A three year monitoring programme (1988-1990) of embayment habitats dominated by the seagrasses Posidonia oceanica and Cymodocea nodosa was carried out in Fornells Bay (Menorca, Western Mediterranean), where gilthead seabream (Sparus auratus) was artificially reared. Responses ranging from shoot density decrease to total seagrass demise were observed. Posidonia oceanica was more drastically and quickly affected than Cymodocea nodosa. In 1990, a total of 2 ha of seagrass were significantly degraded. Increased light attenuation, sedimentation rates, and grazing pressure are proposed as the principal factors accounting for seagrass regression.