Mrigal:
Mrigal feeds on detritus with a quite narrow range in food variety. It is a bottom feeder subsisting mainly on decayed vegetation but it can, occasionally, feed on the water column as an omnivore.
Young fish feed mainly on plankton followed by semi-decayed organic matter and the reverse in the larger fish measuring over 560 mm lengths. Considerable amounts of sand and mud are observed in the gut of mrigal, representing up to 35% of the gut contents.
Under aquaculture conditions, catla can reach a weight of 1.8 kg after one year growing, 2.6 kg after 2 years and 4.0 kg after 3 years. Mrigal is reported in most cases to attain its first maturity when 2 years old in ponds (the males can attain their first maturity at the end of the first year).
Relative fecundity of mrigal ranges from 100 000 to 400 000 ova.kg- 1 of female body weight. The spawning season of mrigal corresponds to the Southwest monsoon in India, Bangladesh, and Pakistan and its duration varies in different regions of the Sub-continent from April to September.
Tilapias:
The tilapias belong to the Tribe Tilapiini, an exclusively African group of fish within the family of Cichlidae. Previously regarded as members of a single genus, Tilapia, three main genera are now recognized, based on the last taxonomic revision (1983).
In addition to anatomical characteristics, the criteria for generic distinction are based on the reproductive biology: Oreochromis (maternal mouth-brooders), Sarotherodon (paternal or bi-parental mouth-brooders), and Tilapia (sub-strata spawners).
Almost 100 species of fish are referred to by the common name tilapia but only three species feature significantly in aquaculture: Oreochromis niloticus, O. mossambicus, and O. aureus.
Nearly all the large tilapias most suitable for aquaculture belong to the Oreochromis group and within this group, Nile tilapia Oreochromis niloticus, is the most important species on account of its fast growth rate, adaptability to a wide range of culture conditions and high consumer acceptability.
For these reasons, over the past 40 years, it has been transferred throughout the world to over 100 countries to become the mainstay of tilapia farming in many different culture systems, at all levels of intensification, from subsistence production to highly intensive farming.
All these tilapias species transfers, inside and outside Africa, as well as hybridization, which has been widely used in aquaculture lead to some confusion concerning the origin and the purity of the commercial strains.
It is therefore most important to know the exact genetic make-up of the strains used for aquaculture production. Various biochemical and bio-molecular techniques (allozymes, mitochondrial DNA, and micro-satellites) have been developed for accurate genetic identification.
Many of the problems associated with tilapia farming come from the exceptional mode of reproduction of the Oreochromis species. In addition to being a mouth-brooder fish, it attains its sexual maturity very precociously ( within 6 months and even below 40 g of weight). This diversion of energy from grow with into reproduction becomes a real constraint under culture conditions.
Once mature, tilapia females produce multiple batches of eggs, the oral incubation of each egg batch being followed by only a short period of recovery before she is ready to breed again: each reproductive cycle lasts for about 1 to 1.5 month. On average, O. Niloticus produces from several hundred to about 2.000 eggs per batch and can breed up to 10 times in one year.
For aquaculture purposes, two main problems stem from this reproduction behavior:
- Spawning by Oreochromis tilapias is not synchronized among females, so that a population of brood fish will produce continuously, but at a low rate which is a key problem for the tilapia hatchery producers, combined with the cannibalism of young fry by the older recruits remaining the ponds which is another major cause of declining fry output in brood stock ponds
- Precocious and continuous breeding of tilapias lead rapidly to overcrowding and stunting The main challenge for tilapia brood stock management is to develop conditioning and breeding systems as well as a very frequent removal of swim up fry from breeding systems in order increase the production of fry per unit area of hatchery pond.
Authors:
Lionel Dabbadie and Jerome Lazard
