A method of determining the effect of a spill on a marine environment, in which physical and chemical sensors are arranged with living organisms (4, 6) fitted with instruments, at an appropriate distance and position from the object (1) which is to be monitored, wherein the response of the living organism (4, 6), measured online in real time, is compared with known patterns of response, and where upstream and downstream measurements are compared to determine whether a change in response may be caused by a spill from the object, the significance of the real time measurements being validated through sampling of the organisms on a regular basis or as required, in order to analyze the effects on the health of the organisms, biomarkers, for the purpose of assessing the significance of the environmental effect.