Production of eels in recirculation systems
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The early development of recirculation technology for freshwater has been closely related to eel farming.
And for many years in Europe recycling technology was only applied for eels.
These fish can tolerate a very harsh environment, and the optimal temperature is around 26 - 27 degree C.
They were then very suited for taking part of the early development, which was often based more on a try and error concept, than on scientific research.
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Eels feeding.
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The eel farming in closed recirculated systems, took its start around 25 years ago, and was primarily
developed in Denmark and Holland. The systems were simple. Reliable mechanical filters were then not
yet developed, and all organic waste from the fish including particulate material, had to be removed
through sedimentation and in the biofilter. The cyclone was then often used to remove particulate material.
A simple device but little efficient.
In the 70'ies, two slightly different biofilter concepts were applied.
The most simple was using a simple trickle filter, where the water was lifted to the top of a tower
made of a plastic media, the bacteria layer would then sit on the top of this media, and absorb the
organic waste and the ammonia. The additional effect of this filter, was the CO2 stripping and oxygenation.
The stripping of CO2 was a side effect little recognised, because at that time, there was little attention
to the importance of CO2.
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System with simple sedimentation and trickling filter.
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The other biofilter type used, was a combination of a submerged filter, and the above described trickling
filter. Typically the two filters, were two cylindrical or square towers/tanks, where the first tower was
a submerged filter, where the water was pumped from the bottom and up, reaching the top, the water then
flew to the trickling filter to driple downwards entering a pump sump at floor level, from where the water
was pumped through an oxygen cone before returning back to the fish tanks. The later type with the submerged
filter, was more dependent on some kind of removal of particulate material, as it would alternatively
easy block up. Both systems were expensive to operate in terms of energy costs, partly due to the large
lifting head required by the filters, and partly due to the head loss in the oxygen cones.
The head loss in the filters themselves was typically 4-5 meters, where modern filters operate
with a head loss of less than a half meter.
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System with cyclone, submerged filter and trickling filter.
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In that kind of system, the water quality was very poor, and had a very high turbidity.
Moreover, the ammonia level was out of control, as the organic load would inhibit the nitrification.
The CO2 level was also relatively high.
Some of these systems are still in operation today, and the same technology is actually still offered today,
although improved with a mechanical filter. These systems are though still mainly for eels, as the water
quality is normally not very good.
At present, the CO2 levels on eel farms in Holland and Denmark, is typically between 60 and 100 mg/ltr.
This is far to high to obtain optimal growth, as the levels should be kept less than 30 mg/l to facilitate
optimal growth. The consequence is, of course, a high variation in growth among the fish, and a relatively
bad fish health situation.
Considering that it is actually possible to create a very favourable situation regarding fish health,
if one is using recirculation technology.
The high Ammonia level, has the consequence that the farms have to be operated with very low pH levels,
some with pH levels below 5. Recall, that the higher the pH, the more nitrogen will be on the highly toxic
NH3 form. Such a low pH, is not optimal for the fish, and has the consequence that the nitrification
process will work even worse due to pH inhibition of the involved bacteria, adding to the ammonia problem.
Due to poor water quality and water treatment technology, the fish are also very sensitive to the
ectoparasites, dactylus and tricudina. Operating with low pH levels, makes life difficult to the fish,
but also to the ectoparacites, of which the latter effect is seen as an advantage to farmers.
It has even been said, that the fish feeds better at the low pH. This might well be right, but only due
to an even poorer feeding caused by the toxic effect of the ammonia, at a higher pH.
The consequence is, that the farmers are battling in the middle of these bad circles.
Simply because the farms have not been well dimensioned and designed from the beginning.
In the future, one can expect, that the increasing awareness on fish welfare will set higher standards
regarding the water chemistry, for eel as well as for other cultured species.
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UNI-Aqua recommended maximum levels of compounds/parameters for eels
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| Ammonia (NH3) |
< 0.04 mg/l (NH3-N) |
| Nitrate (NO3-) |
< 150 mg/l (NO3--N) |
| Carbon dioxide (CO2) |
< 30 mg/l |
| pH |
Between 5.5 and 8.0* |
| Fish density |
300 kg/m3** |
*Today some farms operates with pH levels as low as 4.5.
**However, density does not seem to be very important for eels as long as the water quality is OK.
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A recognised problem in freshwater eel farming, is that the meat develop a muddy taste due to bacteria
(Streptomyces). This is an indirect consequence of the high turbidity in the water in most systems.
One solution would of course be to reduce the turbidity. An alternative solution would be to use a
saltwater system for production of the eels prior to harvest, as this bacteria does not tolerate seawater.
Today the problem is partly solved transferring the fish tanks with clean water or potentially salted water,
where the fish are left for 1-3 weeks prior to harvest.
Much can be achieved in the future, regarding water quality, growth performance and flesh quality
in the eel farming industry. But the time has come for an upgrade of existing plants, and construction of new plants with
the new technology, that will reduce production costs, and improve the quality of the product.
UNI-Aqua, we have staff with solid experience in eel farming, and we can dimension and design a system
for eel farming, and secure an optimal water quality. UNI-aqua has recently been helping Denmark's largest
eel farmer (see pictures below) to upgrade their recirculated facility in order to improve water quality, capacity and profit.
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Lifting large drum filter in place at a 1,000 ton recirculated eel farm.
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Non-corrosive propeller pumps lifting water to the CO2-stripper.
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Bottom view of the abowe pumps. Capacity: 1,600 m3/hr each.
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