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PART I
During the last decades many different recirculation systems have been constructed in Denmark. Primarily systems for production of rainbow trout, Oncorhynchus mykiss. First of all the recirculation systems were made in order to meet the strict environmental demands, but they have also shown to be very efficient and labour-saving.
In most cases the construction of the recirculation system have been made without any regard to the veterinarian conditions.
The diseases in the recirculation systems are more or less the same as in the conventional flow through systems. However, prevention and treatment of the diseases is often a lot different.
Diseases in Danish trout farming most often occur when the water-quality parameters are fluctuating.
The most important are:
1. Fluctuation in water flow
2. Fluctuation in water temperature
3. Fluctuation in levels of oxygen
4. Fluctuation in levels of organic material
5. Fluctuation in levels of iron
6. Fluctuation in infection pressure
7. Fluctuation in other soluble substances (pH,CO2,nitrite,ammonia)
Optimum fish health is obtained when the trout farmer adapt:
· Feeding intensity
· Stocking density
· The way you handle the fish
· Hygiene
· Vaccination
· Immunity status
to the previous mentioned fluctuations in water quality.
In conventional flow through systems it is primarily fluctuations in water-quality parameters number 1 to 6 of the above mentioned, which have great significance.
It is completely different in the recirculation systems, where it is parameters 6 to 7 which have great significance.
Obviously, you have to consider different parameters in the recirculation systems, which also means that the adjustments of the above-mentioned operations are different.
Our current knowledge of optimal prevention and treatment in the recirculation systems is very limited.
In the near future more knowledge of the factors, which influence on the health of the fish is required.
The most important factors are:
1) How important is the construction of the recirculation systems considering the prevention of diseases?
Construction related to disinfection possibilities, size of the recirculation units, pond/tank versus raceways, different oxydation systems. It is essential to investigate the significance of various biological filters. How do various types of feed have an effect on the filters?
2) How important is the immunity status of the fish before being put into the recirulation system?
Most systems have been constructed in a way that fish with different immunity status are mixed, i.e. fish from different types of systems and different age groups and sizes. Is the solution all in/all out systems like the ones we know from the pig and poultry industry?
3) The optimum use of “traditional remedies” in the recirculation systems.
In Danish trout farming a series of “traditional remedies” are being used to disinfect the water. The use of these additives is completely different in the recirculation systems compared to conventional flow through systems. We must find out which types of “traditional remedies” are best? What are the correct doses and exposure times?
Conclusion
A lot of recirculation systems have already been built in Denmark, and during the next years many more will be added. Our current knowledge of prevention and treatment of diseases in these systems is limited. It is evident very soon to obtain knowledge of this subject.
Niels Henrik Henriksen - Veterinarian (1994). Veterinarian consultant of the Association of Danish Trout Farmers since 2000.
Contact Details: Association of Danish Trout Farmers, Vejlsovej 51, 8600 Silkeborg, Denmark. E-mail: niels@dambrug.dk
PART II
The diseases encountered in high temperature, intensive recirculated systems for eels are few in numbers.
Virus: All recirculated systems are infected with IPN (Infectious Pancreas Necrosis) which is harmless in eels, and a Herpes virus (Red Head Disease) which normally causes mortalities between 1% and 10%.
Bacteria: Glasseels are sometimes infected with Vibrio sp. on arrival. Untreated the mortality is 100% within a week. Eelfarms that use salt water from the sea, can be infected with Vibrio vulnificus, a vaccine has been developed.
Parasites: The two parasites Trichodina sp. and Pseudodactylogyrus sp. are present in all eelfarms. Pseudodactylogyrus causes considerably production losses in most eelfarms.
The major problem with parasites is caused by the more and more intensive recirculation techniques that are developed. More fish in the tanks, lower pH, higher values for NH3+ and maybe most, the elevated content of gasses in the water.
The focus of research on recirculated systems, has to be moved from minimum water usage to maximum water quality.
Christian Graver is a master in Biology from the University of Copenhagen. He has been working wit eels; Anguilla anguilla for 22 years. First 12 years as biological manager on 3 different recirculated eelfarms (from 20 to 500 tons production) situated in Denmark, Norway and the Netherlands. The last 10 years, he has been working as a biological and technical advisor for the Danish Eelfarmers Association.
Contact Details: Agertoften 16, 6760 Ribe, Denmark. Phone. +45 74 86 71 62. E-mail: cgraver@adslhome.dk
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