Introduction
In Europe, freshwater pond aquaculture, which has high potential to meet the growing consumer demand for fish, is facing environmental restrictions, leading to a decrease in the sector’s production. Monoculture and polyculture systems are facing many challenges to become more sustainable, that is, more productive, resilient, robust and mature, as well as providing more support for natural and cultural values, human beings and biodiversity (Dong et al., 2022). Current aquacultural research recommends combining multiple species in polyculture, with one reference model being carp polyculture, first established in China. This type of polyculture originally focused more on the fish than on the other trophic compartments of the ecosystem. The SEPURE project aims at improving understanding of complex trophic interactions in pond ecosystems in order to identify innovative assemblages of fish species that use and recycle nutrients better. In this study, trophic web models were built for several polyculture fishponds with different management practices to develop a method to assess the structure and functioning of aquacultural ecosystems.
Materials and methods
Using data for 10 fish ponds (with fish fed or unfed) studied in different regions of France in 2021 and 2022, a model of the trophic web of each pond ecosystem was built using Ecopath software. Each pond was considered an independent case study. The ponds differed in feeding, fertilization, the amount of bird predation and the number of trophic compartments represented. The fish assemblages varied, including the common carp Cyprinus carpio and/or grass carp Ctenopharyngodon idella, roach Rutilus rutilus, rudd Scardinius eruthrophtalmus, tench Tinca tinca, largemouth bass Mycropterus salmoides, pikeperch Sander lucioperca, northern pike Esox Lucius or white sturgeon Acipenser transmontanus. The species selected and their ratios differed among ponds. Using Ecopath software, which models the balance of mass circulating among the compartments, each compartment’s contribution to ecosystem productivity was studied. To assess each pond ecosystem, performance indicators such as ecotrophic efficiency (EE), total system throughput (TST), net system production (NSP) (i.e. respiration minus primary production) and system omnivory index (SOI) (i.e. the complexity of feeding interactions between trophic levels) were calculated. Indicators such as Finn’s cycling index (FCI) (i.e. the percentage of TST recycled) and the ratio of primary production to respiration (PP/R) were then used to assess the maturity of the system.
Results
Initial model outputs predicted a variety of dynamics among the ponds. EE remained £ 1 for all trophic compartments of each pond, which is necessary for a model to be considered a satisfactory representation of a trophic web. The EE were high for macro-invertebrates and zooplankton in most ponds, indicating that they were key factors that limited fish growth in the ponds, even when the fish were fed. TST ranged from 513.8-1095.0 g/m²/cycle, indicating that the size and activity of the ponds differed, with the largest values indicating the highest turnover in the systems. SOI ranged from 0.17-0.21, which indicated many, and thus more complex, feeding interactions among the trophic compartments. NSP ranged from 140.5-264.4 g/m²/cycle, with the larger values indicating more immature systems that are still developing. In addition, PP/R ranged from 4.64-9.40, likewise indicating the immature state and small size of the ecosystems, with no clear differences between systems with fed vs. unfed fish. FCI ranged from 4.4-10.2%, which seemed to indicate high stability and resistance to external disturbances.
Discussion and conclusions
This study is a preliminary approach to build accurate reproducible mass-balanced Ecopath models of European freshwater fishpond systems. SOI, NSP and PP/R indicated that the structures of the food webs during the cycle were incomplete, which means that the primary production could not be fully used. The high variability in performance indicators seems to indicate room for improvement in management practices, fish assemblages and their interactions. Nevertheless, the model results provided guidance on better understanding the functioning of each ecosystem and reflecting on how to improve management of the systems. Analysis of the variety of fish assemblages provided new knowledge to help reconsider polyculture characteristics and potentially develop future experimental designs and recommendations for sustainable management of aquaculture.
Acknowledgements
This study was conducted within the framework of the SEPURE project (https://www6.inrae.fr/sepure), financially supported by the European Fund for Maritime Affairs and Fisheries (FEAMP). We thank all SEPURE project partners and fish pond producers for the data they provided.
Reference
Dong, S.P., Shan, H.W., Yu, L.Y., Liu, X.B., Ren, Z.W., Wang, F., 2022. An ecosystem approach for integrated pond aquaculture practice: Application of food web models and ecosystem indices. Ecol. Indicators. 141, 9.