David Little reports
Dr David Little, seconded to the Asian Institute of Technology by the Overseas Development Administration UK, reports
RESEARCH, over the past eight years, at the Asian Institute of Technology (AIT), near Bangkok, Thailand, has refined techniques for producing predictable quantities of Nile tilapia seed that are suitable for hormonal sex reversal. The work built on earlier interest and studies in both the USA (Auburn) and the UK (Stirling) and is now producing results in the field with commercial hatcheries using the methods and stimulating the widespread adoption of sex reversed tilapia by farmers.
The Nile tilapia is a huge, and largely unsung, success in Thailand. Government statistics show that after its introduction in 1965 the Thai tilapia, known as the Chitralada strain, now ranks as the most important cultured freshwater fish in the country. The evidence is also on the streets and in the markets, with this low-cost fish present in large amounts and sold at a cheap price. Tilapia appears to be filling an important niche in providing low-cost animal protein to poorer urban and rural people.
Quality is a problem, however, with growers reporting inconsistent performance and seasonal shortages of seed fish.
A recent seminar organised by the Thai Department of Fisheries and Charoen Pokhaphand company indicated the great interest in export of tilapia and a general upgrading in culture techniques.
The sex reversed fish has established itself over the last two years among commercial farmers, particularly in Bangkok’s ‘tilapia belt’ – the provinces surrounding the capital.
Two large hatcheries, both producing 2-5 million fry/month, have developed the market for the fry using AIT’s technical assistance.
A standard hormonal sex reversal technique is used, in which 17 alpha methyl testosterone is incorporated into a good quality, fine feed and fry are raised in hapas (originally simply inverted mosquito nets) over the first few weeks of life. After such treatment, fewer than 1 per cent of the fish are functional females, which effectively controls breeding in the pond as the fish mature. The wholesale price of MT-treated seed is approximately five times the price of untreated fish of the same size (US$0.8/100 fry compared to US$0.16/100 fry).
The large numbers of first-feeding fry required to make this process economic are produced by the frequent harvest of seed from incubating female broodstock after natural spawning in large (120m2) nylon hapas. The undeveloped egg and yolk-sac fry are then incubated under hatchery conditions until the first feeding stage, when hormone treatment begins.
This is different to the way most tilapia seed are currently produced in Thailand, whereby vast numbers of large fry are seined from shal-
low earthen ponds in the border area between Chonburi and Chacoengsao Provinces in Eastern Thailand and sold via fish seed trading networks all over the country. These fish are of variable age and size, and unsuitable for sex reversal. Earthen ponds can be used to produce seed suitable for treatment, if first feeding fry are skimmed very regularly from the edges of ponds, but output/unit area is low and management needs to be intensive to maintain regular output and quality.
There are many reports of producing tilapia seed in more intensive tank systems, and such methods have been compared with hapa-inpond and pond methods at AIT. Greater control over water quality allows the condition of broodfish and intensity of breeding to be maintained for longer periods, but start-up costs are much higher than the other two methods. In areas constrained by a lack of land and water, tank systems may well be viable, though.
Hapas have been used most in the Philippines for spawning and holding tilapia fry. Normal practice is to remove fry after natural spawning and incubation. This limits the productivity of individual female fish and increases early losses, particularly through cannibalism. It has long been appreciated that early ‘robbing’ of eggs from incubating females reduces the interval between spawning, but this practice has only recently been scaled up to commercial proportions at AIT. The low female fecundity of tilapia means that any boost to individual productivity through improved broodfish management can have a great impact on system output. Large numbers of broodfish are, nevertheless, required for mass production and this influences the size of spawning units and their management.
Traditional hapas are small and use considerably more hapa material and harvesting labour than the long and narrow ‘jumbo’ hapas used at AIT. The shape and size facilitates both the concentration of broodfish and removal of seed, as well as reducing costs. The locally made nylon material is cheap and durable and costs around US$1/m2 of hapa area.
A major constraint on the commercialisation of such systems has been poor success during artificial incubation of Oreochromis eggs. Earlier research at the University of Stirling had shown the eggs to be very sensitive to infection, particularly by bacteria common in hatchery systems, and that this was exacerbated by poorly designed incubators. The UV light sterilisation units and inverted, round bottomed (soft drink) bottles developed to solve these problems have since been scaled up at AIT to handle the large numbers of eggs produced under commercial conditions.
Poor incubation can be related to variable and often low egg fertilisation during natural spawning (usually around 60 per cent). The infertile eggs quickly encourage a bacterial broth in any incubator with a poor water exchange and/or poor filtration. Oreochromis eggs are heavy and yolky and suffer mechanical trauma if movement in the incubator is too turbulent, particularly close to hatching. High egg densities in suitably designed incubators are desirable, as the egg to egg contact promotes physical cleaning, rather like pebbles on a beach, and is more akin to the incubation process in the mother’s mouth.
Twenty litre plastic drinking water bottles have been modified into incubation vessels and an improved version is under development. Recirculated water for the incubators is passed through slow sand filtration to maintain water of high bacteriological quality.
After hatching, the fry are held until yolk-sac absorption in shallow, aluminium trays for first feeding. The natural reproductive biology again favours intensification, since fry are crowded during natural oral incubation at this stage of the life cycle. Fry density and flow rates are regulated to maintain gentle movement and the shallow tray design promotes rapid water exchange and optimal water quality.
These techniques produce fry of a proven quality for sex reversal, as well as high seed productivity for a given pond area and number of broodfish. Losses through cannibalism and incomplete harvesting are minimised, as broodfish are handled intensively. In addition to careful management of egg and larval incubation, the sustained high output of fry requires the use of good quality broodfish feeds and the maintenance of good water quality.
important niche in providing low-cost animal protein to poorer urban and people” rural
Below: for seed harvest. Opposite (top): being sold. More than 50 tonnes are handled daily at Bangkok’s Sapan Bla (below): from the mouths of