Optimization of hatchery protocols for Macrobrachium rosenbergii culture in Vietnam

The giant freshwater prawn is commercially one of the most important crustaceans, being widely fished and reared in ponds and rice fields throughout its natural distribution and beyond. Freshwater prawn farming can be conducted by unskilled rural people on small establishments and prawns are consumed domestically by all social classes. It is an activity that can play a role in poverty reduction and the empowerment of women. It has aided in increasing job opportunities for women and increasing their contribution to household income in developing countries. However, a major constraint in the large scale aquaculture of this species is the adequate supply of seed at required quantities and in all locations where the majority of seed used in grow out farming comes from hatcheries. Existing hatcheries are not producing up to their installed capacity due to various constraints. Availability of healthy and high–quality seedlings has always been a major obstacle in the expansion of M. rosenbergii culture. The low yield because of mass mortality of hatchery–reared seed may be caued by several factors such as broodstock management, larviculture zootechniques, microbial control,… Therefore, the current study investigated several factors which affect larval performance in terms of survival and larval quality in order to improve larviculture production and optimize the efficiency of larval rearing. A first study investigated performance of M. rosenbergii broodstock from different sources. The broodstock origin had significant effects on the offspring quality. Larval quality of Chinese and Vietnamese pond–reared breeders was markedly better than that of Vietnamese wild and Hawaiian pond–reared animals in terms of larval development rate, survival and ammonia tolerance. For prawn larval rearing techniques, the best initial larval stocking density was 100–150 l–1 when using a constant–water volume, while this could be increased up to 400 l–1 in case a rearing method with increasing water volume is used. A feeding frequency of 6 times per day by first–stage Artemia nauplii proved better than feeding only twice daily. Wet or dry artificial diets can replace 25% of the Artemia nauplii ration from larval stage V onwards. Subsequently, this could be increased up to 50% from stage IX, without any negative effects on growth, survival and quality of the prawn larvae compared to a live food control. The use of enrichment cultures (ECs) of N–acyl homoserine lactone (AHL)–degrading microbial communities proved a promising tool to control potential pathogens in prawn larviculture. The addition of ECs into the larval rearing water and via enriched Artemia nauplii used for larval feeding protected the larvae from Vibrio harveyi and/or opportunitistic bacteria and had a positive effect on larval survival and larval quality. Feeding larvae of the giant freshwater prawn with β–hydroxybutyrate (PHB)–containing Artemia nauplii significantly increased survival and growth of the larvae. It proved possible to combine delivery of PHB with bio-encapsulation of Artemia nauplii with a lipid emulsion, which is a commonly used technique in the larval rearing of many fish and crustacean species. In conclusion, several factors need to be addressed in order to optimize the larval rearing techniques of M. rosenbergii and to improve larval survival and larval quality. These include amongst others broodstock source, larval rearing zootechniques and microbial control. However, the optimization of the hatchery process should also be based on the specific conditions of the location and therefore it is not possible to draft standard protocols for all locations. The results of the present study may be applied in freshwater prawn seed production, however many remaining challenges still require further research