by Kanyembo Robert

Aquaculture in history

Aquaculture, which is the growing of aquatic organisms and plants, was first coined in China before BCE 1 000. The Zhou dynasty that ruled China from 1 112 to 221 BCE and the then described as a politician, Fan Li (BCE 500), cultivated carp (Cyprinus carpio) in freshwater for food. The fish (carp) was a symbol of good luck and fortune. However, during the Tang dynasty (BCE 618), the then Emperor Li banned the use of carp in fish farming and as food because his name means ‘carp’. To continue with the new way of having fish regularly available, fish farmers had to look for alternatives. They resorted to using other species in the Cyprinidae family where the most cultured fish species belong to, such as Oreochromis niloticus, Oreochromis andersonii, Oreochromis macrochir, Oreochromis tanganicae and Tilapia rendalli to mention but a few. During this era, the first integrated agriculture-aquaculture systems emerged, as well as the first form of mixed fish farming known as polyculture.

As civilization spread, the art of aquaculture reached Europe, where it was first practiced in Rome. The Romans were lovers of seafood such as oysters and crustaceans. They adopted the Assyrian kind of swimming pools called vivaria, where they farmed fish and crustaceans caught in the lagoons. Their guests could choose from the pools (fish pond) the kind they liked. These vivaria were commercialised by the Monastic Orders and Aristocracy in the Middle Ages as they were the main users of freshwater, due to the fact that they monopolised the land, forest and water courses.

During the Renaissance period, several treaties were agreed and published that provided in-depth detail in technique in pond construction and management, choice of species, their diet and their disease. However, carp still dominated the artificial ponds of Eastern Europe. Artificial breeding was first discovered in Germany, brought about by the diminishing fish population in the natural water bodies.

To run the clock much faster, fish farming was first introduced to Africa by the Anglo-Belgian colonists. They introduced the art of fish farming for leisure fishing, as food and also as a way to control the spread of malaria (Cyprinidae ate malaria mosquito eggs and larvae).

During the 1950s, the fish feed industry revolutionised aquaculture. Today we have several stock feed producers in Zambia, producing a wide range of animal feeds and fish. Novatek Animal Feeds is by far the biggest feed producer in Zambia, providing fish farmers with 12 different types that follow different stages of fish development.

Novatek provides fish farmers with the cheapest, high quality fish feeds on the market, produced in conformity to international standards. That is why Novatek Animal Feeds exports products to the Great Lakes regions, Malawi, Mozambique, Botswana, Angola, the Congo, and new markets that are opening up daily.

The feed industry supported the new technologies that were developed in the 1970s, like cage culture practiced in open systems. Aquaculture has reduced pressure on capture fisheries (natural rivers, lakes and oceans) and has provided employment for many in the value chain. Aquaculture is deemed the fastest growing industry in the world. The largest aquaculture producer in the world is China, and Egypt is Africa’s biggest. Nigeria is the largest catfish producer.

Aerators; the new trend in aquaculture

The new technology – cage culture – developed in the 1970s, was a milestone in fish farming. Fish cages had the capability to hold a lot more fish per cubic metre than a conventional fish pond. They had a much higher stocking density and fish grew faster than in ponds due to the fact that they had enough oxygen as they were afloat with constant circulation of freshwater. Fish cages are mostly found on rivers, lakes and oceans, which are basically open systems. In an open aquaculture system, the rate of water exchange is extremely high and likewise is the rate of oxygen. Oxygen availability in open systems can be as high as eight mg/ℓ. This, coupled with water temperatures of about 28˚C, can promote faster growth of fish.

Owing to the fact that open systems are mostly shared water bodies, they are governed by bilateral and domestic laws. To set up a cage on a lake, one needs an environmental impact assessment issued by environmental management agencies. To get one costs money and time. With the demand for fish and fish products on the increase, necessitated by the increase in human population, aerators were introduced in closed systems to help improve yields per square metre. Closed systems include fish ponds, reservoirs, fish tanks, aquariums, dams, etc.

