At the request of the companies, a number of researchers at the Science Faculty of the University of the Basque Country have analysed what type of contaminants are released as a result of extraction of crude oil in the North Sea and how these contaminants affect living things in this marine environment. For this research, mussels were studied. Throughout the world, these invertebrates are often used for investigating the state of the coastline and, so, there is a lot of information on the effects generated by the contaminants. This is why, although these molluscs are not inhabitants of this marine environment, they are regularly used for analysing the environment health status at the open sea. Network of cages A number of cages are submerged at high sea and then are distributed around the oil platforms, normally in two opposite directions. The first cages are positioned close to the platform, at some 500 metres for example; the next set are placed somewhat further away at 1000 metres distance and, finally, the third group of cages are located at quite a distance 10,000 metres from the platform. After 4 to 6 weeks, analyses are carried out measuring biomarkers in the mussels. Biomarkers indicate the degree of response to environmental insult at cellular and molecular levels - early warning signals of the effects of pollution in the ocean. Amongst other parameters, the mussels are analysed to determine whether they have accumulated metals or hydrocarbons, whether they are stressed, have the immune system weakened, have suffered damage in the reproductive system, abnormalities occur in subsequent generations, and so on. Nevertheless, the actual endponit of these studies is not to determine the health state of the mussels these molluscs are used as sentinels of the situation of the marine ecosystem. Moreover, given the complexity of the ecosystem in the North Sea, herring and cod fish have also been used in the research. Umbrella effect According to the results obtained from the measurements, it is still too early to determine the effect of the platforms on the mussels. Nevertheless, the researchers have been able to demonstrate that those mussels installed close to the platforms are hardly affected and, in fact, the influence of the oil platform is more noticeable at a certain distance. This means that the pollutants discharged during the crude oil extraction are dispersed from deep waters in the form of an open and inverted umbrella. But that is not all. Frequently, the cages installed on one side of the oil platform do not register the same effects than those on the other side, although they are located at the same distance from the rig. This might due to the fact that the currents displace the "umbrella from the platform axis at the ocean bottom. Anyway, the investigators say the research has to be confirmed in order to draw definitive conclusions. Above all, further parameters have to be taken into account and monitoring over a longer period of time has to be undertaken. Furthermore, the North Sea is not the easiest zone for this study given the fact that the high seas make the work of the scientists difficult.