The contribution of mangrove leaf litter to juvenile shrimp (Penaeus monodon) production in mangrove-shrimp aquaculture systems

Résumé

Intensification of shrimp farming has been identified as one of the main causes for mangrove destruction in the coastal region of many countries. The idea to develop mangrove-shrimp co-management was developed during the last decades of the 20th century as an effort to help restore the mangrove ecosystem and make shrimp culture sustainable. The present research is part of a broader effort to introduce silvo-aquaculture, combining mangrove and shrimp, to Bangladesh. Mangrove- shrimp co-management has very high potential from an ecological and economical point of view. Mangrove leaf litter are shown to enhance shrimp production by enhancing natural food production. In addition, the nutrient use efficiency of artificial feed is found to be enhanced by providing leaf litter as extra nutrient source, a form of synergism between natural and artificial feed. Therefore the main aim of this thesis is to assess the impact of leaf litter from different mangrove species on pond performance, water quality and natural food availability in mangrove shrimp nursery tanks and ponds. First of all, the nutrient and anti-nutrient content in leaf litter of selected mangrove species and their effect on shrimp performance were determined. The observed effects on post-larval shrimp differed between mangrove species, showing a clear ranking of mangrove species in terms of their ability to enhance shrimp growth. Leaf litter from different mangrove species, not only led to different concentrations of plankton in the PL nursery tank system, but also correlated with shrimp growth performance. Secondly, the effects of interactions between leaf litter from the four chosen mangrove species and pelleted feed on shrimp growth and survival in shrimp PL nursery tanks were explored. Leaf litter and feed combined, resulted in a 21 to 33% higher weight gain of shrimp PL than based on the additive contributions of only leaf litter or only feed, indicating a synergistic effect of the two food sources on shrimp growth. Among the different mangrove species tested, S. apetala (Sa; 23.1%) contributed the most to total weight gain followed by A. officinalis (Ao; 21.6%), S. caseolaris (Sc; 21.6%) and H. fomes (Hf; 10%). The lower feed conversion ratio (FCR) (0.18–0.27) in the treatments combining leaf litter and supplemental feed as compared to the feed-only treatment (0.41) indicated that leaf litter (directly or indirectly by stimulating natural food production) contributed to the nutrition of the shrimp. The observed synergistic effect between supplemental feed and leaf litter is an opportunity for farmers to reduce shrimp production costs and simultaneously raise benefits. Thirdly, effect of different combinations of leaf litter from different mangrove species on shrimp larval performance in tanks and small (mesocosm) ponds were also explored. Three 3-mangrove-species and one 4-mangrove-species combinations of mangrove leaf litter were tested. Under controlled conditions in tanks, mixed leaf litter and feed resulted in 22 to 32% higher weight gain of PL than based on the additive contributions of only leaf litter or only feed, indicating a similar synergistic effect of the two food sources on shrimp growth . Although the nutrient input level and PL stocking density in ponds were less than half the input or density in tanks, the shrimp grew 3.5 times larger in the mesocosm ponds. The different combinations of mangrove leaf litter employed influenced water quality and stimulated the production of phytoplankton and zooplankton food, which allowed greater shrimp weight gain. Then, effect of leaf litter on shrimp growth, color and product appeal to farmers, exporters and local consumers was identified. The body color of shrimp not exposed to leaf litter was lighter than of shrimp grown in tanks receiving leaf litter. The majority of Sa-reared shrimp at harvest were significantly darker (P < 0.05) in body color than larvae reared with leaf litter from other mangrove species. The shrimp body color of shrimp not exposed to any leaf litter at all was lighter than of shrimp reared in presence of all types of leaf litter tested. Within each category of shrimp value-chain actors interviewed, more than 50% of respondents preferred dark colored shrimp, and none preferred pale-colored shrimp. 100% from among exporters and local consumers, respectively, 100% and 60% linked dark body color to higher price, while the opposite was the case among non-mangrove farmers. Aside from production volume, mangrove leaf litter was found to strongly enhance the color properties and hence perceived quality of black tiger shrimp to the consumer. Inclusion of mangroves in and along shrimp culture ponds is recommended as a valuable way of improving both the commercial profitability and sustainability of shrimp aquaculture in Bangladesh and elsewhere.

Finally, the effectiveness of mangrove leaf litter application in combination with pelleted supplemental feed in PL nursery systems is reviewed against existing concepts and the functioning of mangrove-shrimp rearing systems.

The main identifications are:

• Application of mangrove leaf litter contributes to individual growth and total production in shrimp nursery systems.
• When applied at 1 kg m-3 and 0.56 kg m-3 culture volume in tanks and mesocosm ponds, respectively, the anti-nutrient content in leaf litter does not negatively impact shrimp performance.
• Crude fiber content strongly affects the decomposition rate of mangrove leaf litter.
• Leaf litter decomposition at the described leaf litter loading rates releases nutrients that stimulate plankton production, which in turn enhance the total system performance of shrimp nursery systems.
• For the input range and culture duration, applied in this Thesis project, joint application of supplemental pelleted feed and mangrove leaf litter created synergy for a more than additive positive effect on shrimp growth.
• The traditional shrimp farmers in Bangladesh need to be introduced to and trained in mangrove-shrimp co-management rearing techniques to ensure they benefit from mangrove-shrimp aquaculture.

Overall, our results show that inclusion of mangroves in shrimp pond culture has great as yet largely untapped potential to enhance pond productivity and make shrimp farming more environmentally sustainable. A paradigm shift is needed with respect to mangroves vis-a-vis shrimp pond aquaculture. Whereas these areas today are largely denuded of mangrove trees, these should be seen as valuable resource for the shrimp farmer of which the leaves can serves as a healthy and inexpensive source of shrimp food. Proper management of mangrove trees in and around shrimp ponds is an opportunity with which to make shrimp farming more resilient, with important benefits for the coastal communities and the coastal ecosystem.