Natural reproduction of cultured tilapia species occurs in one of two ways: mouth brooders or substrate brooders. The ease with which tilapias spawn and produce offspring makes them a good fish to culture. However, this trait creates problems. Survival of young is high and grow-out ponds can become crowded. Fish become stunted as the supply of natural food organisms in the pond is depleted. Fagbenro (2002) reviewed the several effective methods used to control such undesirable tilapia population and the advantages and disadvantages of these control methods were presented, of which very few have progressed from use in experimental studies or development trials to widespread adoption by farmers (Agbebi and Fagbenro, 2006). Where a thorough assessment of user (farmer and consumer) perspectives are considered, the use of local predatory fish species to control such undesirable tilapia recruitment in ponds is one of the most effective and practical methods.
Density control of tilapia populations by predators is not thoroughly researched in Nigeria as only few indigenous predators have been tested. Unlike clariid catfishes, most predators have some drawbacks (Table 2); hence the combined production of tilapia and clariid catfishes has attracted considerable attention, particularly in Nigeria (Fagbenro, 2000, 2004). The hybrid clariid catfishes, H. longifilis x C. gariepinus and H. bidorsalis x C. gariepinus, and their reciprocal crosses grow faster than their parental species and have high propensity for piscivory, suggesting that they could be used to control tilapia recruitment in ponds. Choosing an efficient predator of a specific size with a recommended optimum predator-tilapia ratio represents a constraint to the success of this technique. Apart from the proper stocking densities and ratios, the effectiveness of combined culture of tilapias with predators is determined by many interrelated factors: adequate good-quality supplementary feed for tilapias; availability of predator fingerlings for stocking; dietary habits of predator; appropriate time of introduction of predator.
Table 2: Predatory fishes used to control tilapia reproduction in Nigeria.
- a prolific breeder with short generation time (5-6 months)
- poor market value due to small adult size
Source: Fagbenro (2000, 2002, 2004)
Even with the use of predators, the main drawback to tilapia culture remains the excessive recruitment in ponds, which result in low yields of harvestable size. Presently, the use of less expensive and appropriate technology in solving the problem of uncontrolled reproduction in tilapias using biological inhibitory agents is being advocated. Plants with antifertility properties may offer solution as they are easy to obtain and can be incorporated into tilapia feeds. Plants that have been tested and proved for their antifertility properties in Nigeria include Quassia amara, Alloe vera, Hibiscus rosa-sinensis, pawpaw (Carica papaya), neem (Azadirachta indica) and morinda (Morinda lucida) (Raji and Bolarinwa, 1997; Udoh and Kehinde, 1999; Uche-Nwachi et al., 2001; Kusemiju et al., 2002; Oderinde et al., 2002; Adebiyi et al., 2002, 2003; Raji et al., 2003; Yinusa et al., 2005; Jegede, 2010; Ellah, 2011). In Nigeria, extracts of pawpaw seeds, neem leaves, had been investigated as fertility control agents in O. niloticus, and T. zillii and their contraceptive efficacies in combating the problem of tilapia overpopulation in ponds have been established (Ekanem and Okoronkwo, 2003; Jegede, 2009).