Abstract
Freshwater polyculture can increase nutrient use by rearing species from different trophic levels. The aim of this study was to quantify nutrient balances and determine the food sources of fish using nitrogen (N) and phosphorus (P) nutrient balances and carbon (δ13C) and N (δ15N) stable isotopes. The contributions of food sources to fish diets were estimated using stable-isotope mixing models. Three treatments were compared in mesocosms: fishonly (F treatment: common carp, roach and largemouth bass), fish-macrophyte (FM treatment) and macrophyteonly (M treatment). Fish were fed with commercial pellets. The results showed that adding macrophytes to the fish did not improve water quality. Chlorophyll-a concentration in F and FM treatments was significantly higher than that in treatment M (8.1, 8.3 and 0.6 µg l−1, respectively). Regardless of the treatment, sediment contained most of the total N and P at harvest (93.5 and 89.7 %, respectively). Carp, roach and bass had a similar specific growth rate in F and FM treatments (1.19, 0.36, 0.94 % day−1, respectively). Carp consumed 90 % commercial feed and 8 % macrobenthic invertebrates, while roach consumed 50 % commercial feed and supplemented their diet with macrophytes (15 %), macrobenthic invertebrates (12 %) and large zooplankton (11 %). Juvenile roach and bass had higher δ15N values than carp and roach did (12.31, 14.55, 7.54, 10.44 ‰, respectively). Bass consumed mainly juvenile roach (59 %), followed by macrobenthic invertebrates (25 %). Combining the nutrient balances and stable isotopes showed that sediment was the main nutrient pool, but that organisms in the mesocosms consumed little sediment organic matter.