Effect of mixed leaf litter of four mangrove species on shrimp post larvae (Penaeus monodon, Fabricius, 1798) performance in tank and mesocosm conditions in Bangladesh

Mangrove leaf litter is a potential source of nutrients for shrimp post larvae. To introduce mangrove trees in shrimp farms it is necessary to identify the combination of trees which is most beneficial for shrimp. The present study evaluated the effects of mixed leaf litter of four mangrove species (Avicennia officinalis (Ao), Sonneratia apetala (Sa), S. caseolaris (Sc) and Heritiera fomes (Hf) on shrimp post larvae performance and water and soil quality. Leaf litter with and without supplemental feed was applied to shrimp culture tanks according to a 4 × 2 factorial design and followed by testing a subset of treatments in mesocosm pond conditions. Shrimp post larvae of 15-days old (PL15) with an average weight of 0.01 g were used for both experiments, each with a 4-week duration without water exchange. Under controlled conditions in the tanks, leaf litter and feed resulted in 22 to 32% higher weight gain of PL shrimp than combined weight gain realized when receiving only leaf litter or only feed, indicating synergism. Based on this, the pond experiment was designed with combined application of leaf litter and feed. The pond experiment resulted in higher shrimp weight gain than realized in the tanks. In tanks, the highest average individual weight gain of PL was observed for the leaf litter mixture SaAoHf (0.23 g)followed by SaScHf (0.21 g), ScAoHf (0.21 g) andSaScAoHf (0.20 g). Paralleling the results of the tank experiment, SaAoHf leaf litter also gave the highest average individual weight gain (1.2 g) of PL shrimpin the ponds but other leaf litter treatments followed by SaScAoHf (0.95 g), ScAoHf (0.84 g) and SaScHf (0.69 g) leaf litter. The different mixtures of mangrove leaf litter also resulted in significant differences in biological oxygen demand (BOD5), phytoplankton and zooplankton concentrations in pond water and organic carbon in soil. Both phytoplankton (P < 0.01, Pearson correlation r = 0.910)and zooplankton (P < 0.05, r = 0.535) abundance was positively correlated to shrimp weight gain. The low feed conversion ratio (FCR) in the treatments combining leaf litter and supplemental feed as compared to treatments using only feed indicated extra food benefits for shrimp PL from decomposing leaf litter. Overall, mixed mangrove leaf litter had a positive effect on shrimp performance and this effect was highest for SaAoHf leaf litter