Effects of different calcium and magnesium concentrations separately and in combination on Macrobrachium rosenbergii (de Man) larviculture

The optimum concentrations of calcium and magnesium for the combinations tested in hatchery water for Macrobrachium rosenbergii larviculture were evaluated. Experiments were conducted in two stages: in the first, triplicate treatments of calcium (120, 180, 240 and 300 ppm) and magnesium (300, 400, 500 and 600 ppm) were separately evaluated in a closed recirculating system (60-l). During the second stage, combinations of the two best calcium and magnesium concentrations determined in the first stage were compared. This stage used a static system (10-l) with minimal water exchange. In both stages, initial larval density was fixed at 100 larvae l− 1 and larval stage index, larvae dry weight, survival and time of the first postlarvae appearance were the determined parameters. The results of the first stage revealed that increasing concentrations of Ca and Mg showed a non-linear and a linear response respectively. According to these results, concentrations of 180 and 240 ppm calcium and concentrations at 300 and 400 ppm of magnesium were evaluated during the second stage of the investigation. In this stage, the results showed that after the 6th stage of larval development, interactions between both calcium and magnesium affected (P < 0.05) the larval quality parameters. The combination of these factors showed that a balance of 240 ppm calcium and 300 ppm magnesium with Mg/Ca ratio about 1.25 are optimal for larviculture. This balance at 30 days post hatch showed the highest larvae survival (40 ± 2.6%) at least 15.4% higher than the other treatments.