Aquaculture (farming fish under controlled condition) in Kenya is a new technology striving to satisfy a growing market for food fish and alleviate poverty in rural areas. Currently it is one of the fastest growing sectors of agriculture in the World. Farmed Fish is increasing becoming popular and profitable. The main warm-water fishes grown in Kenya are Nile tilapia (Oreochromis niloticus) and the African Catfish (Clarias gariepinus. These two species are fast becoming the new "cash crops" of the counties. Thus growing public demand for a healthy tasty and affordable protein food is stimulating the "boom"in this industry. The decline in wild fish populations as a result of overharvest and water pollution (in Lake Victoria and other water bodies) has therefore promoted the culture of farmed fresh fish that are grown in contaminant-free waters in static pond, tank and cage systems.
Recirculation aquaculture systems (RAS) is a new and unique way to farm fish in this country. Instead of the traditional method of growing fish outdoors in open ponds and raceways, this system rears fish at high densities, in green houses or in tanks or in raceways in a "controlled" environment. The system filters and cleans the water for recycling back through culture tanks. Occasionally, new water is added to the tanks only to make up for evaporation and for water used to flush out waste materials. Fish grown in RAS need a continuous supply of clean water at a temperature and dissolved oxygen content that is optimum for growth. A filtering (biofilter) system is necessary to purify the water and remove or detoxify harmful waste products and uneaten feed. The fish must be fed a nutritionally complete feed on a daily basis to encourage fast growth and high survival.
RAS have been in existence, in one form or another, since the mid 1950’s. However, only in the past few years has their potential to grow fish on a commercial-scale been realized. New water quality technology, testing and monitoring instrumentation, and computer enhanced system design programs, have been incorporated and have revolutionized our ability to grow fish in tank system. Nevertheless, despite its apparent potential, RAS should be considered a high-risk, experimental form of agriculture at this time but still parallels other forms of agriculture and Livestock production in Kenya. RAS conserve both water and land because they maximize production in a relatively small area of land and use a relatively small volume of water.
The RAS Design
Fish can be grown in tanks of nearly every shape and size. Fish tanks typically are rectangular, circular, or oval in shape. Circular or oval tanks with central drains are somewhat easier to clean and circulate water through than rectangular ones. Rectangular tanks are usually built with or set upon inclined floors to facilitate cleaning and water circulation. The functional parts of Mwea AquaFish RAS include a: (1) Eight growing tanks, (2) one large pond (600m2) for particulate removal, (3) biofilter tank, (4) oxygen injection aeration device and pump and (5) a 3Hp water circulation pump.
Water Supply and Parameters
A good supply of water, adequate in both quantity and quality, is essential to a successful fish farming enterprise, RAS or otherwise. Testing the quantity and quality of the available water supply is one of the first steps for a prospective fish farmer to take to insure an adequate supply of high quality water. Mwea AquaFish, ground water comes from a borehole that has a depth of 85 metres deep, while at rest, with a recovery period of 11m per hour. A submersible pump was placed at 75m delivering 20m3 of water per hour. This water was found to be free of pollutants, the pH averaged 7.71, NTU at 1.7 and conductivity at 629 μs/cm. All these parameters were within acceptable WHO standards for human use. Other sources of water, particularly surface waters are from a small stream and River Nyamindi used in other areas of the farm but not in the RAS. Surface waters may contain fish diseases, parasites, pesticides, and other pollutants that can kill or slow the growth of fish. We have designed the RAS to recycle most of the water and so it consumes considerable less than other types of culture and are especially well adapted to areas with limited water supplies.
The required quantity of water needed to grow fish varies with the species of fish selected, size of the culture system, and investment size. As a general rule, a minimum water volume of 5-25 litres is needed for every 500g of fish reared and minimum water flows. The biological filter (biofilter) is the heart of the RAS. As the name implies, it is a living filter composed of a media (corrugated plastic sheets or beads or sand grains) upon which a film of bacteria grows. The bacteria provide the waste treatment by removing pollutants.
The two primary water pollutants that need to be removed are (1) fish waste (toxic ammonia compounds) excreted into the water and (2) uneaten fish feed particles. The biofilter is the site where beneficial bacteria remove (detoxify) fish excretory products, primarily ammonia. The amount of ammonia excreted into a tank depends on a number of variables including the species, sizes, and densities of fish stocked and environmental conditions (temperature, pH).
Ammonia loading can be roughly estimated from the biomass (weight) of fish in the tanks or it can be based on the weight of feed fed each day. On average about 25 mg (milligrams) of ammonia per day is produced for every 100 grams of fish in the tank. Therefore, in a tank holding 500 pieces of Nile tilapia each weighing 150g (75kg total fish weight), the daily ammonia load produced by all the fish would be 18,750 mg (18.8 g). To remedy excessively high ammonia levels, add freshwater, eliminate feeding or reduce the density of fish in the tank.
Resulting from preliminary trials that we held in June through August 2014, the eight-beehive shape tanks whose volume is 20m3 each holding 500 pieces of Nile tilapia that average 150g. Ammonia loading from these tanks averages 15g. Since the biofilters are not adequate, we replace over 40% every two days, and flushes 60% water from each tank every two weeks. The main challenges are feed and power costs.
There are eight tanks with a total volume of 160m3 (each tank is 20m3) that forms the first phase of the RAS complex. Based on the efficiency of the system, stocking has been done at 50 fish per m3. In one tank therefore stocking is at 1,000 fish expected to grow and be harvested at an average weight of 250g. One tank will hence hold 250kg of fish giving a total of 2MT at each production cycle. With two cycles in a year, projected production stands at 4MT of Tilapia from this unit. This would double to 8MT a year if average individual fish growth moved to 500g. Gross income from 4MT would be Kshs 1.6million or from 8MT it would be Kshs 3.2 million respectively. Preliminary data showed that a 200g fish increased its weight by 160g in 80 days giving an average growth of 2.0g per day.