The study was carried out in a grass carp cage culture located in thermal effluent canal at the Fisheries Research Station of the Department of Aquaculture, Agricultural University of Szczecin, within 18 June – 6 September 2002. The study involved fry of the grass carp aged 1+ and mean individual weight of 135 g (±15g), stocked at a density of 30 individuals per cage (10 individuals m^-3 water). The experiment was aimed at determining possibilities for growing out the grass carp in cooling water, on a diet consisting exclusively of water hyacinth, and at comparing growth rate and efficiency of food utilisation in the fish kept on a mixed feeding schedule (commercial feed + water hyacinth). In Treatment A, the fish were fed the commercial feed (Aller Master) only; Treatment B involved a mixed feeding of 3 days on the Aller Master and 1 day on water hyacinth; water hyacinth only was fed in Treatment C. All the fish in all the cages were weighed at 8-day intervals to adjust food rations and to calculate the food conversion ratio (FCR). Chemical analyses of the fish body, performed on termination of the experiment, allowed to calculate the apparent net protein utilisation (aNPU), energy retained (ER), and apparent lipid retained (aLR). The values obtained revealed water hyacinth to be ineffective food for the grass carp of the size used in the experiment. This was demonstrated by exceptionally high FCR values and very low body weight gains (6.4%). Increasing contribution of water hyacinth in the diet was accompanied by reduction in the body lipid content. The body weight growth of the fish fed the commercial feed was decidedly higher than that of the grass carp fed exclusively water hyacinths. The commercial feed offered at a 2% metabolic weight ration allowed to obtain a satisfactory growth rate at a cost-effective feed utilisation.

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