While the overall performance of the water recycling fish rearing plant is satisfactory and much has been learned during the first few months of operation, certain deficiencies were noted. The following are suggestions for correcting these deficiencies and for general improvement of operations.
At present there is no way to drain the water from the rearing tanks directly into the sewer, except through small floor drains. It is recommended that these drain lines be enlarged to accommodate the combined flows from several tanks. This would allow the direct disposal of water containing disease treatment drugs or chemicals, disinfectants or other substances out of the recycle system.
Cleaning the clogged biofilters is accomplished by loosening the waste material collected in the filter media by compressed air from blowers and backflushing the wastes over the overflow weir to the drain with clean water supplied from the storage tank. Since the water depth from the media surface to the overflow weir is approximately one metre, it is virtually impossible to remove all wastes before the backflush water supply is exhausted. It is recommended that additional media be placed in the biofilter to decrease the water depth over the media to approximately 25–30 cm. For best results, the present clinoptilolite should be removed from the filter, the new media of somewhat larger particle size placed on the bottom and the clinoptilolite replaced after thorough washing and sieving to remove undersize particles. Efficiency of backflushing should be increased considerably by the decreased water depth.
The recycle system has become contaminated with at least two fish pathogens, Myxobacteria and Aeromonas and should be sterilized before commencing new experiments with disease-free fish. The results of tests conducted with infected fish are likely to be distorted and therefore misleading.
Before the water recycling plant is treated with a sterilant, a decision must be made as to the disposition of the fish already in the system. The first choice would be to remove the fish and not return them to the recycle system after the treatment is completed. If, however, the fish are judged to be of sufficient value for retention then they should be treated with appropriate drugs and chemicals to rid them of pathogens and isolated in disease-free water. They could then be returned to the system after it is sanitized.
Sanitizing is facilitated by the removal of as much organic material as possible from the system. All tanks and troughs should be cleaned, drain troughs brushed to rid them of bacterial slime, and the settling tanks flushed to remove all sludge. The biofilters should be thoroughly backflushed.
The chemical sanitizer, sodium hypochlorite, is added to the recycle system in sufficient quantity to produce an overall concentration of 200 mg/l free chlorine. If the crystalline form of hypochlorite is used it should be dissolved in warm water before adding to the system. The mixture should be circulated by the pumps until complete distribution of the chemical is assured. Chemical analysis of water samples withdrawn from several locations in the system should be made to confirm that the desired concentration has been attained. All pumps should run, all rearing tanks filled, all piping and drains exposed to the sanitizing solution. Fish culture tools such as brushes and dipnets should be placed in the solution and it would be desirable to mop or flood the floor of the rearing hall.
After the solution is thoroughly distributed, the pumps may be turned off and the system allowed to stand for several hours, preferably overnight. The chlorine in the water can be neutralized with sodium thiosulfate (Na2S2O3) or the water drained into the waste canal where organic material will quickly neutralize the chlorine. The recycle system can then be refilled with fresh water and start-up procedures initiated.
Reasonable care should be exercised in handling sodium hypochlorite since it is a strong oxidizing agent and is irritating to skin, eyes and nasal passages. Good ventilation should be provided where the concentrated solutions are prepared and, if necessary, protective masks should be worn.
Systematic determination of several water quality parameters is necessary to insure safe operation and to provide basic information for the interpretation of experimental results. It is recommended that the various parameters be determined as follows:
A thermograph should be utilized to record the temperature of the water in the rearing tanks continuously. If more than one temperature regimen is used, additional thermographs should be provided.
Determination of the levels of dissolved oxygen, pH, ammonia nitrogen (NH3-N) and nitrite nitrogen (NO2-N) should be performed twice daily. One sample should be taken from the rearing tank inflow and another from settling tank inflow. One sample should be taken early in the morning before fish feeding begins and the other taken as late in the afternoon as the work schedule permits. This programme should insure that the levels of the parameters measured will be near minimum and maximum for the day.
Other chemical parameters which should be determined periodically include alkalinity, nitrate nitrogen (NO3-N) total organic nitrogen and phosphate (PO4-P).
Ammonia nitrogen should be determined by the most precise chemical method commonly used. Values should be converted from NH3-N to NH3 since it is the gaseous form of ammonia that is toxic to fish. Appropriate conversion tables, which utilize pH and water temperature, are available in the laboratory.
