Artificial feed and fertilizers are the main sources of nutrients supporting fish growth in aquaculture ponds. The majority of the added nutrients are lost to the sediment, where they are no longer available for natural food production. By increasing resuspension of the sediment through the introduction of benthivorous fish, nutrient loss may be reduced, because of the remobilisation of nutrients from the sediment. The effects of addition of benthivorous fish and/or artificial feed in fertilized ponds have mostly been studied separately. Therefore, this thesis focuses on the integrated study of the (interacting) effects of the addition of artificial feed and a benthivorous fish species on overall nutrient dynamics, pond ecology, and growth and production of fish in polyculture ponds. To achieve this we used rohu (Labea rohita Hamilton) and common carp (Cyprinus carpia L.), two cyprinid species, because rohu-common carp polyculture is becoming popular practice in Bangladesh. Common carp is benthivorous, stirring up bottom sediment, resulting in nutrient resuspension, while rohu is planktivorous and an efficient plankton grazer. Three common carp densities (0, 0.5 and 1 individual m-2) were applied with a fixed density of rohu (3 individuals m-2)in both artificially fed and unfed ponds. The aim of the study was to quantify the effects of feed and common carp addition and their interactions on water quality, nutrient accumulation, natural food production, behaviour, growth and yield of the fish. Moreover, we tried to find out whether there would be a common carp density that would be optimal for fish production in rohu ponds. The present thesis was divided into three parts. The first part was a literature review on the status of carp polyculture and role of common carp and artificial feed on biotic and abiotic components in ponds or lakes. The second part analyzed changes of various biotic and abiotic parameters and measured synergistic effects triggered by the rohu-common carp combination. The third part monitored changes in swimming and grazing behaviour, and social interactions between rohu and common carp. . The first part of this study (Chapter 1) focused on the positive and negative effects of benthivorous fishes on nutrients and natural food availability. Benthivorous fishes stimulated mineralization of organic matter, liberation of nutrients from sediment to the water column and primary production. The density of benthivorous fishes affected turbidity and grazing pressure on natural food, which in turn affected natural food availability. Therefore, the density ofbenthivorous fishes will affect the overall performance of any polyculture system. Part 2 includes Chapters 2, 3 and 4. Chapter 2, explored the links between water quality parameters, different types of food resources available, fish diet composition and fish growth/production. Common carp increased bioavailable nitrogen and phosphorus, a process that was enhanced by the addition of artificial feed. The effects of common carp were more pronounced in treatments with 0.5 than 1 common carp m-2. PO4-P concentration was strongly correlated with phytoplankton and zooplankton biomass. One of the major findings was that rohu growth was best explained by natural food intake, while common carp growth was best eXplained by artificial feed addition and negatively correlated with natural food ingestion. Results indicated that common carp benefited directly from artificial feed addition while rohu benefited indirectly from the boost in natural food availability triggered by the fertilizing effect of the artificial feed. In Chapter 3, the effects of different densities of common carp (0,0.5 and 1 individual m-2) on natural food availability, natural food ingestion uptake and fish growth were compared under fed and unfed conditions. Stocking 0.5 common carp m-2 resulted in the highest observed natural food a
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