When size matters
Many fish farmers in South East Asia are now harvesting their ponds knowing that they will have marketable fish of such uniform size that grading will be unnecessary. Others will haul in their catch with less satisfactory results because while some fish will be good enough for market, others will be far too small. The difference lies not in pond management but in the sex of the fish.
Not all fish species show a size difference between the sexes but, when they do, it is worth exploiting. In the case of the Nile tilapia, where it
is the male that grows larger, fish farmers can get about 30%
more useful
biomass at harvest from raising all male fish. In the case of the Silver Barb, it is the female which not only grows larger but is also more popular
among consumers. It may seem contradictory to what nature intended, but maximum productivity, at least in fish farming, is best achieved by
controlling the sex of the fish.
For the people who raise the fish, this is a simple process. They go to the nearest hatchery and simply buy the monosex fry. In The Philippines, for example, some 80 million monosex male tilapia fry will have been sold in 1999, bringing benefits to fishing people who may not even own their land or water but who lease water or act as share croppers. But, for this technology to be reaching some of the poorest people in the land so easily, much work has previously been invested.
For some years, DFID has been funding the Fish Genetics Research Programme (FGRP) which is managed by The University of Wales, Swansea, working with partner organizations in S.E. Asia. The FGRP has developed breeding strategies to produce broodstock which are the parents of the single sex production fry. It is all a question of controlling the composition, in genetic terms, of the sex chromosomes in the broodstock. In the case of Nile tilapia, YY males - so-called super males - are produced so that when they are crossed with ordinary XX females, all the progeny are XY, in other words male. This means that the production fish are completely normal - something which is likely to reassure the consumer. It is interesting to note that YY males are not viable in mammals but, in fish, they are both viable and fertile.
In The Philippines there is now a network of 45 officially accredited hatcheries where the monosex male tilapia fry may be bought. Production, in 1999, is expected to be 6,000 mt with an estimated added value of £1.23m per annum and this does not take into account the additional value in the hatcheries and other work opportunities arising from the increased production.
Another aspect of the work is to promote the benefits of this technology, especially to those who are poor and therefore wary of adopting new ways. The single sex tilapia fry require no more management than fry of mixed sex and are perfectly suitable for the very low input systems under which they are usually raised.
The work does not stop with this success. Exciting developments are under way which could bring aquaculture more safely into Africa by introducing sterile aquaculture fish species. And, although the Nile tilapia is the species of choice for much of Africa, in the southern part of the continent, the Mozambique tilapia is favoured. Work is now progressing, in conjunction with the South African authorities to develop the genetic technology required to produce monosex male Mozambique tilapia. With marine fisheries in decline, much rests on the successful development of aquaculture and, in particular, on that most marketable of species, the tilapia.
