Mapping and monitoring of the seagrass Posidonia oceanica in Croatian part of the Adriatic Sea showed significant decline in meadows density in the area influenced by inshore capture-based tuna aquaculture. The results of Posidonia meadows survey in the area of Fulija Islet, which are under the influence of tuna farms, are presented. This paper proves that nutrient enrichment and the risk of eutrophication is a significant issue in this area. Aquaculture wastes are a source of nutrient pollution and stimulate blooms of phytoplankton and algae population, as well as organic matter enrichment in the sediment. The condition and density of Posidonia oceanica meadows under the influence of tuna farms (at Fulija Islet) and out of its reach (two control stations at Iž Island and Mrtovnjak Islet) have been analyzed. Physico-chemical and biological variables have been examined in order to evaluate the impact of a tuna farm on the littoral near Fulija Islet. Besides biological and standard basic chemical parameters, such as the concentration of dissolved nutrients, the influence of fish farming on the distribution of dissolved organic carbon (DOC) and surface active substances (SAS) in the water column were also investigated. Significant differences were found between nutrient concentrations among the investigated stations under the influence of tuna farm (Fulija Islet) and out of its reach (control stations). The mucous aggregates of the benthic algae were observed during the spring and summer on the P. oceanica meadows. Shoot density of the P. oceanica meadows decrease at the stations close to the tuna farm, which confirms that the tuna farm activity strongly affects the surrounding meadows.