The controlled environment of the water recycling system enables researchers to carry out a wide variety of experiments. Two parameters, optimum stocking rates and feeding levels, must be closely approximated before meaningful experimental results can be obtained. High priority should be given, therefore, to obtaining this basic information.
Suggested subjects for research projects are as follows:
Development of a complete formulated starting diet for European catfish, (Silurus glanis).
To determine optimum protein, carbohydrates and fat levels in diets for common carp (Cyprinus carpio), catfish and eel.
To determine vitamin and mineral requirements for the above species.
To determine the optimum ratio of plant protein versus animal protein in diets for common carp, catfish and eel.
As practical diets develop, to test new ingredients for possible replacements or supplements for other diet ingredients.
To determine maximum safe levels of un-ionized ammonia and nitrite nitrogen for common carp and other cultivated species.
To determine long-term effects of sub-lethal concentrations of un-ionized ammonia on the growth rate and state of health of common carp.
These are only a few proposals for experiments under the broad headings of nutrition and physiology; research in fish genetics is also an important subject to be considered. As a number of species are reared in the Institute, it would be possible to carry out a variety of experiments as part of projects 2 to 6 proposed above.
Priority of the experiments must be assigned by the research team involved.
Investigation into the feasibility of producing marketable fish in the recycling system should be carried out if not all of the rearing space is required for experiments. The fry of the European catfish were successfully reared in the recycle system despite lack of experience and less than ideal diets. Experiments on rearing of market-size catfish have also been started. This fish has a high market value in Hungary and proceeds from the sale of fry, fingerlings or table fish could help defray the operational costs of the water recycling system. The European eel which also has a high market value is another possible species for production rearing. The nursing of elvers, including starting on formulated feed diets, was successful during 1978 in the water recycling system. Any rearing space not needed for experiments could be utilized for production due to the flexibility of arrangement of tanks and troughs in the rearing hall. It is suggested that up to 25 percent of the total space be devoted to production.
Fish cultural practices in water recycling systems are more demanding than those generally practised in open systems. Quantities of the food fed to the fish should be carefully controlled and efficient tank cleaning methods developed and practised. It is suggested that a feeding schedule be developed for each group of fish, specifying the size and type of food, amount to feed each day and frequency of feeding. Tanks should be cleaned regularly and excessive amounts of uneaten food should be siphoned from the tank. Fish culturists should be trained to observe the fish for disease symptoms. Loss of appetite and lethargy, for example, are often early indications of infection.
It was emphasized in the first report (1976) that it was essential to keep the water recycling system disease-free. Reliable experimental results cannot be expected if the fish are infected. The following procedures are recommended to minimize the chance of contamination:
All stages of fish rearing, i.e., egg incubation, fry and fingerling rearing should be carried out in disease-free water. A part of the old wet laboratory could be used for this purpose and the necessary water supplied from the overflow of the recycling system or a well. If supplemental heating is required, the thermal artesian well water could be utilized. Experiments to be conducted in the water recycling system should be planned far enough in advance so that the desired species, numbers and sizes of disease-free fish are available at the proper time.
Quarantine of fish destined for the recycle system is necessary until it is reasonably certain that they are disease-free. It is suggested that the fish be treated with appropriate chemicals to remove ectoparasites and closely observed to determine if they are harbouring any other parasites or diseases. It would be advisable to sacrifice a sample of fish from each lot for intensive examination for diseases. While the quarantine method provides a reasonable level of confidence, it does not provide any assurance of freedom from disease since some bacterial infections are very difficult to detect except in the acute stages.
Continuous disinfection of the circulating water. The water flowing from the primary settling tanks appears to be clean enough to be sterilized quite easily with either ultraviolet radiation or ozone with little pretreatment. Sterilization at this point in the system would prevent any fish pathogens from entering the biofilters and, consequently, the rearing tanks.
The use of ozone for sterilizing was investigated during the initial planning of the water recycling system but sufficient funds for the equipment were not available. The cost of ultraviolet radiation equipment is now being investigated by FCRI staff. Either method of sterilizing would increase the cost of operating the recycle system considerably.
Periodic total system sterilization with chlorine. This method, while sometimes unavoidable, is the least desirable since it increases costs, increases labour and necessitates starting a new nitrifying bacterial culture. It is also difficult to achieve complete sterilization. Experience with large recycle systems in the USA has shown that a small quantity of contaminated water dead-ended in a pipe or trough and untouched by the chlorine will quickly contaminate the whole system when it is reactivated.