One might ask, what are aerators? An aerator is a device that helps with the introduction of oxygen into an aqua-ecosystem or a body of water. In a closed aqua-system like a fish pond, oxygen demand might surpass the supply, depending on the number of fish per square metre, the size of the fish, temperature and time of the day. Oxygen demand is greatest during the night, owing to the fact that natural oxygen production seizes as photosynthesis seizes, while all aquatic biota consumes oxygen. Aerators therefore help to cushion this deficiency.

Importance of aerators

The main purpose of aerators in aquaculture systems is to infuse oxygen into the water. The movement of water in an aqua system helps oxygen to diffuse into the water. There is a restriction on the flow of water in a closed system. This affects the rate of oxygen diffusion into the water as well. With an aerator, it helps with the circulation of water to aid the introduction of oxygen into the system. Aerators have been incorporated in a new aquaculture development trending as Reticulation Aquaculture Systems (RAS). RAS are specialised aquaculture systems that are often utilised in fish tanks and aquariums. It is a new commercial trend of fish farming in tanks, enclosed in tunnels. It uses less water and land compared to fish ponds. In a RAS, the same water is used. Bottom water is sucked into the mechanical filter where coarser materials such as fish faeces and left-over fish feed are physically removed. The water then moves into a second filter called a bio-filter in which nitrogenous wastes are digested by bacteria, and then moves back into the main tank where it is oxygenated by aerators.

Aeration through aerators encourages the growth of aquatic microbial organisms. Aerobic bacteria consume ammonia, phosphates, nitrates and other nitrogenous compounds that help in maintaining a stable water chemistry. A stable water chemistry limits the growth of algae blooms that benefits from consuming the elements mentioned. Aerobic bacteria convert ammonia and nitrites into nitrogen gas and nitrates that are less harmful forms of nitrogenous breakdown.

Adequate aeration helps in raising the stocking density per square or cubic metre in an aquaculture system. The conventional stocking rate in a fish pond is three to five fish per square metre and five to eight fish per square metre with frequent water exchange. With an aerator, you can stock between eight to twelve fish per square metre, or even higher. The same principle applies in increasing stocking density in a cage system.

Another great importance of aerators is that they help break up thermal stratification in an aquaculture system. Temperature can cause a water body to form heat layers depending on weather, season and latitude. In a fish pond or pump-sump dam, temperature forms three basic thermal strata, namely epilimnion (upper), metalimnion (middle) and hypolimnion (bottom). A thermocline, which is the metalimnion, is a thin but distinct layer between the epilimnion and hypolimnion in which the temperature changes rapidly with increase in depth. Depending on the season and depth, an aerator will help break the stratification formed by circulating the water in the system. For example, if the period in state is summer, the upper layer (epilimnion) is warmer and this reduces the oxygen affinity to the water.

In other words, when water is warmer, the rate of oxygen dissolving in water by wind, waves and turbulence reduces because oxygen dissolving in water is directly proportional to temperature. An aerator would then mix the warmer upper water with the cooler bottom which then lowers the temperature in the entire system. The reduction in water temperature will increase the oxygen affinity to water. The availability of oxygen in the aquatic system promotes rapid fish growth and the general health of the system through proper water chemistry or water quality.

Conclusion

Aquaculture has become a way of life for many people, providing employment and a cheap source of protein. It has evolved a great deal to now being run as a business. With a business mind, time factors are important to meeting the deadlines. Providing supporting services in order to quicken the growth periods of fish and also prevent massive mortalities through oxygen deficiencies, aerators have been incorporated in fish farming enterprises. Aeration processes play a crucial part in a fish farmer’s setup, as it helps increase the stocking density of a growing system and improves the water chemistry of a production unit by encouraging aerobic bacteria to convert nitrogenous wastes such as ammonia and nitrites to less harmful forms such as nitrogen gas and nitrates. In the most recent developments in aquaculture, RAS have come to entirely rely on aeration systems. It is therefore important to have an aeration system as part of the aquaculture practice because it brings with it fewer risks of unexpected fish mortality due to reduced oxygen in the system. It also improves fish growth.

You can contact Wiehan Visagie at wiehanv@novatek.co.zm or Marné Meyer at nutritionist@novatek.co.zm for any information on feeding and feeds.