In aquaculture, animals can be continually exposed to environmental stress factors that put their health and even survival at risk. Two experiments were performed to evaluate the impact of different stress conditions (acute crowding and anaesthetic) on the natural haemolytic complement activity in serum and skin mucus of gilthead seabream (Sparus aurata L.). In the first experiment, fish were subjected to 10kgm−3 (low density, control group) and 50kgm−3 (high density, crowding group) during 2, 24 and 48h. In the second experiment, fish were unexposed (control) or exposed to 40ppm of MS-222 or 5ppm or 10ppm of clove oil for 1h. In fish maintained in acute crowding conditions only an increase of the haemolytic complement activity was observed in the skin mucus after 24h of exposure. However, a similar statistically significant increase was observed in serum and skin mucus of fish exposed for 1h to the lowest concentration of clove oil (5ppm) tested. The results point to a new and alternative way to assess stress in farmed fish by using skin mucus instead of blood serum and confirm that the measurement of natural haemolytic complement activity serves as an indicator of stress in fish.