Reproductive physiology of female tilapia broodstock

Tilapiine fish have become one of the most commercially important groups of cultured freshwater fish with in excess of 850 000 tonnes produced annually in a range of countries (e.g. Thailand, Taiwan, China, Philippines, Belgium and the USA). However, poor broodstock productivity, owing to low fecundity and asynchronous spawning cycles, remains one of the most significant outstanding constraints upon commercial tilapia production and its future expansion. This paper reviews current understanding of the reproductive physiology of tilapia with particular emphasis upon those factors deemed critical to successful broodstock management. Special emphasis is placed upon factors such as fecundity, egg size, spawning periodicity, ovarian development and the exogenous and endogenous regulatory mechanisms involved in their control. Mouthbrooding species exhibit higher levels of parental care but fecundity is lower and egg size larger than in substrate-spawning species. Fecundity, the number of eggs oviposited per spawning act, can be < 350 in mouthbrooders such as Oreochromis mossambicus (Peters) but can exceed 12 000 in substrate-spawners such as Tilapia zillii (Gervais). Eggs produced by mouthbrooders normally exceed 2 mm in diameter whilst those of substrate spawners average only 1.5 mm. Interspawning intervals of mouthbrooders generally average 30-50 days whether the fish are held in natural or artificial conditions, although substrate-spawning species such as Tilapia zillii can re-spawn after only 6 or 7 days. The dynamics of the asynchronous pattern of ovarian development adopted by tilapiines are complex and result in the ovary being dominated by late-vitellogenic/maturing oocytes as early as 8-10 days after spawning. Advancement of our understanding of these key areas will be essential if the present constraints on tilapia culture are to be overcome.