Bivalve aquaculture is an important sector if the economy. Croatia is the only world producer of the European flat oyster Ostrea edulis. Cultivation technologies in use are extensive and depend on the conditions of the ecosystem in which cultivation takes place. Live bivalves in human food pose a health risk, their cultivation is strictly regulated by regulations. Recirculation systems in aquaculture enable precise monitoring of water quality parameters and control of conditions in breeding ponds. They imply intensive feeding with phytoplankton. An increase in sea temperature can negatively affect primary production, cause a decrease in the amount of food available for bivalves, and consequently reduced growth and the condition index of bivalves. The aim of this work is to investigate the influence of the intensity of feeding oysters with live plankton and commercial food for shellfish grown in a recirculation system on the change in the condition index of oysters. The purpose of the paper is to examine the possibilities of mitigating the negative consequences of the decrease in primary production. Two experiments were conducted in which oysters were fed in recirculatory systems. The experiments lasted three weeks, two recirculation systems were used and daily feeding was carried out. In the first experiment, the daily ration was 3% of dry oyster weight, two groups of oysters were fed different types of phytoplankton. In the second experiment, the same mixture of several types of phytoplankton was used for both groups, but a different daily ration, 3% and 6% of oyster dry weight. The target increase in fitness index was not achieved. The results indicate a possible influence of feeding intensity on the change in the condition index, but this cannot be claimed with certainty. Insufficiently clear results are a possible consequence of the small number of individuals in the sample. The fundamental conclusion is the need for a higher level of daily ration and further long-term research into the possibility of intensive feeding of oysters in a recirculation system.