Penaeus monodon post-larvae and their interaction with Rhizophora apiculata

B.T. Nga

Research output: Thesisinternal PhD, WU

Abstract

In recent years, expansion of shrimp aquaculture in Vietnam has brought considerable financial benefits to farmers and local communities. In the coastal provinces in the Mekong Delta, brackish shrimp aquaculture is the major economy activity. Extensive shrimp-mangrove culture systems are popularly practiced here. Although the average shrimp production is low, due to over-exploitation and destruction of mangrove forests and salt marshes, these systems are of special interest in view of the problems of sustainability of intensive aquaculture (Naylor et al. 2000 Nature 405: 1017-1024). Several studies demonstrated that mangrove swamps are highly productive ecosystems providing food, shelter and nurseries for various aquatic organisms, many of which are commercially important. The tiger shrimp, Penaeus monodon , is a clear example in this case. Natural shrimp production in these areas is believed to depend to a large extend on the presence of mangroves. However, the complex of mechanisms through which mangroves affect shrimp production is still poorly understood. The work in this thesis is an attempt to unravel some of the key-processes involved. It confirms the picture that mangrove litter represents a formidable input of organic material and nutrients into the aquatic system, and reveals how this input may have positive as well as negative effects on growth and survival of post-larval shrimp.Mangrove stands of different age have been studied for one year with respect to their litter fall and nutrient input (chapter 2). Litter fall consisted for 70% of leaf litter and organic matter accounted for 90% of the dry weight. Litter fall declined with the age of the mangrove stands, and also nitrogen and phosphorus levels were considerably higher in the leaf litter of younger stands (7 and 11 years) as compared to the older stands (up to 24 years). Thus, both the amount and the quality of litter input to the aquatic systems are highest in younger mangrove stands.As a next step key factors affecting the decomposition of mangrove leaves were analyzed (chapter 3). Decomposition rates tended to be highest at lower salinities, and reached an optimum at 5 ‰. The decomposition rates were also highest in the wet season, and this may well be due torelatively low salinities in this period. Wet season salinity in the Camau area was in the range of 4 - 9 ‰, close to the optimum for decomposition derived from laboratory experiments. Our studies also indicated an effect of humidity per se. We found that the decomposition rate was higher for leaves submerged in the ditches, than for leaves incubated near the roots of mangrove stands in the open air, where decomposition rates were higher in the wet than in the dry season. We also analyzed the dynamics of nutrient concentrations in decomposing litter. Nitrogen and phosphorus levels in decomposing leaves increased during the decomposition period. This enrichment indicates an increase of food quality over the first period of decomposition .The following chapters show that the effects of decomposing mangrove leaves on shrimps can be positive but also negative (Chapter 4 and 5). The amount of decomposing leaves appeared key. At high concentrations of leaves negative effects prevailed. These effects were probably due tothe release of nitrite and sulphide, and a decrease in dissolved oxygen concentration. On the positive side, mangrove moderate concentrations of leaves promoted growth of Penaeus monodon post-larvae, and apparently served as a shelter and as a food source.The fact that micro-organisms growing on the leaves, rather than the leaf material itself may be important as food was illustrated by the result that shrimps feeding on mangrove leaves grew better when a periphyton layer covered these leaves (chapter 5). A somehow surprising positive effect of leaves was the apparent prevention of excessive concentrations of ammonium and nitrite. The results suggest that adding conditioned mangrove leaves might ameliorate negative effects of high protein pellets on the water quality. The high C/N-ratio of leaves tends to balance the stochiometry of the system which may otherwise be dominated by the excessive N-input through CP pellets.In the final chapters the interaction among the shrimp larvae themselves, i.e. the effects of stocking density and the release of crowding chemicals and possible alarm pheromones on the shrimp populations are addressed (chapter 6, 7). A strong effect of crowding on shrimp growth and survival was shown. Physical interference stress and cannibalism could be excluded as causal factors. It was thus clear that the effects were caused by other water quality variables. Temperature, pH, salinity, dissolved oxygen, chlorine, nitrite and nitrate appeared of minor influence. However, ammonia toxicity could not be excluded as the causal factor for the observed mortality and reduced growth of P. monodon post-larvae in our experiments.On the other hand, alarm cues, as released by crushed conspecifics had negative effects on post-larval survival at high concentrations (100, 70, 50 and 30 crushed shrimps.l -1 ). Surprisingly, low concentrations of crushed conspecifics (1 crushed shrimp.l -1 ) were shown to have rather stimulatory effects on body size and dry weight.Put in an applied perspective, this study suggests simple ways to improve the management of mangrove-shrimp systems. Clearly, mangrove leaves can promote the survival and growth of shrimp post-larvae. However, at high leaf concentrations negative effects may prevail related to a drop in dissolved oxygen and the release of sulphide. A straightforward way to ameliorate such negative effects may be to increase the water flow. This will reduce the risk of local anoxia, and may help spreading the litter over the area, thus avoiding accumulation of these leaves at some sites. The reduction of potentially toxic nitrite and ammonium concentrations by decomposing leaves suggests that mangrove leaves may serve as a useful complement to CP pellets in semi-natural production systems.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Scheffer, Marten, Promotor
  • Roijackers, Rudi, Co-promotor
Award date19 Nov 2004
Place of PublicationWageningen
Publisher
Print ISBNs9789085040934
Publication statusPublished - 19 Nov 2004

Keywords

  • penaeus monodon
  • shrimps
  • rhizophora apiculata
  • mangroves
  • interactions
  • aquaculture
  • shrimp culture
  • shellfish fisheries
  • population dynamics
  • mortality
  • litter
  • nutrients
  • toxicity
  • vietnam

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