Development of a cost-effective system for ovoviviparous production of Artemia nauplii at low-salinity as live food for the larvae of the African catfish (Clarias gariepinus: Burchell, 1822)

[摘要] ENGLISH ABSTRACT: The brine shrimp Artemiais a small crustacean used as live food in fish and shell fish culture. Among other live food types which are commonly used in aquaculture, Artemia is widely known and accepted mainly due to the ease with which it can be used in hatcheries either in the form of nauplii, hatched from cysts, or as decapsulated cysts which are sold as off-the-shelf products. Supply of cysts has been hampered due to discontinuity of Artemia distribution in natural biotopes, meteorological fluctuations, climatological changes and possible overharvesting of resources. This has led to instability and resulted in fluctuations and sometimes reduction ofthe global production. Hence regional cyst supply has been insured in many places through artificial inoculation and man-managed production in salt ponds. Hatcheries in sub-Saharan Africa have so far relied on importation of product of variable quality, thereby adversely affecting fish production efficiency and reducing the competitiveness of aquaculture in this region.This research work contributes to the knowledge on the alternative ways for the supply of Artemia as live food for use in hatcheries. Firstly, literature was reviewed focusing on the general overview of the status of aquaculture globally and within the sub-Saharan region of Africa; the taxonomy, biology, culture and applications of Artemia; on the taxonomy, biology and aquaculture production of the African catfish Clarias gariepinus, and the requirement of live food for their larvae. Among other challenges which aquaculture faces in Africa, a common re-occurring factor identified is the insufficiency or non-availability of fish seed which in turn has been partly associated to the scarcity of live food which is an essential component in the hatchery production of the larvae of many cultured fish species.Secondly, data are presented on a study of low salinity (0 – 32 g L-1) culture of Artemia using four strains from two bisexual [Great Salt Lake (GSL), Vinh Chau (VC)] and two parthenogenetic [Tuz (TUZ), Balikun (BLK)] populations. Laboratory tests were performed, firstly, to measure axenically the survival of instar I and II at low salinity during a 48 h period. The use of the two instars was based on the hypothesis that ambient salinity does not affect the embryo and instar I nauplii of Artemia, and to assess if using instar II nauplii to inoculate culture would affect survival. Similarly, survival of the Artemia at two inoculation ages (i.e. nauplii and pre-adult) was tested xenically over a nine days period at these low salinities. The two inoculation ages were used based on the hypothesis that mortality rate usually reduces as Artemia attains maturity. Results of both tests showed that the low salinity influenced survival with respect to the strain types. With respect to the two instar developmental stages, no difference was observed as a result of the low salinities tested. With regard to the inoculation ages, no differences were also observed among the salinities tested, except atlower (5 g L-1) salinity where the lowest survival was recorded. Furthermore, a test on the effect of low salinity on the reproductive and life traits showed that both salinity and strain had an influence on the parameters measured. Generally, high survival was observed at those low salinities where survival occurred, but the highest was recorded at 20 and 32 g L-1 (control) salinity. Overall the best performance in terms of the reproductive and life traits measured was observed with the Great Salt Lake (GSL) strain. In particular, the GSL strain had the highest total offspring as well as the number of offspring produced as nauplii. We conclude that 20 g L-1 salinity and the GSL strain are the most suitable for use in mass culture.Subsequently, this research work also presents a study assessing the effect of five locally sourced agricultural based materials (Oat pellet, canola pellet, barley pellet, whole barley grain, whole wheat grain) as sole diet on growth and survival, and their suitability for maintaining a reproducing population of GSL Artemia franciscana at low (20 g L-1) salinity. Two separate tests were conducted which involved firstly a nine days small scale (using 500 mL glass bottles) feeding experiment in order to screen and select suitable feed material(s), followed by up-scaling and a mass culture of Artemia from the nauplii stage and through the reproductive stage (43 days), using the selected feed material(s). Both the screening and the up-scaled biomass tests showed that feed type influences the growth and survival of the animals, whereby feeding a stock suspension prepared from barley pellet resulted in better performance than feeding with the other feed materials. The up-scaled biomass test confirmed that the barley pellet diet could be used for the culture of actively reproducing biomass of the GSL strain of Artemia franciscana. During the reproductive period of the biomass daily nauplii harvest was achieved over a period of 22 days, after which the culture was terminated when nauplii production reduced consistently. Despite low levels of nutrients of the different feed materials, the adults fed with the test feed and the nauplii produced ovoviviparously were found to contain appreciable levels of protein, essential and non-essential amino acids as well as saturated, mono-unsaturated and poly-unsaturated fatty acids in their tissues. In order to achieve higher nauplii production, however, feed manipulation, contrary to feeding with sole diets may yield better results.The suitability of the ovoviviparously produced nauplii was tested by feeding them directly to catfish Clarias gariepinus larvae in comparison with oviparously produced nauplii and decapsulated cysts. Higher survival, better protein efficiency ratio and food conversion ratio were observed in catfish larvae fed with the ovoviviparous nauplii. We conclude that the ovoviviparous nauplii could serve as an alternative live food for larval fish.The economic viability of using the ovoviviparously produced nauplii at the current production capacity of the developed pilot system versus the use of the imported Artemia cysts was analyzed. Cost analysis of producing an individual C. gariepinus larva fed with the ovoviviparously produced Artemia nauplii was found to be 2.15 USD while the cost involved to produce the fish larva fed with locally shopped imported decapsulated Artemia cysts was approximately 0.002 USD. The result of the analysis clearly shows that at the current capacity of the developed nauplii production system, the cost of feeding anindividual C. gariepinus larva is far higher than when the imported decapsulated cysts are utilized. However, there is still a lot of room for optimisation of many aspects of our production system, so that the result of the above comparison may be altered in favour of the use of ovoviviparous nauplii. Moreover, the overall benefits of using the ovoviviparous nauplii should not only be anchored on monetary terms but also on the associated benefits such as fish larval growth, survival rate and postlarval quality, constant and predictable quality of the live food offered to the fish larvae, self-reliance of live food production etc.Finally, the overall results of this work are discussed in the framework of its objectives. Some limitations and their implications, which may have interfered with validity of the results, are highlighted. In order to enhance production of the nauplii, it is recommended for the system to be optimized and more studies to be conducted on various aspects such as stocking density of the culture, the extent to which the feed types offered affect the performance of the maternal population in terms of fecundity and type of reproduction, and the nutritional quality of the ovoviviparous nauplii.

[发布机构] Stellenbosch University