Feeding strategies

Newly-hatched M. rosenbergii larvae are around2to2.2 mm in length. They normally start feeding about 1 day after hatching and initially require live zooplankton. Their prey should be of a suitable size for ingestion; the optimal size is estimated at 300 to 500 |im. Newly hatched Artemia nauplii are considered the most successful starter diet but, after the first rearing week, are usually fed in combination with prepared diets.

Sick & Beaty (1975) maintained an Artemia density of 0.1 nauplii/ml in an early experiment that involved inert feeds for freshwater prawn larvae. However, New & Singholka (1985) pointed out that they did not actively

Larval stages

Fig. 6.8 Ingestion rates of M. rosenbergii larvae feeding on Artemia nauplii and M. micrura. (Source: modified from Alam etal. 1995a.)

Larval stages

Fig. 6.8 Ingestion rates of M. rosenbergii larvae feeding on Artemia nauplii and M. micrura. (Source: modified from Alam etal. 1995a.)

seek food and recommended the maintenance of 1 to 5 nauplii/ml, depending on the size of the prawn larvae. Furthermore, these authors suggested that the level in the rearing water should never fall below 1 nauplius/ml to ensure maximal encounter possibilities. This practice is commonplace in Thailand but 5 to 15 nauplii/ml were used in Hawaiian hatcheries (New 1990). Many failures in early freshwater prawn hatcheries were associated with inadequate supplies of feed because hatchery operators did not realise the essentiality of providing food organisms close to the prawn larvae (M. New, pers. comm. 2008). Feeding rates of up to 100 nauplii per prawn larva by day 15 (=~5nauplii/ml) were applied by Daniels et al. (1992). Ingestion rates of live food for the various larval stages of freshwater prawns were reported by Alam et al. (1995a; Fig. 6.8). Ensuring that Artemia are available throughout the day, to avoid either over- or under-feeding, is essential (New 2002). Energy intake in M. rosenbergii is directly proportional not only to Artemia concentration but also to Artemia size. Sick & Beaty (1975) demonstrated that stage VII to VIII prawns attained a maximum energy ingestion of 0.0066, 0.062 and 1.014 cal/mg animal DW/h, respectively, when fed 0.7, 1.5 or 5.5 mm brine shrimp. More recently, Barros & Valenti (2003b) also evaluated ingestion rates for the different larval stages relative to prey density ranging from 2 up to 12 Artemia nauplii/ml. Ingestion rate significantly increased as food density increased. The results of this study furthermore suggested that four levels of ingestion occur during larval development, ranging from about 40 nauplii/day in stages II, III and IV, 55/day in stages V and VI to 80 to 100/day in stages VII and VIII. From stage IX onwards, maximum ingestion exceeded the load capacity of the medium, indicating the necessity to supplement Artemia with an inert feed. As highlighted in section

6.2.5, it is furthermore recommended that Artemia nauplii should be enriched with HUFA/vitamin C-fortified products to enhance their nutritional composition prior to their administration to the prawn hatchery tanks.

Although greenwater (Chapter 5) culture systems, designed to enhance the availability of natural food organisms through fertilisation, as well as to improve rearing water quality, were in favour in many freshwater prawn hatcheries in the 1970s (New 1990), almost all small and large hatcheries operate on clearwater systems because they are easier to manage (New 1995; see also Chapter 5). Some exceptions were noted in the early 1990s in Malaysia (Brown 1991) and Mauritius (Thompson et al. 1992). The rearing water of commercial hatcheries therefore currently has minimal inherent nutritional value; the larvae are almost entirely dependent on the live food and inert feeds that the hatchery operators supply. Several investigators have discussed the potential role of unicellular algae in the successful culture of Macrobrachium sp. However, the cause and effect relationship between the algae and larval nutrition has never been established. Maddox & Manzi (1976) studied the effects of seven species of unicellular algae on rates of survival and metamorphosis and concluded that all algal species enhanced the successful culture of larvae, with the diatom Phaeodactylum tricornutum as the most effective species. On the other hand, Sick & Beaty (1975) could not demonstrate a nutritional effect of phytoplankton added to larval culture systems, while Joseph (1977) was not able to detect any fatty acid assimilation from algal populations in M. rosenbergii rearing tanks. Cohen etal. (1976) confirmed that M. rosenbergii larvae did not ingest 14C-tagged algae. Also, enzymatic studies showed that first-feeding prawn larvae are carnivorous rather than herbivorous (Jones et al. 1993). Thus the contribution of unicellular algae may be rather a greenwater effect than a nutritional one (i.e. they assist in maintaining acceptable water quality conditions, antibiotic activity, etc.).

In order to economise on hatchery feed costs, Artemia nauplii may be supplemented with Moina sp. The use of farm-made larval feeds is commonplace and some freshwater prawn hatcheries use commercially prepared larval feeds. Aquacop (1983), Daniels et al. (1992) and Valenti et al. (1998) suggested the use of brine shrimp nauplii as a sole food up to day 10 to 12 for maximal hatchery output. After that, the use of an inert feed as a supplement to, or a total substitute for, Artemia is applied for cost-effective reasons, and without affecting survival and growth, in the larval culture of M. rosenbergii. Alam et al. (1995a) claim that a complete replacement of brine shrimp by Moina micrura, a cheap live food source, is possible from stage V onwards, without affecting growth or survival of the prawn larvae. This has, however, not been confirmed in commercial operations in Thailand, where a 20% lower postlarval

Table 6.8 Variations of food amount allowed per larva per day during larval rearing. (Source: Aquacop 1983.)


Artemia nauplii (No.)

Pellets (|g DW)

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