Food web interactions and nutrients dynamics in polyculture ponds

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 re-mobilisation 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 ( Labeo rohitaHamilton) and common carp ( Cyprinus carpio L.), two cyprinid species, because rohu-common carp polyculture is becoming popular practice inBangladesh. 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 of benthivorous fishes will affect the overall performance of any polyculture system.Part 2 includes Chapters 2, 3 and 4.àChapter 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 . PO 4 -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. àIn 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 availability, the highest natural food ingestion, the highest fish growth (rohu: 1.55% body weight day -1 , common carp: 1.59) and the highest total fish production (3,532 kg ha -1 137 day -1 ). The effects were less pronounced when stocking 1 common carp m -2 . Rohu shifted from phytoplankton to zooplankton when the latter became more available in response to artificial feed addition. àChapter 4 investigated the effects of common carp and artificial feed addition on water quality parameters and on the accumulation of nutrient in different types of natural foods present in the ponds. In all treatments all water quality parameters remained favourable for fish growth. The overall nitrogen and phosphorus concentrations were highest in treatments with 0.5 common carp m -2 followed by treatments with 1 and 0 common carp m -2 , respectively. The oxygen concentration decreased with increasing common carp density. More nitrogen and phosphorus accumulated in fish, phytoplankton and zooplankton in treatments with 0.5 common carp m -2 , followed by treatments with 1 and 0 common carp m -2 , respectively. The nitrogen and phosphorus accumulation in sediment was lower in presence of common carp than in the common carp free treatments. Concentrations of all nitrogenous and phosphorus compounds were higher in fed than in non-fed ponds except for NO 2 -N. A larger fraction of the input nutrients accumulated in all natural food types in non-fed than fed pond whereas the opposite were observed for the fraction of nutrients accumulating in the sediment. ·     In the third part of this study (Chapter 5) the resting, grazing, swimming and social interactions of rohu and common carp were observed in purpose build tanks for the same treatments as applied in part 2. Rohu decreased intra-species and increased inter-species interactions with increasing common carp density. Presence of common carp reduced the resting time and increased swimming and grazing time of rohu near the tanks bottom. Rohu benefited from spending more time close to the bottom and wall in presence of common carp by ingesting more zooplankton, as in these areas zooplankton biomass was higher than in other parts of the tanks.A general discussion of the experimental results is given in Chapter 6. Both addition of common carp and artificial feed affected the pond ecology, fish growth, total production and fish behaviour. Stocking 0.5 common carp m -2 was better than stocking 1 m -2 . The strength of this study is that it looked at the combined effects of stocking benthivorous fishes and feed addition considering water quality, natural food availability, feed intake and behaviour. Results show that behavioural observations are equally important as water quality and feed intake monitoring to explain synergism in polyculture. Similar studies, adding a third species, are recommended to further elucidate changes in the food web dynamics when synergism is observed in polyculture ponds.