Other larval feeds

6.3.1 Non-Artemia live feeds

In section 6.1 of this chapter we have already mentioned that rotifers (Brachionusplicatilis) are not suitable as a sole feed for M. rosenbergii larvae. Nevertheless, in an attempt to reduce reliance on Artemia for prawn larval rearing, Thomaz et al. (2004) replaced Artemia nauplii (5 ml/day) with frozen enriched rotifers at different proportions. While total replacement of Artemia led to total larval mortality, thus confirming earlier reports, the authors showed it was possible to replace up to 60% of the Artemia ration with rotifers without significant detrimental effects.

Moina is occasionally used as a non-Artemia live feed in the larviculture of freshwater prawns. Whereas Artemia is an Anostracan, Moina belongs to the sub-order Cladocera, which are small crustaceans that almost exclusively live in fresh water. A thick carapace encloses the whole trunk, except the head, and there is also a saddle ridge on the posterior part of the body; both of which may interfere with the digestibility by predators. Moina thrives in freshwater ponds and reservoirs but primarily inhabits temporary ponds or ditches. The period to reach reproductive maturity takes 4 to 5 days at 26°C. Females are bigger than males (1.0-1.5 versus 0.6-0.9 mm) and carry only two eggs enclosed in an ephippium which is part of the dorsal exoskeleton. Moina can also be mass cultured using various waste products as a food source (Punia 1988; Shim 1988), and produced biomass has been successfully used in the larviculture of several fish species and the giant freshwater prawn. The nutritional composition of Moina depends on the diet administered, but is generally high in proteins and low in n-3 HUFA. Enrichment can be carried out using the direct method, i.e. by culturing them on baker's yeast in combination with emulsified marine oils. Experiments have shown that Moina takes up n-3 HUFA in the same way, although slower, than Artemia nauplii, reaching a maximum concentration of 40% after a 24 hour feeding period (Delbare & Dhert 1996).

Generally, most freshwater prawn hatcheries prefer Artemia over Moina as a live food source because of its higher nutritional value and digestibility, its better availability, and because it thrives well in brackishwater conditions. Moina does not tolerate salt water and survives only for

5 to 10 minutes in 12p.p.t. water (Tattanon & Ruangpanit 1978). The dead Moina sink to the bottom of the tank where they become unavailable for the prawn larvae and, hence, their decomposition results in deterioration of the rearing water quality. They can, however, be adapted to salinities up to 7p.p.t. (Aniello & Singh 1980) and S. Pitipornchai

6 B. Nuang-saeng (pers. comm. 1999) reported maximal survival for 4 to 6 hours at that salinity.

Direct replacement of Artemia by Moina micrura without affecting production conditions for M. rosenbergii is only possible from stage VI onwards: earlier stages apparently have low ingestion rates of Moina, which may be related to the difficulty in capturing and ingesting the relatively large live prey (Alam et al. 1995a). This confirms earlier findings of Alam et al. (1993a) who reported high mortalities in prawn larvae fed Moina during the first 7 days of culture. Nevertheless, S. Pitipornchai & B. Nuang-saeng (pers. comm. 1999) reported that in Thai commercial hatcheries, even at a reduced salinity of 6 to 8 p.p.t. and after a 1-week Artemia feeding period, the substitution by Moina still resulted in a 20% lower postlarval survival, i.e. 40 to 60%. Alam et al. (1993b) suggested that the n-3 HUFA composi tion of Moina affected the larviculture output. A good tool to overcome this problem could be the addition of marine oils to the egg custard that is co-fed during the daytime to the prawn larvae (Alam et al. 1995b).

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