STRATEGIES FOR DEVELOPMENT OF ROTIFERS AS LARVAL FISH FOOD IN PONDS

Strategies to sustain rotifer peak biomass, distribution of rotifer re sting eggs in the sediment, and relationship between rotifers and larv al fish growth were studied in a series of pond experiments, After the ponds were filled with water, herbivorous rotifers (e.g., Brachionus calyciflorus) developed first, but were gradually replaced by predator y rotifers (e.g., Asplanchna). Subsequently, herbivorous cladocerans ( e.g., il Moina sp) eventually replaced rotifers and dominated the zoop lankton community. The occurrence of Asplanchna and Moina indicated th e decline of B. calyciflorus. Peak rotifer biomass developed 8-10 d af ter the ponds were filled with water at 20-25 C, 10-15 d at 17-20 C, 1 5-20 d at 15-17 C, 20-30 d at 10-15 C, and >30 d at <10 C, The abundan ce of resting eggs in the top 5-cm sediment varied from 6 to 83/cm(2). About 25% of resting eggs were buried in the top 5-cm sediment but th e number of resting eggs decreased with increased sediment depth. Opti mum rotifer biomass for silver carp Hypophthalmichthys molitrix Larvae stocked at 1,500,000/ha was 20-40 mg/L. High rotifer biomass (>20 mg/ L) usually lasted 3-5 d, but could be prolonged by pond fertilization or cladoceran controls, A weekly application of dipterex at 0.05 mg/L reduced cladoceran biomass but enhanced rotifer biomass. Our results i ndicate with a careful management plan it is possible to synchronize t he rotifer development with larval fish stocking.