The release of nutrients from fish farms may cause environmental problems and is traditionally monitored by analysis of nutrient concentrations in the surrounding waters. As concentrations vary strongly over the day this requires intense and expensive sampling. The aim of this study was to demonstrate the use of bioassays as an alternative low cost monitoring strategy that may document the distance from the farms where pelagic primary production is affected. We used a macroalgal and a phytoplankton bioassay to monitor the release of nutrients from four fish farms located in Cyprus, Greece, Italy and Spain. The macroalgal assay consisted of Ulva sp. contained in net cages and the phytoplankton assay was surface water, filtered through a 50 μm filter to remove grazers and then incubated in dialysis bags. Growth in both types of bioassay was highest closest to the fish cages as a result of nutrient release from the cages. The 2 types of assays agreed in most cases and growth exceeding background levels in both types was only found less than 150 m from the fish cages. This indicates that pelagic primary production was stimulated by nutrient release from these 4 fish farms up to a distance of 150 m downstream in the dominant current direction. The content of N, but not of P, in Ulva tissue was highest closest to the cages. This indicates transfer of N to the primary producers. The N : P ratio of Ulva tissue was > 40 in all cases, indicating a severe phosphorous limitation in agreement with the general notion of the Mediterranean being P limited. The content of N and P in Ulva tissue was at or below literature values of the minimum nutrient content necessary for growth. However, Ulva did grow and subsistence quota from the literature may not hold for Mediterranean Ulva. It is thus recommended that the variation in threshold concentrations for a given Ulva population is investigated, before a firm translation of internal nutrient concentrations to growth conditions is made.