Production of salmon postsmolt in recirculation systems
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Postsmolt
Until recently, it was first of all salmon up to smoltification, that were produced land based, and in freshwater.
The trend is to grow the fish beyond smoltification land based, and then transfer the fish to the cages
around 3-500 gram. This is a consequence of the lower salmon prices and higher pressure on the producers to
reduce production costs or to find ways to gain higher prices for their products.
Why then, is land based farming of salmon beyond smoltification an option?
- Fish that are transferred to the cages at 300-500 gram, have proven to perform extremely well,
when they are transferred to the sea in respect to growth and decease problems.
- The reduced production time, bigger harvest size and reduced losses improves the overall economy.
- That the fish when released to the cage at a size of 3-500 gram, can all be of uniform size obviously
also has a positive impact.
- Growing the fish on land, it is a choice between flow through or recirculation are the options.
The energy costs operating with flow through are higher, where as the capital costs are the same or higher
with recirculation. An important factor is though the decease control which comes with the use of a properly
designed recirculation system.
- Only recirculation technology gives temperature control, and the logistical tool that involves.
Heat pumps are not an option in that respect, as not only are the energy costs higher,
but so are the investments, and that without any kind of decease control.
When working with post smolt, we are talking seawater, therefore, the ammonia levels have to be under strict control,
and the CO2 stripping must be very efficient. Here we claim that the UNI-Aqua multi-step filter and the
CO2 stripper, both developed for seawater, are the best options on the market - the result of a development
process initiated by Dr. Bent Urup in 1992. Technologies that have proved the ability to secure a steady water quality
optimal for fish growth and health in seawater. Apart from aspects regarding ammonia and CO2,
it is further very important to secure that the systems are designed so that any sulphide levels will never reach the fish -
a serious risk when dealing with seawater. Further that nitrogen super saturation will not take place.
Recommended max levels for post smolt production in seawater:
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Recommended Max levels of compounds/parameters
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| Ammonia (NH3) |
< 0.02 mg/l (0.04 mg/l)* |
| Nitrate (NO3- |
< 70 mg/l (NO3--N) |
| Nitrite (NO2-) |
< 1 mg/l (NO2--N)** |
| Carbon dioxide (CO2) |
< 20 mg/l |
| Oxygen (O2) |
Between 65 and 120% saturation |
| Hydrogen Sulfide (H2S) |
< 0.001 mg/l |
| Chlorine residuals |
< 0.001 mg/l |
| pH (fresh water) |
Between 7.1 and 7.8*** |
*Peak levels can be accepted up to twice as high levels, if levels are very stable.
**Not too critical in seawater. During start up of biofilter, levels of more than 8 mg/l can be found, without risk.
***Optimal in recyc systems, though lower than in natural waters.
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Production models
The growth potential post smolt is shown here for different temperatures.
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An example of a production model with 3 annual intakes of 70g smolt is
shown here, where the fish grows up to 500g in a recirculation system at
14°C before it e.g. is moved to cages for further growth. It is estimating
a non-sales period in December-February, which is likely in the northern
hemisphere. For southern hemisphere this is different and more flexible
production strategies are possible.
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The impact of temperature regimes on further growth either in cages following
a lower actual temperature level, or in land based recirculation units with
a higher constant temperature gives a high potential in accelerating growth
dramatically. This is illustrated in the following graph.
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The introduction of recirculation technology thus gives a high productivity
potential and a high degree of safety at a highly cost competitive basis.
In close collaboration with the client, UNI-Aqua can provide the necessary
basis for decision making regarding investment into recirculation technology.
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