The availability of common carp fry is an obstacle to the development of rice-fish culture in the Malagasy highlands. Survival of juveniles during the rearing phase is generally low (< 10%). This is linked to several factors, including insufficient zooplankton as live prey as initial feed. To remedy this situation, we assessed the multiplication of a small freshwater parthenogenetic cladoceran, Moina sp., which is widely produced in Asia to feed farmed fish larvae.
A local strain of moina found in the Malagasy highlands was used. Three tests were carried out to evaluate 1) the effects of the concentration of pig slurry (0, 0.5, 1, 2, 3 g L-1 wet weight, ~50% dry matter), 2) the effect of the prior adaptation of the moinas to the culture conditions and 3) the use of a phytoplankton solution “green water” as culture media. The green water was previously produced with pig slurry during 5 days and partially diluted (67%) before its use for moinas culture. Each treatment was applied to 10 tubes filled with 40 mL and initially stocked with 8 moinas (200 moinas L-1). Water temperature was kept constant (~27.8°C) and permanent artificial lighting was applied. An exhaustive count of moinas was done in all tubes, every day for 14 or 21 days.
With clear water and adapted moinas, the pig slurry concentration 2 g L-1 was optimal. After prior adaptation, 1 adapted moina produced an average of 51 moinas during the 3.7 days increase phase while 1 non-adapted moina produced only 30. The numbers increased rapidly from day 2 to day 5, reaching 402 moinas tube-1 (10 050 moinas L-1), before collapse. In both experiments, the increase phase was preceded by a latent phase lasting 1.7 days. Tube collapse was accompanied by a deterioration in culture conditions with nitrite up to 3.7 mg L-1. Green water allowed to extend the culture duration (Figure 1A). An experiment is ongoing to test if phytoplankton improved the water quality.
Local moinas have good multiplication potential when fed pig slurry. An estimate show that 1.3 kg of dry slurry would produce ~1 kg of fresh, drained moinas. Future experiments should test production at a larger scale with a combination of slurry and green water.