Turbot, sole and Japanese flounder in recirculation
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When it comes to flatfish culture, the use of land based recirculation technology is by far the best option.
Already known is that only land based farming works well with flatfish culture.
The main problem with flat fish culture, is the tank area requirements. So in any case the investments for
a production system for flatfishes will always be relative high per ton of fish produced.
Compared to round fish, the cost of the water treatment system for a flat fish production,
is relative small compared to the investment in total - simply due to the investment in the big
tank bottom area and related infra structure.
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A section of a recirculated turbot facility delivered by UNI-Aqua in China.
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Turbot during grow out, China.
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By using recirculation, we can reduce the required tank area dramatically (see below),
as the production per m2 will be much higher due to the improved growth rate, and uniform growth over
the year.
This means, that the savings on tanks, infrastructure and building area, will usually more than cover the
cost for the water treatment system.
Moreover, energy is saved, because lifting head in the water treatment system, on average, typically
will be much lower than the lift in a flow through system.
Another clear advantage and potential saving, is that the facility does not have to be located right
next to the sea, as the water amounts of water will be much less.
A 500 ton system, will have an internal flow of approx. 12.000 m3 per hour, but the actual water exchange
of new inlet water will only be 60 m3/hr. This means it is not a big deal to put a piping, say 2000 meter,
to the sea.
The faster fish growth in a recirculation system, means that less capital is bound in in standing stock
and tank facilities.
There are two elements which makes the fish grow much faster in a closed recirculated system:
- Constant optimal temperature, and optimal water chemistry. Principally in respect to Oxygen,
Ammonia and CO2/bicarbonate
- Avoidance of maturation
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Turbot in 18oC recirculation, and in simulated natural temperature regimes/flow through.
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Growth curve in recirculated water with a constant 18 degree,
and optimal levels of
oxygen, ammonia levels permanently below 0,02 mg NH3/l.
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UNI-Aqua recommended maximum 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) |
< 15-20 mg/l |
| Oxygen (O2) |
Between 60 and 120% saturation |
| Hydrogen Sulfide (H2S) |
< 0.001 mg/l |
| Chlorine residuals |
< 0.001 mg/l |
| pH |
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.
***Optimal in recirculated systems, though lower than in natural seawater.
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Maturation
The problem with maturation is partly that the growth will be reduced for a period of time, further
the overall feed conversion will be poor, as energy is lost during maturation, and finally it will cause
fluctuations in meat quality.
In a recirculation system, we can make the fish grow to a larger size before entering maturation, due to
the combination of rapid growth, and photo control.
The maturation (of turbot) is first of all triggered by a reduction in light which will trigger a hormone
production. Also the temperature has some influence, but if temperature is maintained at a constant level, then it is possible to control the timing of maturation by light alone. If we were focussing on a broodstock for turbot, we would maintain a temperature around 13 degree, but in the growout system, our focus is to let the fish believe they are in the summer period all the time, so therefore we maintain a temperature of 18 degree, and long sun hours.
Eventually some of the fish would mature anyway. But all we need is to extend the maturation period a
number of months, that combined with the improved growth rate, will make the majority of the fish
reach market size before maturation.
Fish quality
Another element in using recirculation technology is the quality of the meat.
The quality of the fish meat is best when the growth conditions for the fish are optimal. And optimal
conditions is what can be secured by using recirculation technology.
Furthermore, flatfish species are passive fish, and will have similar activity pattern, whether in
captivity or not. The fish feed used nowadays is based fishmeal of marine origin - the same as the fish is eating in
the wild. This means that, fish of a quality which is superior to even wild fish,
can be produced in a recirculation system.
In addition, to obtaining optimal fish quality, it is very important that the fish is handled the right way,
also post harvest. On a land based fish farm, the logistical management will secure that the fish is
handled the correct way, and correct temperature regimes are maintained.
The UNI-Aqua technology for turbot culture
UNI-Aqua has its early roots in turbot culture. The development of the technology which UNI-Aqua are using today,
is a further development of a technology that was originally developed for production of turbot.
The development of the technology we use today, took its start back in 1992.
At that time Bent Urup, now director in UNI-Aqua, was managing the most successful turbot hatchery of that time,
Maximus A/S in Denmark. It was not only then the biggest, but also the hatchery with the best quality of fry.
Back then, producing approx. 40 % of the total turbot juveniles produced world wide.
In to days terms maybee a modest little more than 700.000 juveniles per year.
This hatchery, was using a first feeding technology developed by Bent Urup, where the larvae were fed with
cultured copepods.
Importantly, the juveniles were free from deformities and mal pigmentation, and they had a very
uniform growth.
Read Fish Farming International's article about the hatchery (pdf).
In 1992 Maximus was short of space due to the increasing production and logistical problems related to sales.
The company then hired a freshwater recirculation facility, located at a beach. This facility was turned into
seawater recirculation, based on the knowledge then available.
The fish were surviving well, but the growth was modest compared to the known growth curve for these fish.
This in spite that all known parameters were OK.
Other systems, as well, were build in Scandinavia during the 90'ies where freshwater technology was used in
seawater aquaculture, and they all failed the expectations.
By monitoring the flow of carbon, ammonia and phosphorous in the system (operated by Dr. Bent Urup), it was
found that it was the build up of bicarbonate, and fluctuations in ammonia levels that caused the problems.
Bent Urup left Maximus after yet another successful production season in 1993.
After then he dedicated his research into finding a way to design a recirculation system for seawater which would
secure optimal growth of the fish.
The first larger pilot system based on the new concept was build in Chile in 1997. The system worked very
successful, with growth performance above flow through systems, at same temperature.
The system was simply build, not meant to last, as being considered as a pilot system. The system was though
still kept in operation until 2003.
The person locally in charge of that project, Patricio Urrutia, is now in charge as production manager, of a
new facility in Spain for production of Sole and Turbot. When fully completed, this facility will produce
8,000-10,000 ton of turbot and sole per year. This facility will be using UNI-Aqua technology, from hatchery to growout.
The latest version of the UNI-Aqua technology operates with three layers of tanks in the same building,
with full operation head between each level of tanks. All the tanks in one production unit,
will be on the same water treatment system.
The head between the levels are used optimal for the water treatment system. Hence the additional energy
requirements for operating the system in three layers is minimal.
The savings are on buildings, and the required area of land is obviously reduced.
Obviously the system is designed with semi-automatic feeding, fish transport and grading.
Many older systems operates with very low productivity, with less than 10 ton production/year per staff.
Where the production is more than 50 ton/year of flatfish per staff with the present technology.
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Example of a 500 ton production unit, in top-view, and from the side.
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