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Dynamika rybničního ekosystému z hlediska produckční ekologie
RUTEGWA, Marcellin
With the rise of environmental concerns, fish farmers are facing the challenge of maximizing fish production while maintaining the ecosystem integrity of fishponds. In semi-intensive ponds, fish production is based on the utilisation of the natural production potential of the pond ecosystem and supplementary feed. Understanding the sources and the fate of inputs in fishponds is crucial to assess the utilisation efficiency of these inputs and hence reduce nutrients and organic matter losses. Ecological integrity of ponds is not regularly assessed in the Czech Republic, where pond aquaculture is an important economic activity. Therefore, the quality of inflow, pond water and effluent were monitored in the Dehtář pond from 2014 to 2018. Nutrient and organic carbon budgets were used to quantify the sources and fates of nutrients and organic matter inputs in Dehtář. Assessment of ecosystem metabolic rates, plankton community composition, utilisation efficiency of nutrient and organic carbon inputs and nutrient retention were used to indicate the effect of fishery practices on the structure and functioning of the pond ecosystem. The transboundary layer model was used to determine and compare the levels of dissolved and diffusive methane (CH4) from two types of fishponds (main and nursery) differing in individual fish size of cultured common carp. The results showed that the Dehtář pond became hypereutrophic due to fishery management, namely high fish stock, associated weed fish, manuring and supplementary feeding. The phytoplankton assemblage was dominated by eutrophication tolerant species, including toxic cyanobacteria. Larger sized zooplankton disappeared from the pond due to the high grazing pressure of fish stock already in the spring. The major sources of phosphorus, nitrogen and organic carbon inputs to the pond were feed, water inflow and primary production, respectively. Accumulation in water and sediments were the main fate of nitrogen and phosphorus whereas respiration was the main fate of organic carbon. The utilisation efficiency of nutrient inputs was comparable to results from other studies but the utilisation efficiency of organic carbon inputs was rather lower. In general, this efficiency was low due to the poor quality and low digestibility of cereals used as fish feed and the inability to transfer the primary production to fish. The potential of the Dehtář pond to retain nutrients is comparable with other temperate lakes and reservoirs. This may be explained by a high load of nutrients, their assimilation by fish and phytoplankton, sedimentation and denitrification. However, during the fish harvest, part of the retained nutrients is released and discharged. Furthermore, it was found that unused organic matter partly fuels the production of methane, which is then released into the atmosphere. The methane concentration was higher in nursery ponds than in main ponds, but the diffusive methane emission did not differ much between the two types of ponds. Based on our present findings we highly recommend to conduct more research activities heading to more sustainable pond management practices that can maximize fish production while minimizing pollution risks in the fishponds and their catchments.
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Carbon metabolism and nutrient balance in a hypertrophic production fishpond
Hejzlar, Josef ; Potužák, J. ; Drozd, B. ; Rutegwa, M.
Semi-intensive production of fish in pond culture is based on natural food in the form of zooplankton or zoobenthos and supplemental feeding and manuring applied with balanced nutrient budget. Measurement of the metabolism (i.e., primary production and respiration by the method of continuous measurement of dissolved oxygen in the water column) and the input / output balance for C, N and P in the semi-intensive Dehtář fishpond in the 2015 growth season have shown that additions of fish feeding and manure were from the point of view of utilisable fish feeding in a 3 – 4 fold excess, the efficiency of nutrient transfer through fish food chains was low and that most of the primary production remained unused for fish production, resulting in eutrophication problems with accumulation of biomass in water, high respiration of phytoplankton and microbial chains, hypoxia conditions, and intense N and P cycling.
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