Decomposing mangrove litter supports a microbial biofilm with potential nutritive value to penaeid shrimp post larvae

The use of fish meal in shrimp culture not only contributes to the decline of wild fish stocks, but also undermines its profitability and enhances ecosystem pollution. There is an urgent need for alternative natural food supply in shrimp cultures. The present study investigated the potential of mangrove litter from Rhizophora mucronata and the associated microbial biofilm as food for shrimp post larvae of Penaeus indicus and Penaeus monodon in a community-based ecological shrimp farm in Mtwapa creek, Kenya (3°57'S; 39°42'E). Senescent mangrove leaves were incubated together with shrimp post larvae, PL 15–25, for 6 weeks in shallow mangrove pools. Leaf litter degradation, carbon and nitrogen nutrient remineralization, bacterial community structure and algal biomass in the periphytic biofilm were investigated weekly for 6 weeks. Food uptake and assimilation was assessed by comparing fatty acid profiles and d13C isotope values in the shrimp tissue, litter and biofilm. Post larvae from the open creek were used as a control. Decomposing mangrove litter supported the growth of microalgae and bacteria in the form of periphytic biofilm with a maximum growth at the 3rd and 4th weeks when the litter was 43% decomposed. Bacterial community varied in structure with the progress of litter decomposition by declining in abundance after the 3rd week toward a minimum at the 6th week. The diversity of bacterial colonies also changed from a high dominance, at the early stages of litter decomposition, to evenly diverse colonies in the litter decomposed beyond 5 weeks. Shrimp stocked in mangrove forest had 1) highest levels of linoleic acid, linolenic acid and highly unsaturated fatty acids (HUFA) in the 3 to 4 weeks old litter, 2) lower fatty acid levels compared to the shrimp from the creek and 3) were isotopically close to seagrass and biofilm. In terms of nutritional value, mangrove litter supports penaeid shrimp post larvae with a periphytic biofilm during the early stages of decomposition (weeks 3–4). The results of this study suggest that optimal nutrient supply to ecological shrimp aquaculture in mangrove systems could be optimized by controlling residence time of mangrove litter in shrimp ponds and selecting sites linked to other ecosystems such as creeks and open sea