| The present study firstly described a grass carp, silver carp and common carp polyculture pond ecosystem using an Ecopath with Ecosim (EwE) model, aimed at characterizing trophic structure, the interactions among ecological groups and assessing the efficiency of the pond ecosystem to carrried out a preliminary exploration of EwE model in the application of aquaculture ecosystems. Based on the results, a grass carp, silver carp and shimp integrated aquaculture pond ecosystem was researched for further exploration, and suggestions for aquaculture ponds structure optimization was provided. Subsequently, three grass carp aquaculture management systems were compared in terms of ecological, economic and environmental factors based on the EwE model results. Finally, pond ecosystem changes were simulated under different stocking density and feeding rates according to the established EwE model in grass carp, silver carp and common carp polyculture pond ecosystem, which evaluated the feasibility and capacity of using EwE model as a powerful tools and methods in aquaculture ecosystems. The results are summarized as follows:1. The modeling analysis of the structure of the grass carp-silver carp-common carp polycultured pond ecosystem:A grass carp, silver carp and common carp polyculture pond ecosystem was described using an Ecopath with Ecosim (EwE) model, aimed at characterizing trophic structure, the interactions among ecological groups and assessing the efficiency of the pond ecosystem. Fourteen functional groups were incorporated into the model. The results indicated that this polyculture pond ecosystem was mainly composed of 3 aggregated trophic levels. The trophic flow analysis suggested that the trophic flow of trophic level I was up to 56.90% to the total system throughput (TST), and the trophic flow from trophic level Ⅱ to trophic level V decreased with the increase of the trophic level, or rather 34.45% for trophic level Ⅱ,8.20% for trophic level Ⅲ、0.44% for trophic level IV and 0.003% for trophic level V. The prey overlap index between cladocera and rotifera was close to 1, and the prey overlap index between copepod and cladocera, copepod and rotifera were about 0.6, implying that these groups share a similar diet. The predator overlap index between copepod and cladocera, protozoa and rotifera were over 0.8, implying that these groups share a similar pressure from predators. The main way of the trophic flow in the grass carp-silver carp-common carp polyculture pond ecosystem included pastoral food chain (starting from phytoplankton), rotten food chain (starting from detritus) and feed chain (starting from feed). The ecotrophic efficiency (EE) of most functional groups were relatively high (exception for gastropoda, the EE was nil), implying a high efficiency of the ecosystem. Detritus plays an important role in the ecosystem, its’main source were bacteria, protozoa and phytoplankton. The EE of detritus were relatively high (0.903 for detritus in water, 0.551 for detritus in sediment), indicating most of detritus consumed and passed up the food web.2. The modeling analysis of the structure of grass carp-silver carp-shrimp integrated aquaculture ecosystem:A grass carp (Clenopharyngodon idellus) cultured pond in East China was described using an Ecopath with Ecosim (EwE) model, aimed at characterizing trophic structure, the interactions among ecological groups and assessing the efficiency of the pond ecosystem. Fifteen functional groups were incorporated into the model (including artificial feed). Values for production, consumption and respiration were compared with the total system throughput. The results indicated that both primary and secondary productivity were not well utilized in the pond ecosystem, resulting in an accumulation of detritus. The maximum trophic level of any functional group in the model was only 3.14. This indicates a short food chain, typical of simple or immature food webs in terms of structure and dynamics. The different size fractions of phytoplankton made different contributions to the system, and larger sized phytoplankton were easier to eat by organisms at higher trophic levels. The study indicated that introducing a polyculture system would increase the efficiency and outputs from the pond ecosystem.3. Assessment of integrated multi-trophic aquaculture modes based on ecological modeling software:example using three grass carp aquaculture modes: Three grass carp aquaculture management systems were compared in terms of economic and environmental factors. These consisted of grass carp only (G), a polyculture of grass carp and silver carp (GS), and a polyculture of grass carp, silver carp, and shrimp (GSS). Measured outcomes were economic (output and market value), ecological (ecotrophic efficiency, primary production, and transfer efficiency), and environmental (detritus flow and residual detritus biomass).Data were analyzed using Fcopath with Ecosim (EwE) modeling software. The economic and ecological outcomes of the GSS system were significantly better than those of the G and GS systems. The ratios between trophic level Ⅱ and primary production in G, GS, and GSS were41.9%,44.1%, and 61.5%, respectively. The trophic level Ⅱ transfer efficiencies of G, GS, and GSS were 15.9%,19.1%, and 23.5%. The detritus flow and detritus residual biomass of G, GS, and GSS were 16020,13210, and 10060 g/m2 per season. Thus, the residual detritus biomasses of G, GS, and GSS were 2381.0, 748.3, and 990.1 g/m2 per season. Environmental pollution associated with the polyculture systems was markedly lower than that of the monoculture.4. Simulation of different stocking density and feeding rates on grass carp, silver carp and common carp polyculture pond ecosystem:effects of different stocking density and feeding rates on grass carp, silver carp and common carp polyculture pond ecosystem was assessed based on the established EwE model. According to the current aquaculture mode, the stocking density consisted of 100%, 200%,300%,400% and 500%, and the feeding rates consisted of 75%,100%,125%, 150%,175% and 200%. The results showed that the biomass of functional groups increased by different levels as the stocking density increases. In which, phytoplankton had the biggest increase, the second was bacteria in water, and copepods had the smallest increase. The trophic flows of phytoplankton had a significant increases, the total primary production (TPP) was increased from3298 g m-2 year-1 to 15940 g m-2 year-1 when the stocking density increases from 100% to 500%, but the biomass of phytoplankton flowed to detritus increased from 1288 g m-2 year-1 to 6225 g m-2 year-1, and the biomass intake by trophic level Ⅱ increased from 2010 g m 2 year-1 to 9715 g m 2 year’accordingly. Detritus residues in the environment increased as the stocking density increases, too. There was no big change for the main source of detritus both in water and in sediments, but the contributions of the functional groups to detritus varied. Increasing the feeding rates would bring about complicated impacts on the ecological efficiency (EE) of the functional groups, especially for artificial feeds and detritus in water. The EE of artificial feeds would decrease greatly and caused a big increase in environmental load when the feeding rates increases. According to the current feeding rates, shot feeding rates would in favor of the increase of TPP of phytoplankton. On the other hand, the TPP of phytoplankton would increase if increasing the feeding rates, but the biomass of phytoplankton intake by trophic level II would decrease, and the the biomass flowed to detritus would increase, either. Meanwhile this would cause a big change in the soure of detritus in the aquaculture pond ecosystem. |
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