enough to hold back smoltification, salmon are prone to early smoltification from any of the triggers of smoltification, especially when salmon are large.

Total photoperiod control needed for RAS production of S1 Smolt

Complete light control, with the ability to block out natural sunlight and the ability to implement any photoperiod of desire, is an essential part of good design.

Such design features are widely known as necessary for quality SØ production, but it is not widely known that such is also a must-have feature for RAS production of S1 smolt. The natural increase in day length from the winter solstice toward spring is enough to induce smoltification in RAS systems without the presence of cold temperature serving as a brake.

Likewise, the use of artificial light on a natural light cycle will cause the same. With RAS, winter time photoperiod should not be allowed to decrease below 10:14 (L:D), and slow increases in day length should not be used as spring progresses.

Day-length should be increased only when it is time to induce smoltification, and it should be done in the same way one photo-manipulates SØ smolt, with a sudden increase to 18 to 24 hour day length 4-6 weeks prior to the desired transfer date.

Temperature manipulation

RAS systems will also warm quicker in spring as compared to most flow-through situations. The increase in temperature in early spring promotes early smoltification from both the temperature increase itself from and from the increase in feeding rates associated with it.

In many cases the use of RAS has also been installed due to seasonal water shortages that are more frequently encountered in summer and autumn. Such facilities usually have adequate water to operate with a higher flushing rate in early spring, and when the water source is cold as it usually is, increasing the flushing rate with the progression of spring is a strategy to maintain a flat consistent temperature and avoid the smoltification cue that warming can cause.

Late spring, when smoltification is desired, a reduction in water flushing to promote a temperature increase in the RAS can be coupled with an increase in day length to produce a strong smolt signal with greater levels of smolt synchronization within the population.

A reduction in flushing rate also makes it much easier to manipulate RAS water chemistry in such a way so as to enhance pre-seawater adaptive processes that can be achieved with use of technologies such as SuperSmolt®.


When using RAS for smolt production it is important to track smolt development with monitoring tools such as gill Na+K+ATPase much earlier in the year than one ordinarily does in flow-through systems.

Detecting that pre-mature smoltification has occurred is useful for refining production methodology for future years and for having time to initiate counter-measures, such as the use of SuperSmolt®Technology.

For salmon that have smoltified early or that have already reverted, the use of SuperSmolt® is the operators best bet at having a positive outcome with the exception of when conditions, design features, and water chemistry of the freshwater source may allow for the successful use of blended seawater within the RAS.

The ability to succeed year after year in producing high quality Atlantic salmon smolt with recirculating aquaculture systems is as much a factor of having skilled operators as it is in having the right design features.

Enhanced operational control of water quality parameters can often be used to negate shortcomings in design, but such compensation is much more difficult to counter with operator talent when critical components such as light control is missing.

Production of large S1 smolt as compared to SØ smolt also has other challenges in the autumn preceding transfer such as having the right equipment to vaccinate larger smolt, problems of pumping fish without causing significant stress due to their greater swimming ability to escape pump intakes, and the greater incidence of hemorrhagic smolt syndrome that is associated with larger and faster growing salmon. Such problems are addressable, but require advance planning and equipment investment.

This article was taken from the February 2016 Sustainable Aquaculture Digital.

About the author

David R. Russell is a co-inventor of the SuperSmolt® Process and although he promotes the use of it for certain applications, he does so with no financial gain as he has no ownership in the company that created it, nor has he any rights to residual royalties from its use. He provides consulting services to the global aquaculture industry and can be contacted through his website