Description of the sexual cycle related to the environment and set up of the artificial propagation in Pangasius bocourti (sauvage, 1880) and Pangasius hypophthalmus (Sauvage, 1878), reared in floating cages and in ponds in the Mekong Delta

Abstract

First artificial propagation of Pangasius hypophthalmus was reported in Vietnam in 1980, but results did not allow extension for large scale production of fry. Otherwise, for P. bocourti, no reproduction was yet reported in 1994. Then, investigations were carried out on the reproduction of the two species in two rearing conditions, floating cages and ponds. Climate in South Vietnam is characterised by two alternate seasons, dry (December-April) and rainy (May-November). Development of oocyte shows an annual cycle in the two species. In P. bocourti, maturity of females peaks before the rainy season, while in P. hypophthalmus maturity period is a bit delayed, as it occurs at the end of dry season and the beginning of the rainy season. For the two species, development of oocyte do not differ between the two environments, leading to conclusion that rise of water temperature and/or photo period probably induce the oocyte development. In males, mature fish produce milt when stripped. In the two species, mature male are observed at the same time as the females. However, in P. bocourti, some males can produce milt all year round. In the two species, sexual maturity peaks before, or at the early beginning, ofthe water flow in the Mekong River. As no female was observed with ovulated oocyte in captivity, oocyte maturation was induced with hCG treatment. Females of P. bocourti require a long preliminary treatment with 3 to 10 injections at a low dose (530 VI.kg-1 ) . Afterwards oocyte are larger and more sensitive to the resolving treatment, including one or two injections (2020 or 1460-2070 Ulkg-I ) . By contrast, females of P. hypophthalmus do not require a preliminary treatment, as they have naturally large oocyte sensitive to the resolving treatment, including one (2530 UI.kg-1 ) , two (2530-2520 VI.kg-l ) or three injections (490-1000-1500 VI.kg-I ) . Application of such treatments led to ovulation and ova collection by stripping in 59010 and 88% of the treated females, in P. bocourti and P. hypophthalmus respectively. Ova collected are smaller in P. hypophthalmus than in P. bocourti, 1.0 and 1.9 mm diameter respectively. One gram of ova in P. bocourti and P. hypophthalmus contains respectively 251 and 1437 ova.g'. The relative fecundity is lower in P. bocourti than in P. hypophthalmus, respectively 4.7 103 and 48.8 103 ova per kg of body weight. Influence of the rearing conditions and the fish morphology and fattening are discussed. Ovulation was usually induced once a year but preliminary data showed that 30% of females of the two species can be induced twice per year. In males, compared to the natural spermiation per kilogram of body weight, a single injection of hCG (2000 VI.kg-l ) allows to collect 10 and 5 times more milt respectively in P. bocourti and P. hypophthalmus. In P. bocourti, a single injection of LHRHa (30 ug.kg") associated with domperidone (3 mg.kg") induces also a rise of milt collected, but lower than hCG. Generally, volumes of milt collected from P. bocourti (5495 ul.kg") are lower than in P. hypophthalmus (11-2092 ul.kg"), The same figure is observed about the spermatozoa concentration, lower in P. bocourti (2.4 109-36.5 109 spz.ml') than in P. hypophthalmus (25.7 109-63.4 109 spz.ml"). Milt of both species, diluted twice in 155 mM NaCI solution (9 g.l") buffered at pH 7, can be stored during 24 h without change of its fertility. Motility of spermatozoa is brief and stops before one minute in tap water. So better fertilisation is obtained for P. hypophthalmus with a 34 mM NaCI activation solution (2 g.l"), The optimal dilution of milt in the activation solution is 102_104 and 102 times for P. bocourti and P. hypophthalmus respectively