Modeling trophic webs in freshwater fishpond systems using Ecopath: towards better polyculture management

J Aubin,C Jaeger,M Roucaute,S Gamito,V Baizeau

doi:10.3354/aei00406

Abstract

Freshwater pond polyculture faces many challenges in Europe. Appropriate tools must be developed to better understand and manage trophic interactions in pond ecosystems. The objective of our study was to understand the trophic interactions and make inference on the fish diet in common carp polyculture through a combination of experiments and trophic web modeling. We conducted an experiment in small fishponds of common carp polyculture reared with roach and perch and used Ecopath with Ecosim software to characterize the food web. Two replicates of 3 treatments were performed: a semi-extensive pond with low fish density and no formulated feed, an intensive pond with twice the fish density and formulated feed and an intensive pond coupled with a planted lagoon. Ten trophic groups were defined to describe the food web. The modeling procedure enabled us to estimate the diets of each trophic group. The fish diet in fed and non-fed treatments differed greatly since the carp fed mainly on formulated feed when available. The roach exhibited trophic plasticity by adapting their diet to the available resources. The benthic macroinvertebrates and zooplankton were preyed upon intensively; they became the limiting factors for fish production and depended on phytoplankton availability. Detritus and phytoplankton were the main sources of nutrients in all treatments but were not used efficiently. These results provide several insights for improving polyculture. In particular, they promote better management of zooplankton and macroinvertebrates as food sources for target species and a better balance in fish assemblages for more efficient use of resources.

Highlights

Freshwater pond aquaculture is decreasing in Europe due to increasing environmental restrictions, little appreciation for the taste of pond fish and the abandonment of fish farming for more profitable ventures such as hunting or other recreational activities
The fish assemblage consisted of a polyculture of common carp Cyprinus carpio as the main species, roach Rutilus rutilus and rudd Scardinius erythrophthalmus for their omnivorous and pelagic habit, and Eurasian perch Perca fluviatilis as a carnivore
Treatment had no influence on the mean weight of individual roach, which ranged from 85.4−114.7 g at harvest

Summary

Introduction

Freshwater pond aquaculture is decreasing in Europe due to increasing environmental restrictions, little appreciation for the taste of pond fish and the abandonment of fish farming for more profitable ventures such as hunting or other recreational activities. The reference model of species assemblage in freshwater pond systems is carp polyculture, which has been established in China since the Tang dynasty (Milstein 1992) This system combines cyprinid species of different trophic levels that feed on the varied nutritional resources in the pond. Among the many ways to intensify pond systems, using formulated feed can sustain fish productivity but may cause changes in the trophic web and an overload of nutrients (nitrogen, phosphorus) and organic matter This is a major trend in fish farming throughout the world (Tacon et al 2010), especially in Asia, where non-fed polyculture systems have been abandoned in favor of fed monoculture systems (FAO 2018). The combination of species requires a previous understanding of complex interactions, which may only be feasible with the application of ecological modeling (Reid et al 2020)

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