147 ANALYSIS OF PRECOCIOUS MORTALITY OF PANGAS1US HYPOPHTHALMUS LARVAE (SILURIFORMES, PANGASIIDAE) DURING THE LARVAL REARING AND PROPOSITION OF APPROPRIATE TREATMENTS· Jojo Subagja (1), Jacques Slembrouck (2), Le Thanh Hung (3,4) and Marc Legendre (2) (1) RIFF, Jalan Raya 2. Sukamandi Subang 41256. Java Barat, Indonesia (2) IRD (ex ORSTOM). Catfish Asia Project. Instalasi Penelitian Perikanan Air Tawar, Jalan. Ragunan- Pasar Minggu, P.D. Box 7220/jkspm. Jakarta 12540. Indonesia and GAMET, B.P. 5095. 34033 Montpellier Cedex I. France (3) Aquaculture Research Unit. CIRAD-EMVT, GAMET, BP 5095. 34033 Montpellier Cedex I. France (4) National University ofHo Chi Minh City. Faculty ofFisheries. Thu Due, Ho Chi Minh City. Vietnam Abstract Both in Indonesia and Vietnam, larval rearing of Pangasius hypophthalmus remains problematic due to variable and generally low survival rates obtained. The first week represents the most critical period for these larvae and up to now the cannibalistic behaviour was considered as the main cause of mortality. In the present study, two experiments were carried out in order to better understand the evolution and causes of mortality of P. hypophthalmus larvae from hatching up to 8 days of age and find out measures to improve survival rates. The first experiment was designed to evaluate the importance of cannibalism and differences in mortality when larvae from two different females were reared either in groups of 30 individuals or in isolated condition (30 larvae reared separately). In both cases, the culture was carried out either with or without antibiotic. The aim of the second experiment was to test Oxytetracycline and different disinfectants (Chloramine-T, formalin and "formalin + Malachite Green Oxalate") at different dosages to prevent bacterial outbreaks in the culture. The results indicated that the survival rates of P. hypophthalmus larvae was dependant on the initial quality of larvae or eggs and that larval mortality was more a consequence of pathogenic infection than a direct effect of cannibalism. The present study demonstrated that the survival rates of larvae were systematically improved when rearing was carried out in water containing antibiotic (Oxytetracycline at a dose of 5 to 20 mg.L"), Survival rates and final mean body weights of larvae as high as those obtained using antibiotic were also reached with applications of disinfectants such as Chloramine-T and formalin. The use of these disinfectants is recommended for an application in commercial P. hypophthalmus hatcheries. INTRODUCTION Fish breeding in Indonesia has rapidly developed and is characterised by the emergence of numerous small-scaled hatcheries. Among cultured fish species, Pangasius hypophthalmus (Sauvage, 1878), synonimised with Pangasius sutchi since a recent systematic revision of Pangasiidae (Roberts & Widthayanon, 1991) and originating from the Mekong River, was introduced from Thailand to Indonesia in 1972 (Hardjamulia et al.; 1981). The species has been well adapted to local conditions and is appreciated by consumers. Induced spawning of P. hypophthalmus was initially reported in Thailand in 1976 (Charoen Panil, 1977) and in Indonesia in 1981 (Hardjamulia et al., 1981). However, larval rearing I This paper has been accepted for publication in the journal "Aquatic Living Resources".