Influence of Divided Ponds on Fish Diets in Polyculture: Insights From Carbon and Nitrogen Stable Isotopes in Trophic Webs

ABSTRACT This study analysed the effects of pond‐management treatments on the diet composition and niche overlap of fish species by period of the production cycle in a polyculture system of pikeperch, common carp, roach and tench. Between stocking (February) and harvest (October), the treatments consisted of (i) dividing a pond in half with a net fence, stocking adult fish in one half and removing the fence (July) (‘divided’ treatment) or (ii) keeping a pond open (‘open’ treatment). Each treatment was replicated in three ponds. Fish and food resources were sampled each period, and their stable isotopes were analysed using mixing models to estimate fish diet compositions. The biomass of food resources did not differ between the two treatments, nor between the halves of the divided ponds with or without adult fish. The specific growth rate and net fish yield of each fish species also did not differ between the two treatments. Between the two treatments, the diet compositions of fish species did not differ, but they did differ between July and October, likely due to the presence of tadpoles in July. The niche of pikeperch did not overlap with those of the omnivorous fish species, as pikeperch are carnivorous. Niches of the omnivorous fish species did overlap somewhat, but less so in the divided pond, likely due to resource specialisation, but this overlap was not reflected in the diet compositions, possibly due to the low fish stocking biomass. Thus, using a divided pond design helps decrease niche competition, which may increase fish productivity.