WP/81/SPH/CP-16
by
Charles M. Larsen1
1. INTRODUCTION
In the early sixties, it was generally recognized that extensive hatchery operations can cause pollution problems. The U.S. Fish and Wildlife Service has evaluated several methods of removing pollutants from the discharges of hatcheries. Simple setting of waste appears to be a very satisfactory method of removing most of the pollutants from the discharges.
2. MEASUREMENT AND SAMPLING
Accumulation of waste deposits downstream from hatcheries is considered the major problem however, the water source entering the system should be sampled for pollutants which will disrupt or inhibit production in the facility itself.
All samples should be analyzed for total suspended solids, biochemical oxygen demand (BOD), settleable suspended solids, chemical oxygen demand (COD), total nitrogen, pH, ammonia and salinity.
3. CONSIDERATIONS
Oxygen is easily absorbed into water and if elevation is present, waterfalls are the economical way to aerate water. But what type of waterfall? Is 0.5 m drop better than 1 m drop? Is a heavy spill 10 cm deep better than a shallow spill 1 cm deep? Do splash boards increase or decrease oxygen intake into the water? When, where, how does oxygen enter the water? In short, how should a waterfall be designed to give maximum efficiency in aeration? The answer to this would increase the production of most hatcheries by at least 25 percent.
If elevation is not available and waterfalls are not possible, what is the best means of aeration? Is it cheaper to blow air into water or pump the water into the air?
How can oxygen be injected into water to obtain 100 percent absorption? If 100 percent of liquid oxygen could be obtained, this would probably be the most economical production increase that could be made.
The second environmental factor affecting production is usually ammonia. How can we get rid of it? When is it economical to aerate ammonia out of water? At high pH, at high temperatures, and in soft water, ammonia goes to the gaseous stage where it can be aerated off. How can this be done economically?
If two fish farms are located on a stream, one below the other, what can be done to improve the water quality to the second farm? If plants are allowed to grow in this ditch, the plant consumes oxygen at night and can cause severe oxygen depletion in the hours before sunrise. If the plant is removed, there is nothing to remove the ammonia and the ammonia travels to the second farm. Would another type of vegetation that takes its nutrients from the water but its oxygen from the air solve the problem?
Most of the good water sources have been developed. Additional production is now coming from reusing this water. What can be done to improve the quality of this reuse water? Concrete ditches that move water from one point to the next with minimal plant and silt problems may not be the answer. The engineering of the stream must be coordinated with the biology of the stream to obtain those characteristics of water quality needed by the second farm.
1 Fisheries Adviser. United States Agency for International Development (USAID), Jakarta, Indonesia
What is the ideal velocity in a raceway, and what is the ideal pattern of movement for that velocity? Velocity is used to carry away the waste particles and move freshwater to the farm. How can a raceway be designed to give the maximum benefit from the velocity and provide for the removal of the unwanted with minimum effect on the fish?
Density — if all the answers were known concerning density one would find that twice as much concrete was poured in building most farms than was needed. Density has two dimensions, weight of shrimp per cubic measure of space and weight of shrimp per flow of water. Social as well as physical factors control stocking densities. Why can fish be raised in jars where there are more fish than water and good conversion obtained, but when they are grown in raceways only 2 or 3 pounds can be grown per cubic feet (30–50 kg/m3) of space?
4. SHRIMP/PRAWN HATCHERY EFFLUENT CONTROL PROGRAMMES
Regulation of shrimp/prawn hatchery discharges should be considered. This, however, would require a properly prepared guidance document. Effluent guidelines will probably not be forthcoming in the near future for hatcheries, and treatment beyond the simple settling of cleaning flows will not be required for economic reasons.
If guidelines are promulgated, it is likely that there will be some effort from the fishery management agencies to be responsible in carrying out any implementation plan.
For economic reasons, it is fortunate that the literature shows hatchery treatment beyond simple settling of cleaning water flows should not be required.
5. POLLUTION FROM PESTICIDES
Pesticide pollution has been one of the major environmental threats to the shrimp/prawn hatcheries industry in Asia. Ricefield fish culture has been particularly affected by this problem. Also fish farms situated near ricefields have been affected by drainage from them, or through the use of common water supplies. It is noted that at least some of the countries had prohibited the use of persistent pesticides with a view to protecting fish stocks. Efforts underway in countries such as Indonesia to study the effects of pesticide pollution on fish, including their accumulation in fish tissues, is noted and the need for similar studies in other countries is emphasized.
6. PATHOLOGY AND DISEASES
It is just as important for pathologists to know the fundamental requirements and biological tolerances of a fish species as it is for a design engineer to know these factors for design and construction. When these requirements and tolerances are known, a stress-free environment can be created that will greatly reduce the incidences of diseases and the future large expenditures for diagnosis and treatment.
Many new hatcheries have been stocked with disease-free fish with little or no history of previous diseases. This appears to be a progressive move until one considers that there are few, if any, continuing disease-free environments. In many cases, a year or two of production have been lost because these “disease-free” fish with no resistance have encountered an introduced or indigenous pathogen. Usually the mortality is complete and very costly, and the first few years production are lost.
Would not it be better to exploit disease-resistant strains that have had a cycle or two in isolation and test environments to eliminate disease carriers? If disease does strike the fish, the loses are usually minimal and can be controlled. To complement this approach, there should be a massive effort to clear legally the effective drugs that are now available and reduce the dependence on new drugs that might be effective.
Disease control requires a properly designed, low-stress environment that should be stocked with a good strain of disease-resistant fish. This is an added bonus to start-up and operational costs.
7. CONCLUSION
The supply and quality of water in the future hatchery should be given more consideration. The water flow performs two major functions for the well-being of the shrimp. First, it transports oxygen, which is consumed by the shrimp to convert food into muscle and energy; and, secondly, it carries away the metabolites that, at a certain level, will stress or kill them. Therefore, future hatcheries should be designed around the kilograms of oxygen, which are naturally available or can be made available through aeration, and the pH, salinity, and temperature of the water, which in turn dictate the ultimate limiting factors — the metabolites. The toxic metabolites are usually in the form of unionized ammonia, and perhaps other substances that we are not aware of at this time.
It would seem that the above statements are not very profound and are known by everyone of the fishery biology of fish culture profession. Yet, almost without exception, when hatcheries are designed, preconceived formulas based on previous construction are implemented to design the hatchery with a set number of kilograms of shrimps per cubic meter or kilograms per liter flow. Little, if any, consideration is given to the water quality in terms of temperature, pH, salinity and available oxygen.
Almost invariably, when more shrimp production is desired at an installation, the first, and sometimes the only, consideration is more space. Rarely are water flow and quality assessed to estimate if there is enough oxygen available and if the quality of water flow warrants an increase in production and hatchery size. Usually, an increase in flow and exchange rates without an increase in structural size will accomplish the same purposes.
