SPAWNING AND MANAGEMENT OF GAMETES, FERTILIZED-EGGS AND EMBRYOS IN SILUROIDEI

The structure of gonads and gametes is highly diversified in siluroid fishes; in some groups the testes are composite with an anterior part formed of spermatogenetic tissues and the posterior part of seminal ve sicles which may or may not store spermatozoa. For catfish species of aquacultural interest, ovaries present the same general morphology as in other teleosts with oviducts. Spermatozoa are biflagellated in the Channel catfish (Ictalurus punctatus) and monoflagellated in the Europ ean catfish (Silurus glanis) and African catfishes (Clarias gariepinus , Heterobranchus longifilis). The egg size shows considerable intergro up differences, from 0.8-1.2 mm in some Pimelodidae and Clariidae up t o 15-20 mm in Ariidae. In numerous cases, the eggs are adhesive and de velop either a surrounding sticky layer (Ictalurus, Chrysichthys, Silu rus) or an adhesive attachment disc (Clarias, Heterobranchus). Fertili zation is in general external but internal fertilization is reported i n some species. The annual sperm production was measured in European c atfish and Channel catfish as 1.7 x 10(11) and 1.8 x 10(10) spermatozo a.kg(-1) body weight, respectively. In females, the fecundity ranges f rom about 50 eggs per spawn in ariids up to more than 100000 eggs.kg(- 1) body weight, e.g. in pimelodids and clariids. A large variety of co ntrolled reproduction systems are found in the siluroids. In some case s (Ictalurus, Chrysichthys, Hoplosternum), spawning occurs naturally i n ponds or tanks provided that adequate spawning substrates are availa ble; fertilized eggs are collected immediately after spawning and plac ed into incubators until hatching. But for most species, although natu ral or seminatural spawning could be achieved in captivity, the tenden cy at the present time is to develop techniques using hormonal-induced ovulation and artificial insemination in order to control the various steps of reproduction and to allow gamete preservation and manipulati on. Oocyte maturation and ovulation or spermiation can be induced in m any species by a large variety of hormones (GnRHs, fish gonadotropins, hCG, various steroids etc). Some information is available on gamete b iology and preservation. Sperm motility is short-lived, not exceeding 70-80 s of forward movement as in most other freshwater teleosts. A pe culiarity of the European catfish sperm is the activation by urine dur ing sampling which could be prevented by direct collection in an immob ilizing solution (NaCl 200 mM, Tris HCl 30 mM, pH 7.0). Spermatozoa cr yopreservation was successfully attempted in several species. Ova gene rally survive only a few hours after ovulation and fertilization must be carried out soon after. Methods for gamete collection, insemination and incubation of eggs are described for the most widely cultured sil uroid species.